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Question about DOF with D700 (and other full frame cameras)

ttwwpp

Smyrna, US
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ttwwpp Registered since 21st Nov 2005
Thu 08-Jul-10 12:30 PM

I've been researching DOF between crop and full frame sensors and the more I read the more confused I get... I don't know why it is so hard for me to grasp the difference in DOF between the 2 sensors.. Tell me if I'm getting this.. If I shot a pic with a crop sensor from 3 feet with a 50 mm lens at f1.4, would I get the same DOF with a FF if I shot it at 2 feet with the same lens and aperture?
Thanks for any help..
Tim

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fwellers

Ashburn, US
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#1. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 0

fwellers Gold Member Nikonian since 21st Nov 2008
Thu 08-Jul-10 11:07 AM

the way I understand it you would get less DOF at 2 ft regardless of which camera you use.
DOF is a function of focal length, aperture and distance to subject.
So if you shot a dx and an fx at the same distance, same focal length,and same aperture, the DOF would be the same for both.

But. In order to get the same field of view with the same focal length on the fx, you'd have to move closer which would then give you less DOF.

There is some physics about DOF as it applies to printouts and magnifications that I don't get but what I said above should help.

Peace,
Floyd

elec164

US
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#2. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 0

elec164 Silver Member Nikonian since 15th Jan 2009
Thu 08-Jul-10 11:32 AM

In a word no, but it is a bit more complicated than that.

DOF is dependent on aperture, subject distance and magnification. With the same focal length you have a different FOV between DX and FF. To achieve the same FOV you need to move closer with the FF camera to use the same focal length. By moving closer you altered the DOF making the FF image appearing to have a narrower DOF.

If, on the other hand, you stayed at the same distance with the same focal length and aperture then it would appear that the FF camera had a greater DOF due to the lesser magnification of the scene and the different FOV. But if you crop out the central portion of the FF image to give the same FOV of the DX image and enlarge that to the same print size they would appear to offer the same DOF.

Then again, on the other hand, if you used a 50mm lens on the DX camera and a 75mm lens on FF camera to provide the same FOV shot at the same aperture (that is actual aperture and not f-number) and subject distance then they would have approximately the same DOF. So a 50mm lens at f/5.6 on a DX camera and a 75mm lens at f/8.4 (providing the same aperture)on a FF camera would at the same subject distance provide the same DOF. But at the same f-number (providing a larger aperture on the 75mm lens) the FF would provide a narrower DOF.

Most people make the blanket statement that DX provides greater DOF. But that is misleading for FF can provide the same greater DOF. A better why of stating it would be that DX cameras cannot at times provide the same narrow DOF as FF cameras for a given circumstance.

A good tutorial on DOF can be found at Cambridgeincolour. And to understand how print size and viewing distance can affect DOF you can use the more flexible DOF calculator.

Hope this helps.

Pete

Pete

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briantilley

Paignton, UK
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#3. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 0

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
Thu 08-Jul-10 11:53 AM

This can be a confusing subject, which is probably why there are some slightly misleading statements in the earlier replies...

As well as aperture, subject distance and focal length, DoF does also depend on the dimensions of the recording medium (film or sensor). If you have the same subject, and use a 50mm lens on a DX camera and a 75MM lens on an FX camera from the same distance (to keep the framing the same), at the same aperture, the DX shot will have greater DoF.

This can sound odd until one realises that the traditional DoF tables and calculators assume viewing of a standard-sized print from a standard distance. The DX image, being smaller, will require greater enlargement and this changes the visible DoF, even when other factors are the same.

I would suggest playing around with the calculator that Pete linked to - changing the camera format, aperture, focal length and subject distance - to get an idea of the practical implications.

