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Subject: "why flash compensation with iTTL for regular exposure?" Previous topic | Next topic
ddubyaw Registered since 29th Oct 2005Sat 14-Jan-06 05:35 AM
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"why flash compensation with iTTL for regular exposure?"


Atlanta, US
          

I'm curious why flash compensation is necessary with iTTL if you want the flash to produce a regular exposure.

For a typical lousy indoor snapshot, I can have an SB-800 in TTL mode (normal TTL, not BL) aimed at the subject with 0 flash compensation, and it'll produce a correct exposure for my camera (supports CLS).

If I then aim my flash upwards to bounce it off the ceiling, I'll get drastic underexposure. Obviously this is because I lost light due to the bounce and the extra distance required to make the bounce, but with iTTL, why does that matter? With TTL, shouldn't the camera be able to keep the flash firing longer until a correct exposure made?

Instead, I have to histo-guess at +1, +2/3, etc. flash compensation for the bounce -- seems unnecessary with the technology, or am I missing something?

  

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Replies to this topic
Subject Author Message Date ID
Reply message RE: why flash compensation with iTTL for regular exposu
edmun
14th Jan 2006
1
Reply message RE: why flash compensation with iTTL for regular exposu
hlk
14th Jan 2006
2
Reply message RE: why flash compensation with iTTL for regular exposu
bclaff Silver Member
14th Jan 2006
3
Reply message RE: why flash compensation with iTTL for regular exposu
ddubyaw
14th Jan 2006
4
Reply message RE: why flash compensation with iTTL for regular exposu
sixgun
15th Jan 2006
5
Reply message RE: why flash compensation with iTTL for regular exposu
ddubyaw
15th Jan 2006
6
Reply message RE: why flash compensation with iTTL for regular exposu
ultrafast Silver Member
15th Jan 2006
9
     Reply message RE: why flash compensation with iTTL for regular exposu
ddubyaw
15th Jan 2006
10
Reply message RE: why flash compensation with iTTL for regular exposu
rpyaaunl
15th Jan 2006
7
     Reply message RE: why flash compensation with iTTL for regular exposu
sixgun
16th Jan 2006
11
Reply message RE: why flash compensation with iTTL for regular exposu
blw Moderator
15th Jan 2006
8

edmun Registered since 16th Sep 2003Sat 14-Jan-06 06:44 AM
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#1. "RE: why flash compensation with iTTL for regular exposu"
In response to Reply # 0


eugene, US
          

If your flash is at or near maximum out put for a given distance then bouncing does not work. You have to open up another stop to get it to correctly expose.

ledmun

  

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hlk Registered since 31st Oct 2005Sat 14-Jan-06 08:30 AM
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#2. "RE: why flash compensation with iTTL for regular exposu"
In response to Reply # 0


Perth, AU
          

If you are going to shoot synchro-sun than flash exposure compensation is necessary. That's similar to TTL-BL. It may be TTL or DTTL or iTTL and that depends on the typr of camera you are using.

  

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bclaff Silver Member Awarded for multiple contributions for the Resources Nikonian since 26th Oct 2004Sat 14-Jan-06 04:37 PM
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#3. "RE: why flash compensation with iTTL for regular exposu"
In response to Reply # 0


Vancouver (WA USA not Canada), US
          

Derrick,

Flash exposure computation is no more "fool proof" than ordinary exposure computation.
Under some circumstances the value needs to be "tweaked" by the photographer.

One example would be using flash with a non-D lens.
Since the camera doesn't know the subject distance you might have to adjust the flash power via Flash Exposure Compensation.

Bill

Visit me at My site

  

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ddubyaw Registered since 29th Oct 2005Sat 14-Jan-06 10:39 PM
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#4. "RE: why flash compensation with iTTL for regular exposu"
In response to Reply # 0


Atlanta, US
          

I'm still not understanding why the camera can't compensate automatically. With matrix metering with a D/G lens, in Aperture priority mode, the D2X not only knows my bounced shot is underexposing, but it's also controlling the flash -- so why not tell the flash to output more light?

It seems to me that what we get out of CLS iTTL flash is the camera cutting the flash to prevent overexposure. That's great, but the next step would be to keep the flash firing to prevent underexposure. Maybe that's a more difficult engineering task -- I can see it would be easier to cut the flash short than to extend it, and overcharging the flash always to be able to dynamically handle both over- and underexposure would reduce battery life and increase recycle time. Maybe that's why this sort of thing doesn't work automatically for me.

