Even though we ARE Nikon lovers,we are NOT affiliated with Nikon Corp. in any way.

English German French

Sign up Login
Home Forums Articles Galleries Recent Photos Contest Help Search News Workshops Shop Upgrade Membership Recommended
members
All members Wiki Contests Vouchers Apps Newsletter THE NIKONIAN™ Magazines Podcasts Fundraising

Metering and exposure compensation on the D800...

walk43

Pennsylvania, US
719 posts

Click to send email to this author Click to send private message to this author
walk43 Registered since 07th Feb 2012
Mon 10-Dec-12 04:31 PM | edited Mon 10-Dec-12 05:26 PM by walk43

Another technical question for the many of you who I am certain have a better handle on this than I apparently do.

We all know that the amount of light entering the lens is controlled via shutter speed and aperture. Most of us also remember that the 'speed' of film allowed the same amount of light to record the image on the film either slower or faster based on the ISO rating of the film.

Question 1. Now with digital sensors, the technical ISO adjustment allows us to compensate for the lack of film 'speed'. How does it do that... technically? What actually happens or is adjusted within the camera? Is it the sensitivity or density of image distributed on the sensor that gets changed by the ISO value....which is why it is more noisy?

Question 2. We also have an EV button on the top of the camera that let's us adjust the exposure to lighten or darken the recorded image, regardless of the A,S or ISO settings. I believe this adjustment actually affects the meter by changing shutter speeds in A priority for example...but why do that? is it an 'auto' way to do 'manual settings??

Or to put it more simply... how do the 4 variables (A,S,ISO,EV) work together to set the exposure? What has priotity etc.

My real question I guess is just what does the EV button do as it relates to the other 3...and when?

Dan
(Nikon D800,V2,Sony HX400V,Lumix ZS40)
"I don't read, I just look at pictures" - Andy Warhol

Visit my Nikonians gallery.

Visit my Nikonians gallery.

icslowmo

Surprise, US
613 posts

Click to send email to this author Click to send private message to this author

#1. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 0

icslowmo Registered since 01st Jan 2012
Wed 12-Dec-12 02:07 AM

My answers to your questions as I understand them:

Question 1:

Digital sensors need a certain amount of voltage to read light being picked up. D800 sensor is configured to be equal to iso 100 (Base). Now by increasing read voltage, iso goes up and so does digital noise. How the sensor re-acts is what you see in pics plus any internal noise processing done by the processor in the camera.

Question 2:

Exposure comp. is used to bias the internal light meter in the camera +/- how ever much the operator decides is correct. So if you spot meter on say a black shirt/jacket, the camera will try to expose as close to middle gray as passable and will tend to over-expose. Which would need negative exp. comp. to compensate for what the internal light meter is trying to do.

Hope this helps direct you in the right direction on understanding your questions...

Chris

Visit my Nikonians gallery.

Visit my Nikonians gallery.

ericbowles

Atlanta, US
10639 posts

Click to send email to this author Click to send private message to this author

#2. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 0

ericbowles Moderator Awarded for his in-depth knowledge and high level skills in various areas, especially Landscape and Wildlife Photoghraphy Writer Ribbon awarded for for his article contributions to the community Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015 Nikonian since 25th Nov 2005
Wed 12-Dec-12 09:55 AM

ISO is essentially an amplifier. It's like turning up the volume on a distant radio station. But if there are noisy pixels, they are also amplified. So if you had a radio with a little static, the static would be more easily noticed if it was louder.

Exposure Comp signals the camera to change the exposure by changing one of the variables - Aperture, Shutter, or ISO - depending on which automated mode you have selected. If you are in Aperture Priority with a fixed ISO, exposure compensation changes the shutter speed. With Manual Mode, Exposure Comp just changes the scale.

By dialing in exposure compensation, you can account for scenes or subjects that need more or less light than metering suggests. The meter suggests a neutral exposure based on the metering mode selected. A dark subject dominating the frame would need less light since it needs to be rendered darker than neutral. A subject in a brightly lit scene - such as a snow scene - would need positive compensation to make the snow white rather than neutral gray.

Eric Bowles
Nikonians Team
My Gallery
Workshops

Nikonians membership — my most important photographic investment, after the camera

walk43

Pennsylvania, US
719 posts

Click to send email to this author Click to send private message to this author

#3. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 0

walk43 Registered since 07th Feb 2012
Wed 12-Dec-12 02:18 PM | edited Wed 12-Dec-12 02:20 PM by walk43

Thanks guys, that's much more clear than the internet explanations.

>>Exposure Comp signals the camera to change the exposure by changing one of the variables - Aperture, Shutter, or ISO - depending on which automated mode you have selected. If you are in Aperture Priority with a fixed ISO, exposure compensation changes the >>shutter speed. With Manual Mode, Exposure Comp just changes the scale.
>>

Eric...for me this is an important point!! So when I adjust the EV the camera changes the appropriate other settings and not my priority setting. So that means for example that if I want a deep DOF and am in A mode with a fixed ISO then the EV adjustment will change my shutter speed. And with higher +/- EV corrections the speed will change more than with lower +/- adjustments. Assuming I am correct here, that is a big piece of information I was not aware of. I just tested it out and it seems to work as you explain....no surprise But that means that 5 stops in EV equals 5 stops in shutter speed in 'A' priority ... right? I wonder how many others out there have not been aware of that? Guess it just shows why I keep asking questions...I know the actual exposure is in the VF but I personally never checked it after I adjusted the EV.