Brian
Welsh Nikonian

ttwwpp

Smyrna, US
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#4. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 3

ttwwpp Registered since 20th Nov 2005
Thu 08-Jul-10 11:56 AM

Thanks to all who replied. I finally get it!! I knew this would be the place to ask..
Thanks again,
Tim

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esantos

McAllen, US
13267 posts

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#5. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 2

esantos Moderator Nikonians Resources Writer. Recognized for his outstanding reviews on printers and printing articles. Awarded for his high level of expertise in various areas, including Landscape Photography Awarded for his extraordinary accomplishments in Landscape Photography. His work has been exhibited at the Smithsonian. Nikonian since 10th Nov 2002
Thu 08-Jul-10 12:08 PM

Pete,

Thanks for that thorough and excellent explanation of a very confusing subject. It helped me quite a bit to get it straight in my mind.

Ernesto Santos
esartprints.com Ernesto Santos Photography
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elec164

US
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#6. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 3

elec164 Silver Member Nikonian since 15th Jan 2009
Thu 08-Jul-10 12:48 PM | edited Thu 08-Jul-10 12:59 PM by elec164

>As well as aperture, subject distance and focal length, DoF
>does also depend on the dimensions of the recording medium
>(film or sensor). If you have the same subject, and use a
>50mm lens on a DX camera and a 75MM lens on an FX camera from
>the same distance (to keep the framing the same), at the same
>aperture, the DX shot will have greater DoF.
>
>This can sound odd until one realises that the traditional DoF
>tables and calculators assume viewing of a standard-sized
>print from a standard distance. The DX image, being smaller,
>will require greater enlargement and this changes the visible
>DoF, even when other factors are the same.
>

Hi Brian and thanks for joining this discussion, for you were instrumental in aiding to my better understanding of DOF and the proper usage of terms.

And I agree that the more one begins to study DOF more intently, the more confusing it can become at first.

But as to your comparison to the 50mm on DX and 75mm on FF, I still believe at the same aperture(again real aperture size and not f-number) they would provide the same DOF. The calculator at DOFmaster assumes an 8x10 print from each format therefore the format size is calculated in with the difference in CoC. Again if the true aperture (50mm at f/5.6 and 75mm at f/8.4) is used the result should be the same. Trouble is that you cannot get the exact aperture in that calculator for it is not flexible enough. As stated earlier, 50mm at f/5.6 would equate to a 75mm at f/8.4, but you only have a choice of f/8. So yes there is a slight difference in DOF when using DOFmaster calculator (about 2 inchs at 10 feet), but I attributed that to the fact that you cannot achieve the proper aperture for the 75mm exactly and because of that you wind up with a slightly smaller than needed one creating the difference.

But then again, if you wish to use the same EV camera settings (ISO, shutter speed and f-number), then the FF camera would provide a narrower DOF. But it was not because of the larger format, but because of the fact to achieve the same FOV at a given subject distance you need a longer focal length. And if you wish to maintain the same EV settings, you would need a larger aperture, or the same f-number, as that of the shorter focal length on the DX camera. And at that point the larger aperture created the narrower DOF, not the larger format size, although format size needs to be considered when used in the calculation.

Am I wrong in this thinking? And yes complex indeed. Just when I think I got it, I look again and scratch my head wondering if I really do!!! LOL

Pete

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briantilley

Paignton, UK
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#7. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 6

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
Thu 08-Jul-10 12:57 PM

I'm not sure I understand your concept of "true aperture"; I think you're referring to the actual physical diameter of the diaphragm opening

I'm sure you know this, but an aperture value of f/5.6 on a 50mm lens will let through the same amount of light as an aperture of f/5.6 on any other focal length lens. An f/number isn't an absolute thing - it's the ratio of the diameter of the diaphragm opening to the focal length of the lens.

The standard DoF tables and calculators always work with f/numbers.

Brian
Welsh Nikonian

elec164

US
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#8. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 7

elec164 Silver Member Nikonian since 15th Jan 2009
Thu 08-Jul-10 01:19 PM | edited Thu 08-Jul-10 01:24 PM by elec164

>I'm not sure I understand your concept of "true
>aperture"; I think you're referring to the actual
>physical diameter of the diaphragm opening
>

Yes that is what I am getting at.

DOF tables ask you for focal length and f-number as to be able to calculate the aperture or effective aperture (diaphragm opening). Just knowing the focal length without the f-number, or the f-number without the focal length does not define the actual opening area.