  

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sixgun Registered since 26th May 2004Sun 15-Jan-06 02:51 AM
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#5. "RE: why flash compensation with iTTL for regular exposu"
In response to Reply # 0


Austin, US
          

>nstead, I have to histo-guess at +1, +2/3, etc. flash compensation for the bounce -- seems unnecessary with the technology, or am I missing something?<

Do you find much of a difference between bounce TTL and bounce TTL +2/3?

I used to have a load of problems getting proper exposure on the bounce, the evaluative algorithm just isn't quite as good, remeber its doing a calcuation from a pre-flash which is not the same as letting the light build up and then shut things off. Kind of like trying to graph a trend with 3 data points vs 10.

Anyway the solution I found a while back for wonderfully consistant flash exposure on bounce is to switch you camera over to Manual Mode, shutter speed 1/500, ISO 400, F5.6 or less and put the SB-800 into Manual mode at 1/2 power. Works pretty well up to 12ft ceilings, sometimes you have to drop to F4. Sure your discarding 20 years worth of technological advances in flash and metering, but it works every time. Try it.

  

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ddubyaw Registered since 29th Oct 2005Sun 15-Jan-06 02:54 AM
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#6. "RE: why flash compensation with iTTL for regular exposu"
In response to Reply # 5


Atlanta, US
          

I am able to get good results with ceiling bounces by dialing in +1 or +2/3 on the SB-800 and keeping it in TTL mode. It works -- I was just curious why the technology didn't deliver a little more. I'll give your method a try too and see how that does.

  

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ultrafast Silver Member Charter MemberSun 15-Jan-06 01:11 PM
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#9. "RE: why flash compensation with iTTL for regular exposu"
In response to Reply # 6


New York, US
          

I think there is a fairly technical reason that the flash is not calculated properly when it is bounced. The reason is that the intensity of the flash is decided by a smallish preflash, before the real flash. Often, this small preflash is not strong enough to provide the necessary feedback when a significant distance needs to be covered. I believe that the camera just sees very little return from the preflash so ups the power on the full flash. This however, is probably a very conservative estimate so it may by slightly underexposed.

www.incandescent-earth.com

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Visit my Nikonians gallery.

  

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ddubyaw Registered since 29th Oct 2005Sun 15-Jan-06 04:13 PM
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#10. "RE: why flash compensation with iTTL for regular exposu"
In response to Reply # 9


Atlanta, US
          

Ah, this explanation does make sense to me. I forgot about the pre-flash and was thinking the iTTL control was more realtime with the actual exposure. Thanks!

  

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rpyaaunl Registered since 30th Dec 2005Sun 15-Jan-06 11:22 AM
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#7. "RE: why flash compensation with iTTL for regular exposu"
In response to Reply # 5


JP
          

My experience with the SB-800 is somewhat different...I find that it's so easier to get a good exposure that it's scary!! You can point the thing in any direction you want and still get it perfect. I find it frankly amazing! Combined with the easy lcd check you get with digital cameras (not the F6), and you can know in an instant if the flash has provided the light in the right places...smooth graduations and evenly lit. Often times I have the SB-800 pointing in what would seem a completely insane flash direction and it still exposes well. For example, if you have overhead bounce flash set for a landscape shot, then trying to keep up with a dog or something fast, you move the camera to portrait and snap before thinking that the flash is pointing in some insane direction and then have it expose well. Too easy...a little scary. The SB-29S combined with a digital camera is another story...complete opposite, it confuses me no end!

Cheers, Ryan...

My Daily Updated Photo Weblog - http://blog.palaka.biz

  

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sixgun Registered since 26th May 2004Mon 16-Jan-06 03:29 PM
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#11. "RE: why flash compensation with iTTL for regular exposu"
In response to Reply # 7


Austin, US
          

For good calculated exposure (TTL) with digital you pretty much have to use SB-800/600. It does all of its exposure calculations via a pre-flash which is measured and metered. If it can't do the pre-flash it just can't meter properly because the digital sensor is more reflective than film. It will try, and mostly fail. You pretty much have to use the older flashes on manual.

  

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blw Moderator Awarded for his high level of expertise in various areas Nikonian since 18th Jun 2004Sun 15-Jan-06 12:11 PM
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#8. "RE: why flash compensation with iTTL for regular exposu"
In response to Reply # 0


Richmond, US
          

That's not my experience. I find that as long as I stay within the envelope of the possible (ie don't try to bounce off the ceiling of a dark cathedral at f/16), bounce flash is quite simple and effective. I have even handed my camera to my non-camera sister-in-law with the head tilted up and she got good results too. (Her house has pristine white ceilings, though - ideal for bounce.)

_____
Brian... a bicoastal Nikonian and Team Member

My gallery is online. Comments and critique welcomed any time!

  

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