Assuming this is all the case...why not avoid the EV adj all together and just use the ISO or shutter speed manually. That way I would be more aware of what I am changing and avoid the possibility of having the EV adjusted and forgetting that I did it in later shots.

Dan
(Nikon D800,V2,Sony HX400V,Lumix ZS40)
"I don't read, I just look at pictures" - Andy Warhol

Visit my Nikonians gallery.

Visit my Nikonians gallery.

ericbowles

Atlanta, US
10639 posts

Click to send email to this author Click to send private message to this author

#4. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 3

ericbowles Moderator Awarded for his in-depth knowledge and high level skills in various areas, especially Landscape and Wildlife Photoghraphy Writer Ribbon awarded for for his article contributions to the community Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015 Nikonian since 25th Nov 2005
Wed 12-Dec-12 06:14 PM

<Assuming this is all the case...why not avoid the EV adj all together and just use the ISO or shutter speed manually. That way I would be more aware of what I am changing and avoid the possibility of having the EV adjusted and forgetting that I did it in later shots.>

Probably a better solution is to get in the habit of looking at the settings in your viewfinder. The automation and metering in the camera is really quite good. It's just a matter of learning how to use it.

Most photographers use Aperture priority a very high percentage of the time. Most use a fixed ISO unless there is a good reason. That means Exp Comp essentially adjusts shutter speed.

Eric Bowles
Nikonians Team
My Gallery
Workshops

Nikonians membership — my most important photographic investment, after the camera

txstone12

Texas, US
630 posts

Click to send email to this author Click to send private message to this author

#5. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 0

txstone12 Gold Member Donor Ribbon awarded for his generous support to the Fundraising Campaign 2014 Nikonian since 12th Feb 2012
Thu 13-Dec-12 02:16 PM | edited Thu 13-Dec-12 05:10 PM by txstone12

This is a very informative topic. Thanks, Dan.

I always like to start a new area with a tutorial. Here's one on beginning ISO from one of my favorite photographer/authors, Nasim Mansurov

http://photographylife.com/what-is-iso-in-photography

I always get to the question, so what is it? Nasim says, ISO is the level of sensitivity of your camera to available light - whether we're talking film or image sensor. To me, that even relates to how Chris and Eric think of ISO. Chris - increase read voltage on the image sensor and Eric - increase the amplification at higher ISOs. Increase the voltage, amplification, sensitivity and you can see we're in for a tradeoff. Greater light sensitivity at the expense of noise on this still analog device, the image sensor. BTW, Chris does that mean operating at higher ISO levels in low light conditions, we are creating a greater draw on the battery?

There is a really good explanation of what happens, and in what order, with Auto-ISO - it's either in Digital Darrell's or Thom Hogan's D800 book. I haven't dug it out this morning. I like to use Auto-ISO, but others may prefer not.

If we're in a low light situation, Eric's mention of aperture priority makes a lot of sense to me. Use a fast lens, open the aperture and crank up ISO to try and keep the shutter speed up.

If we choose to use Manual, and even manual ISO, aren't we in the realm of seeing what kind of exposure preview we have available or resorting to exposure tables? Eric mentioned Exposure Comp changes the scale, but I don't understand?

Edited to remove atch remnant

David

Visit my Nikonians gallery


Visit my SmugMug gallery

Visit my Nikonians gallery.

jgould2

Fort Pierce, US
4631 posts

Click to send email to this author Click to send private message to this authorClick to send message via AOL IM

#6. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 5

jgould2 Gold Member Fellow Ribbon awarded for his excellent and frequent contributions and sharing his in-depth knowledge and experience with the community in the Nikonians spirit. Nikonian since 13th Oct 2007
Thu 13-Dec-12 03:41 PM

Hi David.

Looking through the viewfinder or on the top LCD there is an exposure scale whenever we are in M mode or whenever we have applied exposure compensation in A,S, or P modes.

Lets take the snow example, the camera's metering system will try to render the snow as medium gray. We must therefore override the camera's metering system. Lets say by decreasing the shutter speed by one stop.

First in A mode we would use positive exposure compensation of one stop. When we take the picture the shutter speed would have been decreased by one stop (assuming manual ISO). Before the exposure compensation was added no exposure bar would be visible. After the exposure compensation is added the exposure bar appears, and will show one stop of compensation.

Second in M mode we would simply change the shutter speed by one stop. The exposure bar (which is always visible in manual mode) will show that the camera thinks we are one stop overexposed.

Third in M mode we could use one stop of exposure compensation. If we keep the original exposure, the exposure bar will show that the camera now thinks that our shot is one stop underexposed. Once we zero out the meter by decreasing shutter speed one stop we will find we have done the same thing we did in the second case above.

We do not have to use exposure tables in manual mode, just look at the exposure bar. That tells you what the camera would have picked in A mode.