This is where the main confusion in all this laid (for me at least). That is until I finally understood that f-number, in and of itself, is really meant to calculate exposure not DOF. To be used in calculating DOF you need to use it in combination with the focal length to determine aperture size.

A 50mm lens at f/5.6 has an aperture diameter of about 8.9mm where as a 75mm lens at f/5.6 would have an aperture diameter of about 13.4mm. That will provide the same EV and FOV on the two formats, but a different DOF because of the larger aperture. But the 75mm lens at f/8.4 would provide the same aperture area (8.9mm aperture diameter)and DOF at the expense of an increase of ISO or decrease in shutter speed to make up for the loss of light due to the smaller aperture size.

I know you and I have gone round and round on this before. Thanks for your patience.

Pete

Edited to add:

And this is what qualifies my earlier statement of FF providing a narrower DOF rather than stating that DX provides a greater one. If f/5.6 were the largest f-number available on the 50mm DX and 75mm FF lens, then the DX camera would be incapable of providing the same narrow DOF of the FF, but stopping the FF down to f/8.4 would provide just as great a DOF as the DX.

Pete

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LuisGonzalezLT

Macedonia, US
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#9. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 8

LuisGonzalezLT Silver Member Nikonian since 04th Nov 2005
Fri 09-Jul-10 10:28 PM

And while we're at it let's throw out the notion that wide angle lenses don't have any more dept of field than telephoto lenses

Luis Gonzalez
Everlasting Photography, Inc.

Bart B

Loveland, CO, US
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#10. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 0

Bart B Registered since 06th Mar 2007
Wed 14-Jul-10 11:22 PM | edited Wed 14-Jul-10 11:27 PM by Bart B

Years ago on 35mm cameras, there seemed to be a standard for setting depth of field (DOF) aperture marks on lens' barrels. As I remember, that was based on a "circle of confusion" (COF) or "airy disk" of light. This is the diameter of the cone of light for a finite point at the subject both in front of and behind the focus plane of the lens. The dimension of the finite point at on the subject is zero as is the focused image of that point in the focal plane. DOF is the distances at which things closer and farther away from the subject would be focused at the COF points.

The distance of the COF in front of and behind the focal point will be the same amount for a given f/stop regardless of lens focal length. A 50mm lens at f/2 has a 25mm diameter aperture A 100mm lens has a 50mm diameter aperture at f/2. But it's aperture is twice as far away and its cone of focused light from a finite point on the image is half the aperture diameter at 50mm in front of the image plane.

I would think a DX format COF might be smaller than a FX format COF. Perhaps the same ratio smaller than the image size. For example, COF would be 0.1mm on FX and 0.067mm on DX. Which means to me, a 50mm lens on FX would have the same depth of field at the focal plane as a DX format with a 33.3mm lens.

Your comments, thoughts and corrections are welcomed.

Bart


DX DSLR: D40
FX DSLR: D700
Film SLR: FG, N80, F100
DX Lenses: Nikkor 18-55, 18-70, 55-200
FX Lenses: Nikkor 18-35, 24-85, 28-80, 28-200, 35-105, two 50's, 70-300; Series E 28, 100 & 135.
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LuisGonzalezLT

Macedonia, US
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#11. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 10

LuisGonzalezLT Silver Member Nikonian since 04th Nov 2005
Thu 15-Jul-10 12:15 AM

The DX <-> FX DOF difference doesn't change if camera <-> subject distance remains the same using the same lens and the final print magnification of the subject is the same ( e.g. same 50mm lens on both bodies, same camera <-> subject distance, you make an 8x10 with the FX shot and a smaller print with the DX shot ( whatever it needs to be ) so that the prints are merely showing the difference in FOV ( i.e. the crop of the scene ). Once you stray on any of these to yield the same FOV on the same size print ( i.e. use different focal length lenses between the two shots, use the same lense but move closer/farther between the two shots, etc. then DOF will change since you are no longer comparing apples to apples. Your COF has changed because of the final print magnification.