JIM

ericbowles

Atlanta, US
10639 posts

Click to send email to this author Click to send private message to this author

#7. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 6

ericbowles Moderator Awarded for his in-depth knowledge and high level skills in various areas, especially Landscape and Wildlife Photoghraphy Writer Ribbon awarded for for his article contributions to the community Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015 Nikonian since 25th Nov 2005
Thu 13-Dec-12 05:32 PM | edited Thu 13-Dec-12 05:38 PM by ericbowles

Nice explanation, Jim.

This sounds more complicated than it is. Just look what happens through the viewfinder to the scale as you change Exp Comp.

David - My choice of Aperture priority is usually driven by creative decisions rather than light. I'm mentally thinking about DOF and whether I want a lot or a little. And I'll use ISO to make sure that the shutter speed is reasonable.

I use Shutter priority in a similar manner. In practice, I stay in Aperture mode even when I think in terms of shutter speed. I just adjust the aperture and ISO to reach my desired shutter speed.

Eric Bowles
Nikonians Team
My Gallery
Workshops

Nikonians membership — my most important photographic investment, after the camera

walk43

Pennsylvania, US
719 posts

Click to send email to this author Click to send private message to this author

#8. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 7

walk43 Registered since 07th Feb 2012
Thu 13-Dec-12 05:57 PM

Ditto that Eric!!

Dan
(Nikon D800,V2,Sony HX400V,Lumix ZS40)
"I don't read, I just look at pictures" - Andy Warhol

Visit my Nikonians gallery.

Visit my Nikonians gallery.

txstone12

Texas, US
630 posts

Click to send email to this author Click to send private message to this author

#9. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 6

txstone12 Gold Member Donor Ribbon awarded for his generous support to the Fundraising Campaign 2014 Nikonian since 12th Feb 2012
Thu 13-Dec-12 07:12 PM

>
>First in A mode we would use positive exposure compensation of
>one stop. When we take the picture the shutter speed would
>have been decreased by one stop (assuming manual ISO). Before
>the exposure compensation was added no exposure bar would be
>visible. After the exposure compensation is added the exposure
>bar appears, and will show one stop of compensation.
>
>Second in M mode we would simply change the shutter speed by
>one stop. The exposure bar (which is always visible in manual
>mode) will show that the camera thinks we are one stop
>overexposed.
>
>Third in M mode we could use one stop of exposure
>compensation. If we keep the original exposure, the exposure
>bar will show that the camera now thinks that our shot is one
>stop underexposed. Once we zero out the meter by decreasing
>shutter speed one stop we will find we have done the same
>thing we did in the second case above.
>

That's good - I have to try that! Thanks for the explanation, Jim.

I have a goal of trying to use EC better. I mostly fly by the seat of my pants right now.

David

Visit my Nikonians gallery


Visit my SmugMug gallery

Visit my Nikonians gallery.

txstone12

Texas, US
630 posts

Click to send email to this author Click to send private message to this author

#10. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 7

txstone12 Gold Member Donor Ribbon awarded for his generous support to the Fundraising Campaign 2014 Nikonian since 12th Feb 2012
Thu 13-Dec-12 07:29 PM

>
>David - My choice of Aperture priority is usually driven by
>creative decisions rather than light. I'm mentally thinking
>about DOF and whether I want a lot or a little.
>

Hmmm - very interesting. I have to say that's a step (perhaps two) beyond where I am. I usually am only able to think

- DOF - need to do this...
- Low light - an easier cue for me.

I'm afraid I'm still working on putting the elements together to form an artistic interpretation.

I'm going to give your advice some thought and try to move in that direction.

David

Visit my Nikonians gallery


Visit my SmugMug gallery

Visit my Nikonians gallery.

jgould2

Fort Pierce, US
4631 posts

Click to send email to this author Click to send private message to this authorClick to send message via AOL IM

#11. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 9

jgould2 Gold Member Fellow Ribbon awarded for his excellent and frequent contributions and sharing his in-depth knowledge and experience with the community in the Nikonians spirit. Nikonian since 13th Oct 2007
Thu 13-Dec-12 11:41 PM

Hi David.

I forgot to mention that all three cases I mentioned would arrive at the exact same f/stop and shutter speed.

Keep flying by the seat of your pants and all this stuff will become second nature.

JIM

GaryPk

Bailey, US
543 posts

Click to send email to this author Click to send private message to this author

#12. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 11

GaryPk Silver Member Nikonian since 30th Apr 2012
Fri 14-Dec-12 08:23 PM | edited Fri 14-Dec-12 11:33 PM by GaryPk

I have been having fun doing the same thing a little differently ... shooting in Manual Mode / Spot Metering using the Zone System. Same thing really. Meter off something in Zone 6, for example, and add +1 exposure. Lots of ways to get to the same place.
<a href="http://www.flickr.com/photos/30039864@N07/8227911209/" title="Great clouds out the back door tonight ... by LightPaintr, on Flickr"><img src="

" width="320" height="218" alt="Great clouds out the back door tonight ..."></a>

Visit my Nikonians gallery.

Visit my Nikonians gallery.

icslowmo

Surprise, US
613 posts

Click to send email to this author Click to send private message to this author

#13. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 5

icslowmo Registered since 01st Jan 2012
Sat 15-Dec-12 08:06 AM

Sorry for not responding faster but from what I've seen and according to some text, yes higher iso can use up more battery power. I've looked through the CIPA rating test:

http://www.cipa.jp/english/hyoujunka/kikaku/pdf/DC-002_e.pdf

And nothing is said about iso settings except using factory settings, which would be base iso I'm sure. And if you look through this write up:

http://www.digitalphotopro.com/gear/cameras/the-truth-about-digital-iso.html?start=2

It explains that in a digital camera sensor, the amplifications/gains have to be turned up to increase signal/iso to create set iso values according to standards set forth. So the only way to increase gains/amplify signal in electronics, voltage would have to increase. Now we are talking mV more then likely sense cameras run on such small batteries.

The D800 has a 7.0V battery rated at 1.90Ah. You can put 8 AA batteries rated at 1.2v each with 2.00+Ah in a battery grip equal to ~9.6v and ~16.00Ah+/- off rechargeables..... Still not a lot off voltage.

Say on a D800, each base iso pic uses a burst of 500mV, then iso 200 uses 525mV, iso 400 could be 550mV, etc. and say iso 6400 is at 650mV, these small increases will use more battery power. Small increases, but can add up though.

Got a little technical but hope it helps explain why I would say higher iso's can create higher draw on a battery.

Chris

Visit my Nikonians gallery.

Visit my Nikonians gallery.

walk43

Pennsylvania, US
719 posts

Click to send email to this author Click to send private message to this author

#14. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 13

walk43 Registered since 07th Feb 2012
Sat 15-Dec-12 10:44 AM

Chris,

This is an excellent and simple tech write up about how ISO works with sensors and batteries.

Many years ago, Lee Trevino (pro golfer) said that all golfers should play at least one round of golf with only the 5 iron... so that he/she will learn what can be done with a club. I tried it and was a much better golfer after I did.

I think the same is true for cameras. We should learn and experiment with the camera controls so that we know what we can do with what control and how that control affects other aspects of the IQ and camera capability. Short periods of experimentation and understanding how the camera does its stuff can make us better photographers.

Thanks for the post.

Dan
(Nikon D800,V2,Sony HX400V,Lumix ZS40)
"I don't read, I just look at pictures" - Andy Warhol

Visit my Nikonians gallery.

Visit my Nikonians gallery.

walk43

Pennsylvania, US
719 posts

Click to send email to this author Click to send private message to this author

#15. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 12

walk43 Registered since 07th Feb 2012
Sat 15-Dec-12 10:49 AM

< Lots of ways to get to the same place

Gary,

VERY nice pic Gary!! See my response to Chris below. Seems like you are practicing with your '5 ron'

Dan
(Nikon D800,V2,Sony HX400V,Lumix ZS40)
"I don't read, I just look at pictures" - Andy Warhol

Visit my Nikonians gallery.

Visit my Nikonians gallery.

briantilley

Paignton, UK
30235 posts

Click to send email to this author Click to send private message to this author

#16. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 13

briantilley Gold Member Deep knowledge of bodies and lens; high level photography skills Donor Ribbon awarded for his support to the Fundraising Campaign 2014 Nikonian since 26th Jan 2003
Sat 15-Dec-12 11:03 AM

You may be correct, but there are so many other things about a DSLR that must surely draw more current (autofocus, VR, monitor/LCD panels, viewfinder displays, built-in flash, shutter and mirror motion, sensor cleaning...) that I suspect any practical difference in battery life from the greater amplification at higher ISO settings would be minimal.

Brian
Welsh Nikonian

km6xz

St Petersburg, RU
3576 posts

Click to send email to this author Click to send private message to this author

#17. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 16

km6xz Moderator Awarded for his in-depth knowledge in various areas, including Portraits and Urban Photography Nikonian since 22nd Jan 2009
Sat 15-Dec-12 07:30 PM

I think a wrong track is being followed.
Gain does not require higher voltage or require more current to be drawn. Gain comes in two stage types, analog and digital.
The analog amplification occurs right after a pixel is scanned when the accumulated photon count is measured and sent to an analog amplifier the conditions the level and provides isolation(buffer stage) for the optimum dynamic range of the ADC, analog to digital converter.
This takes a snapshot of the analog voltage representing the charge state of the pixel and creates a numerical binary word that represents the analog voltage. The pixel has a dynamic range of the difference in voltage in the pixel with no light, comprising mostly of noise of several types that sets the lowest voltage that a pixel can register as black. The maximum voltage level is the number of photons that be accumulated in the pixel between reads is called "full-Well" value. Any additional photon hitting that pixel will not be registered.
The difference between those two values is the dynamic range of the sensor itself.
Pixels are arranged in rows that are read out by sequentially reading and clearing the values in each pixel in a row when addressed by selecting a row number and a column that uniquely addresses a pixel. Each column has a dedicated ADC that creates the number in binary format representing the voltage level that in an analog of the number of photons detected in the time between the last read of that pixel. All column are converted at once since there are converters attached to each column.
The analog signal needs to be preconditioned before being sent to the analog to digital conversion stage, because the ADC has it own DR and minimum value below which is ambiguous noise. After a number is generated, the signal from then on is in the digital domain and digital gain can be applied by arithmetically multiplying the recorded value by a gain. Any noise present at this point is multiplied as well. There are a number of sources of noise, both analog(amplifier noise, conversion noise, shot noise, 1/f noise). Changing ISO changes a combination of the analog preconditioning and primarily the digital multiplication.
All this is to lay the groundwork for explaining where current drain occurs. The reading of a pixel is a very low current process, due to the very high input impedance of the input amplifier, current varies very little between the lowest idle current of the amplifier and highest voltage output. Minimum idle current is sought because any heat generated increases noise, but not enough idle current and distortion increases in the amplifier. That analog stage can also act as an attenuator so get the low extended ISO values under the natural ISO for the D800 of 100.
To reduce losses, noise and complexity, all the ADC and first amplifiers are etched right on the sensor silicon wafer.