Example for why print magnification, size and viewing distance is a factor is a 20x30" canvas viewed from across the room versus from two feet away. I have a shot like that on one of my walls that, when viewed from across the room everything appears in focus, but when you get within a few feet it becomes obvious the distant objects are a bit out of focus compared to the foreground object. From across the room your eyes simply don't have the resolving power to see the difference ( I guess the COF in your eyes has grown larger than the COF in the print )

My head hurts when I start thinking about this stuff. We're cutting hairs a bit. Out on the field or on a project I seldom have the time to really calculate out the exact DOF I am going to end up with. You jusy eyeball with whatever body/lens/subject/etc you are dealing with at that time and get it how you like, take the shot, magnify it to 100% on the LCD and look around and see what you got. If the DOF it not quite to your liking stop down a bit more or open up, etc, retake the shot, and move on. I you are really in a hurry just set your camera to shutter priority, set the speed so the camera sets the aperture to your "middle" aperture, and fire off a volley of 5 under/over bracketed shots. The camera will roll through a series of F/stops above and below your "middle" aperture and you can look at them later to see which shot has the DOF you like best. I feel digital has really freed us from certain tasks like calculating out DOF tables to the Nth degree before taking a single shot.

Now, if your shooting a film body you have some calculations to do before the shot. IMHO The DOF markers on lenses ( mostly old ones as newer ones hardly have any useful markers ) aren't all that useful. I prefer not to use them at all. They are what the optical engineer of that lens probably thought was appropriate for a 35mm camera with an 8x10 print viewd at whatever distance, and maybe even at a time when film was resolving fewer lines per mm.

I ramble... time to quit.

Luis Gonzalez
Everlasting Photography, Inc.

Bart B

Loveland, CO, US
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#12. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 0

Bart B Registered since 06th Mar 2007
Thu 15-Jul-10 07:02 PM

To me, it would be easier to understand depth of field issues if there was a single standard for all formats; a few millimeters square to 8 x 10 view cameras. This standard might be a percentage of the format's diagonol dimension.

For example, the information covered in this web page is interesting:

http://www.nikonlinks.com/unklbil/dof.htm

briantilley

Paignton, UK
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#13. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 12

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
Thu 15-Jul-10 07:48 PM

>To me, it would be easier to understand depth of field issues
>if there was a single standard for all formats

There is

DoF tables and calculations have always been standardised across all formats - they are based on viewing a standard-size print from a given distance, so the difference in size of the recording medium is automatically taken into account.

Brian
Welsh Nikonian

Baaker

Dumbarton, UK
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#14. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 13

Baaker Silver Member Nikonian since 18th Aug 2009
Fri 16-Jul-10 09:29 AM

Quote

DOF is dependent on aperture, subject distance and magnification. With the same focal length you have a different FOV between DX and FF. To achieve the same FOV you need to move closer with the FF camera to use the same focal length. By moving closer you altered the DOF making the FF image appearing to have a narrower DOF.

Unquote

Like everyone else I have trouble getting my head around this subject. I think that I am about 95% there. The problem that I have is where does magnification fit into all of this? As far as I know there isn't any. Could someone please enlighten me.

http://www.flickr.com/photos/43019448@N04/

elec164

US
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#15. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 14

elec164 Silver Member Nikonian since 15th Jan 2009
Fri 16-Jul-10 12:27 PM

Magnification is the result of the amount of enlargement of the capture medium as well as because of subject distance/focal length.

As Brian noted most DOF tables are based on the average visual acuity of human vision viewing an 8x10 print at a viewing distance of about 10 inches. And it is not a strict number as shown by the disagreement by some for 35mm film. Some would state .03 as the CoC and some would argue that .025 is a more proper figure.

As noted an image taken with a DX and FF camera at a specific focal length, f-number and subject distance will provide a different FOV and give an apparently different DOF for a given print size of the full capture (FF having greater DOF then the DX). But if you crop the central portion of the FF image to approximate the FOV of the DX capture, and then enlarge that to the same print size as the DX, the DOF will appear the same. So as you see, the amount of print enlargement/magnification at a given viewing distance has as much of an impact on DOF as does the aperture and subject distance at the time of capture. Or stated differently, the amount of magnification of the subject via the focal length and subject distance was the same on both the FF and DX sensor, but the amount of magnification for the full frame capture of that subject post capture was different.