Reading a sensor does not increase or decrease current based on light or well level. The only significant current is the switching losses when addressing each pixel. That is constant regardless of the signal level or gain, and is primarily determined by the clock rate of the addressing. Low pixel count sensors have fewer reads per second so switching losses are less, sensors and related electronics would stay cooler. Same with computer memory, run at low clock speeds, chips run cool, and heat increases as clock speed increases regardless of what data is being stored. The rate a sensor is scanned determines the current draw per pixel and more pixels mean more heat(current)independent of light level or gain. The major current draws are fast addressing, the DSP (digital signal processor, Expeed3 in this case, is a very fast throughput device so generates a lot of heat through switching losses. Digital circuits pull almost no current in a steady state but switching from a logic 1 to 0 or 0 to 1 takes a finite time short period where the state if changing in voltage, ramping up or down, in nanoseconds where losses occur due to capacitance. The faster it runs the more current. What the data is, makes no difference. It is a multicore processor so as more cores are activated for some computation intensive tasks like NR, current and heat can spike upwards but the image data makes no difference, just what processes are being applied to it.

So if battery drain changes with photo content or ISO, it is not due to the amplification or reading the sensor, would be from the additional processing that some images might require after the images has already been recorded.

The big current draws are backlights, AF motor current, VR etc and the DSP, assist light, on-board flash and mirror motors.
Stan
St Petersburg Russia

Visit my Nikonians gallery.

walk43

Pennsylvania, US
719 posts

Click to send email to this author Click to send private message to this author

#18. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 17

walk43 Registered since 07th Feb 2012
Sat 15-Dec-12 07:38 PM

Stan...you made me think of a comment a guy made to me several weeks ago. Does a sensor get warm and if it does is it more noisy?

Dan
(Nikon D800,V2,Sony HX400V,Lumix ZS40)
"I don't read, I just look at pictures" - Andy Warhol

Visit my Nikonians gallery.

Visit my Nikonians gallery.

km6xz

St Petersburg, RU
3576 posts

Click to send email to this author Click to send private message to this author

#19. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 18

km6xz Moderator Awarded for his in-depth knowledge in various areas, including Portraits and Urban Photography Nikonian since 22nd Jan 2009
Sun 16-Dec-12 01:10 AM

Yes, the sensor does get warm in extended active scan mode such as video and LV and noise, mostly thermal noise increases. Leaky pixels can pop up while warm and return to normal after a cool down. It is not an issue with VF still mode however, the active scanning is very brief, a very short duty cycle compared to the 100% duty cycle in LV and video mode.
Stan
St Petersburg Russia

Visit my Nikonians gallery.

txstone12

Texas, US
630 posts

Click to send email to this author Click to send private message to this author

#20. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 0

txstone12 Gold Member Donor Ribbon awarded for his generous support to the Fundraising Campaign 2014 Nikonian since 12th Feb 2012
Sun 23-Dec-12 09:10 PM

But, Stan…

First of all, I made the comment to Chris about ‘does operating at higher ISO levels mean we are creating more draw on the battery’? Sorry to divert you to this … ahhh, bumpy track, Chris.

Dan, this is still in the spirit of exchanging information to answer your Question 1.

I’ve spent a good bit of time trying to understand your post, Stan. I’d like to try to put it in context and understand more about how DSLRs operate at high (beyond base) ISO levels. My goal is to understand and illuminate as much as possible how ISO processing relates to Nikon and the D800. I’ve tried to limit my search to noise processing within operating ISO limits (spec values), not long exposure noise reduction. I’ve also tried to limit consideration to that processing done for a single pixel only, not software algorithms that compare pixels over an area.

What I hope to show is DSLRs perform a lot of processing to deal with the complications of noise. I also want to ask for help in finding what the D800 does in terms of NR processing. My contention (as I believe we intended earlier) is that this processing all results in low power consumption, but some power consumption.

A general observation from Nikon

"Raising ISO sensitivity amplifies the electronic signal, which also amplifies any noise in the signal; as a result, the higher the ISO sensitivity, the more obvious the effects of noise on your photographs."

http://imaging.nikon.com/history/basics/13/index.htm

As you mentioned, Stan, Nikon has developed proprietary technology, at least since the FX format CMOS sensor introduced in the D3.

"Nikon has also developed an original method for reading the amplified electrical signal — 12-channel high-speed readout. By reading electrical signals in parallel, high-speed readouts are achieved (about twice the speed of the D2X). At the same time, the drive speed for a single channel becomes inversely lower. This results in better suppression of noise generated in the circuit, and …"

http://www.nikon.com/about/technology/rd/core/optics/cmos/index.htm

A figure labeled 12-channel high-speed readout shows column amplifiers as you mention with 12 channels in parallel for signal readout.

The article below lists the four main sources of noise in DSLRs – Dark noise, Readout (aka Bias) noise, Photon noise and Random noise.

http://photo.net/learn/dark_noise/

The main point I wanted to bring up here is that the article describes
Complicating Factors:

"CMOS sensors allow the placement of both photosites and transistors on the sensor itself. (CCDs cannot have any processing circuitry built into the sensor - just transfer gates and the like, which are controlled by off-sensor control circuitry.) Because of this, CMOS sensors generally have at least the readout amplifier built in to the photosite. There may be other transistors as well, which perform other processing steps. It is now very common for a CMOS sensor to include noise-reduction circuitry directly on the sensor alongside the readout amplifier. In some designs, a sort of small dummy photosite, shaded from light, is used to quantify the likely dark noise level in the actual photosite, and this quantity is subtracted during readout. In other designs, a constant - corresponding to the tested dark current of the sensor - is subtracted from the photosite value during readout. If anything like this is happening, expectations such as "dark noise will double with twice the exposure duration" may turn out to be false."

This article goes on to say,

"Note that both of these kinds of on-sensor processing affect the camera's RAW image. That is to say, the RAW image is not necessarily "exactly what the sensor detected," as is often said. Instead, it is exactly what the sensor detected, plus or minus whatever built-in on-sensor processing is being done in that particular camera. The raw image lacks any post-readout processing, of course - the point is that on CMOS sensors some processing may be unavoidable and its effects will be present in the raw format image."

Since my primary interest is in what technology Nikon is using, that brings us to a review of the Nikon D3X. This review says that Nikon, in addition to the now familiar 12 channel readout, applies on-chip noise reduction and NR algorithms on the image processor.

http://m.dpreview.com/reviews/nikond3x

Among the Key Features are


"• 14-bit A/D conversion, 12 channel readout
• Gapless micro lens array and on-chip noise reduction
• Nikon EXPEED image processor (Capture NX processing and NR algorithms, optimized for D3X, lower power)"

The Nikon information below on both the D3S and D3X, sheds some light on the D3S

http://imaging.nikon.com/lineup/microsite/d3s_d3x/en/image_quality/
TECHNOLOGY:
"14-bit A/D conversion and 16-bit image-processing pipeline
… So, to maintain impeccable accuracy, the D3S and the D3X use 16-bit data transmission throughout the image-processing pipeline."
(More about the image-processing pipeline later)

"Advanced noise reduction at high ISO for natural-looking results
The D3S uses a newly developed image sensor, which has been refined from the D3 to further minimize noise generated by the amplified signals. This contributes to exceptional image quality and spectacular saturation, even at high ISO settings such as ISO 12800. High ISO noise reduction activates automatically when shooting over ISO 3200, minimizing mottling, color bleeding and shadow noise."

What about the current generation Nikon DSLRs, the D4 and D800/E? I haven’t been able to find much on either

D4 - Nikon's original key technology
http://imaging.nikon.com/lineup/dslr/d4/features01.htm

"Optimum use of light: Nikon's proprietary image sensor technology
Such exceptional image integrity across such a wide ISO sensitivity range is made possible through Nikon's proprietary and exclusive sensor technologies. The pixels are spaced at a pitch of 7.3 µm while gapless micro-lenses are employed, and anti-reflective coating is used on various parts – all of which results in minimized ghost and flare. This detailed design gives the D4 an unprecedented ability to channel all available light efficiently and directly into the sensor. The advantage is maintained through improved sensor quantum efficiency, ensuring optimum conversion of light into electric signals, and delivering digital files at ISO 100 to 12800 with a wide dynamic range and an outstanding signal-to-noise ratio. The benefit of a high-efficiency sensor is enhanced by an integrated approach to noise reduction. The layout of electronics within the sensor has been carefully configured to minimize noise. Despite operating at the amazing speed required to realize approx. 11 fps, the D4's image sensor consumes less power, contributing to extended battery life."

And the D800/E

Nikon’s original technology
http://imaging.nikon.com/lineup/dslr/d800/features01.htm

"EXPEED 3 image-processing engine: speed, versatility, and high performance

Even with specialized processing features like Active D-Lighting and high ISO noise reduction, capture speed is not affected."

D800 Key Features
http://www.europe-nikon.com/en_GB/product/digital-cameras/slr/professional/d800

"36.3 megapixel FX-format (full-frame) CMOS sensor with high signal-to-noise ratio, wide dynamic range and 12-channel readout."

And finally, Nasim Mansurov on the ‘image-processing pipeline’

Benefits of a High Resolution Sensor - Jan 9, 2012 BY NASIM MANSUROV
http://photographylife.com/the-benefits-of-a-high-resolution-sensor

"There is a fourth*, very important attribute that very few people mention when talking about pixels and sensors that also plays a huge role; it is the software algorithm run by the image processor that analyses the data from the sensor and runs a series of image processing steps to reduce various artifacts, reduce noise, apply sharpening and more. This is commonly called the “image processing pipeline”…"
* Pixel Size, Pixel Density, Sensor Size and Image Processing Pipeline

"But revenge and larger market capture are not the only reasons why Nikon decided to go with a 36 MP sensor on the D800, in my opinion. There are two more key factors here – high resolution sensors are cheaper to make in the long run for Nikon than low-light sensors. Sounds wrong, but Nikon spends a lot of R&D money on its noise reduction algorithms. And after spending all that time and money, it is painful to see something like the D700 cannibalize its flagship line sales. …"

So, there is a whole lot of noise processing going on, although in a low power environment. It looks like some of this processing may be performed on the EXPEED 3 processor. I would be very interested in hearing what others know about just what NR the D800 does on chip and in the processor.

Back to the list of power consuming operations. Should we update the list to include use of Live View, Stan?

David

Visit my Nikonians gallery


Visit my SmugMug gallery

Visit my Nikonians gallery.

txstone12

Texas, US
630 posts

Click to send email to this author Click to send private message to this author

#21. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 0

txstone12 Gold Member Donor Ribbon awarded for his generous support to the Fundraising Campaign 2014 Nikonian since 12th Feb 2012
Sun 23-Dec-12 09:25 PM

Jim,

I've gone through a couple of practice sessions on Exposure Compensation following your post and wanted to tell you I do appreciate your help. I know a lot more about use of EC. Still need a little more practice to go through your exercise and end up at the same f-stop and shutter speed. My problem is, I don't have snow to practice on.

And Gary,

Your comment didn't go unnoticed - somehow your backyard is a lot prettier than mine.

David

Visit my Nikonians gallery


Visit my SmugMug gallery

Visit my Nikonians gallery.

jgould2

Fort Pierce, US
4631 posts

Click to send email to this author Click to send private message to this authorClick to send message via AOL IM

#22. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 21

jgould2 Gold Member Fellow Ribbon awarded for his excellent and frequent contributions and sharing his in-depth knowledge and experience with the community in the Nikonians spirit. Nikonian since 13th Oct 2007
Sun 23-Dec-12 09:45 PM

Hi David.

Just keep practicing and it will all come to you. Try taking a picture that contains a large percentage of overcast sky.

I think the important thing to remember is that using exposure compensation in A mode can give you the same exact numbers as changing the shutter speed in M mode.

JIM

Gray_star

US
52 posts

Click to send email to this author Click to send private message to this author

#23. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 0

Gray_star Registered since 17th Dec 2012
Sun 23-Dec-12 10:34 PM


>Question 1. Now with digital sensors, the technical ISO
>adjustment allows us to compensate for the lack of film
>'speed'. How does it do that... technically? What actually
>happens or is adjusted within the camera? Is it the
>sensitivity or density of image distributed on the sensor that
>gets changed by the ISO value....which is why it is more
>noisy?
>
>Question 2. We also have an EV button on the top of the camera
>that let's us adjust the exposure to lighten or darken the
>recorded image, regardless of the A,S or ISO settings. I
>believe this adjustment actually affects the meter by changing
>shutter speeds in A priority for example...but why do that? is
>it an 'auto' way to do 'manual settings??

You got a lot of explanations...I figured one more can't hurt!

ISO
In both film and digital photography, ISO represents the reaction to light. With film, this is called the Sensitivity of the film. With digital, this is called the Responsivity of the imaging system.

With film, the ISO represented a level of density. So if you set a specific shutter, aperture, and shine a specific intensity of light through the lens, the film will react by a certain amount. ISO defines that amount. Films with high ISO react more than films with low ISO (and so, films with a low ISO need more exposure to reach the same density.)

With digital sensors, there's no chemical reaction as with film. The sensor, simply "collects" the photons that fall on it, and ISO makes no difference. ISO is applied after collection. When the signals are transferred out of the pixels, they are amplified based on the ISO selection. So if you set a high ISO, the camera will adjust the meter accordingly; when you take your shot you won't collect much light; and when the collected signals are transferred from the pixels, the signals will be amplified to levels similar to that of a low ISO capture. However, as we know, there will be more noise in the image. Noise depends on the size of the captured signal. Small signal, more noise...large signal, less noise.

Noise is a statistical thing. It's equal to the square root of the signal. So a signal of 1 has a noise value of 1, signal of 4 has noise of 2, signal of 16, noise of 4, signal 100, noise of 10, and so on. As you can see, the larger the signal, the smaller the percentage of noise. That's why we want to shoot at low ISO and collect more light.

Exposure Compensation
To quote you, yes, it's "an 'auto' way to do manual settings." However, to view it in that way is a misunderstanding of the control of exposure. But first, the direct technical answer...

How do the 4 variables work together? First lets correct one of your variables. There's A, S, ISO, and...not EV...but light...either luminance (reflected from an area) or illuminance (from a source.) This is why I dislike the Exposure Triangle...it leaves out light.

Exposure Value is an important term to understand. It has two meanings. First, EV is a combination of aperture and shutter. Second, EV is a combination of ISO and light. Now here's where it comes together...when the EV of aperture/shutter equals the EV of ISO/light, you have Standard Exposure.

When you are in M mode, the center of the meter represents the EV of ISO/light. The current position of the Exposure Indicator represents the EV of aperture/shutter. When you move the Exposure Indicator to the center, you have Standard Exposure. But the meter can be fooled by, say, a bright snow scene. So in that case you move the Exposure Indicator to a position, such as +2, on the meter to correct the exposure.

When using auto modes, the camera performs the step of setting Standard Exposure. That is the "auto" of auto modes...and that's all they do. They continuously set Standard Exposure. In order to control exposure for that snow scene, the Exposure Compensation function is used. By applying +2 EC, the EV of the ISO/light combination is shifted by 2 EV.

And now, finally, the answer to your question...how does EC relate to the three settings? It doesn't. The job of EC is to shift the EV of the meter. The camera, now having a new EV to work with, will do whatever its current functionality would normally do with such an EV.

It is important to not take the view that, for example, "in A mode EC shifts the shutter speed." Well, normally yes. But if you have AutoISO enabled and the shutter speed is at your Minimum Shutter Speed already, then apply EC may increase ISO instead. That's because EC doesn't control the camera...it only affects the meter. The camera simply does what its settings tell it to do based on the adjusted meter reading.

Another interesting thing to note is that Nikons are designed so that control of exposure in M mode requires the same process as control of exposure in auto modes. In M mode, if you want +1, you adjust your settings until the meter indicates +1. In auto modes, if you want +1 then you press EC and apply +1. The "meter" in the viewfinder will now turn on and show you +1...exactly like M mode. And there's more. If you were to enable Easy EC, which puts EC on the unused command dial when using auto modes, then control of exposure between M mode and auto modes is practically indistinguishable...that is, the command dial movements turn out to be practically identical.

This is why i say to not look at as a manual way to control exposure, because adjusting exposure in M mode can just as easily be seen as an variation of auto-mode exposure control. It should also be clear that you have just as much control over exposure with auto modes and you do with M mode. They're simply different ways to control the camera...each mode to be used when it is most appropriate for the current application.

jamesvoortman

Durban, ZA
1479 posts

Click to send email to this author Click to send private message to this author

#24. "RE: Metering and exposure compensation on the D800..." | In response to Reply # 3

jamesvoortman Silver Member Nikonian since 06th Sep 2004
Mon 24-Dec-12 08:58 AM

The priority is based on what you choose to control.

Aperture priority - you set the aperture that you want - usuallly to control depth of field either shallow or deep depending on the shot. the camera then adjusts shutter speed to suit the chosen exposure level. In "A" mode the camera does not alter the chosen aperture.

Shutter priority - you choose the shutter speed - usually either to freeze or deliberately blur movement in the frame. the camera then adjusts aperture to suit the chosen exposure level. In "S" mode the camera does not alter the chosen shutter speed

Program Mode - The camera follows a programmed exposure value curve to select the correct combination of shutter and aperture settings. You can "shift" this program up or down using one of the rotary dials to bias the settings towards faster shutter/wider aperture or slower shutter/ smaller aperture to suit your scene but the camera will still endeavour to obtain the same Exposure value based on ambient light. (e.g f4 at 1/125th gives the same exposure value as f8 at 1/60th but f8 gives you more depth of field than f4).

Before deciding on on the shutter or aperture setting the camera first looks at the exposure compensation setting, exposure correction setting and ISO parameters. I will explain how these are used below:

1) Exposure correction - If you find that your camera typically over or under exposes all its shots then you can enter a correction in the menu. This compensates for a light metering systen that gives results either too bright or too dark for your taste. e.g. my D800 is spot-on but my D300 always over-exposed by about 1/3 stop in my opinion. I set the exposure correction to -1/3 on all banks to compensate. When you press your shutter, the camera first looks to see if any exposure correction has been set and adjusts the level of exposure it is aiming for.

2) Exposure compensation - The camera always aims to average the exposure overthe whole frame to a mid grey. This is effective for most scenes in daylight and gives a result that we perceive as similar to what our own eyes would see. However some scenes are naturally much brighter - e.g. on the beach, in snow, in white water, wearing pale coloured clothing - Any scene where a pale shade is dominant. Some scenes are naturally darker - e.g. wearing a dark suit, shooting a subject against a dark shaded background, sunset/sunrise where a dark shade is dominant. In a bright scene you need + exposure comp. to reproduce the brightness and the converse for dark scenes to reproduce the dark tones. The Ev +/- button is conveniently placed to allow shot by shot adjustments of the light metering target value for the camera. After checks for exposure correction in the settings menu, it will also check for exposure compensation settings and will add or subtract exposure value to suit.

Steps 1 and 2 above are used to generate a "setpoint" exposure value - i.e. the exposure value target that the camera wishes to obtain for the shot you are about to take. the camera uses this value in conjunction with the chosen ISO value to calculate aperture and shutter settings. The selection of ISO is discussed further in point 3 below because it is a bit more complex.

3) This is a complex topic, possibly requiring a separate post - I will discuss very briefly here: ISO - represents the "amplification" of the sensor output and just like stereo equipment, it comes with the penalty of increased noise at higher amplification settings - it amplifies everything, including imperfections in the response of individual pixels to light exposure. If you have fixed the ISO setting (e.g. 200) then the camera merely takes note of this in its exposure calculations. However, if you have set Auto-ISO the camera checks to see if the chosen settings will result in a shutter speed slower than the parameter you selected in your Auto-ISO setup. Under Auto-ISO - the camera automatically increases ISO (within preset limits) until a sufficiently fast shutter speed is obtained. This allows you to use your camera handheld (or freeze subject movement) in considerably poorer lighting conditions than would be possible at normal ISO settings.

Visit my Nikonians gallery.

Visit

G