And this is where CambridgeinColour’s flexible DOF calculator comes in handy. If plug in the numbers for the same focal length, subject distance and f-number along with the same subject magnification for the FF print as the DX print on the long side(DX at 10 inches and FF at 15 inches) at the same viewing distance, then they will appear to have a similar DOF.

At least I think I got that correct.

Pete

Pete

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gkaiseril

Chicago, US
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#16. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 15

gkaiseril Gold Member Nikonian since 28th Oct 2005
Fri 16-Jul-10 12:42 PM

I find DOF Master as a good resource for calculators, tables and articles about Depth of Field.

The data needed for the calculation include:

  • Camera model or CoC of the recording medium, film or digital
  • Focal length of the lens
  • Selected f-stop
  • Subject distance

Depth of Field Equations provides information on how to calculate the Hyper focal distance, near and far focused distances.

The site also includes references if you want to do more research abut this subject.

George
My Nikonian Galleries

Tongariro

London, UK
404 posts

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#17. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 16

Tongariro Registered since 14th Jul 2007
Fri 16-Jul-10 02:24 PM

I've got dof master on my iphone. Cost me £1.19. Very handy & saves taxing my brain too much. It doesn't have the D700 listed, so you have to use the D3 setting. That I can cope with!

Bridget

Patrick28

Port Jervis, US
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#18. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 14

Patrick28 Silver Member Nikonian since 22nd Apr 2010
Sat 17-Jul-10 03:47 PM

Some 60 years ago, I worked out an empirical formula for an easy calculation of the hyperfocal distance for any focal length at an aperture of f11. Though it appears to be involved, it is trivial to work out in your head in the field and is easily adjusted to any other f-stop.

You eliminate the FX/DX consideration by using the effective focal length of the lens on either camera. I used a CoC of 0.02 inches. Because we always had to manually set our focus, we were willing to accept some fudging in the interest of simplicity. The results were certainly more useful than the DOF scale on our lenses.

Setting our focus to the hperfocal distance, we assumed that the DOF extends from 1/2 the hyperfocal distance to infinity.

(The calculation adjusts for the fact that the focal length is expressed in mm while the result is expressed in feet.)

The formula for f11: h(ft) = 0.015 times the focal length (mm) squared.

e.g. 50mm: .015 x 2500 = 37.5 feet. But it's easy to do in your head
!
Easy method: Square the focal length: 2500 feet
Drop the two zeroes => 25 feet
Add .5 times that result: .5 times 25 feet => 13 feet
(Note: Just use 1/2 instead .5 if that's easier to think of.)

For a result of h(50mm, f11) = 38 feet.
Your DOF is 19 feet to infinity.

80 squared = 6400. Drop the two zeroes => 64 feet
Add 1/2 of 64 => 32 + 64 => h(80mm, f11)= 96 feet.
DOF extends from around 48 feet to infinity.
Close enough for field work.

200 squared = 40000 Drop two zeroes => 400
Add 200 => h(200mm, f11)=600 (Instead of the expected 611)

OK. That's for f11.
For f8 add 1/2 the value for f11 (f8=57). For all other full f-stops going down, double the value for two f-stops previous.
For f16 add 1/2 the value for f11. For f22, double the value for f11.

For a 50mm lens, you would get h(x) of
f11=38 f8=57 f5.6=76 f4=114, f2.8=152

I know this sounds complicated but it becomes extremely simple and obvious as you work through it.

Trust me!

(This all works because aperture works by the power of 2 (doubled or halved). The log of 2 is 1.4 and I used 1.5 as an approximation.)




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briantilley

Paignton, UK
30235 posts

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#19. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 18

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 17-Jul-10 05:14 PM

It's an interesting idea, but I'm not sure how accurate it is in practice...

>You eliminate the FX/DX consideration by using the effective
>focal length of the lens on either camera.

Maybe I'm misunderstanding, but I don't think that works. For example... according to the standard tables, on FX a 75mm lens at f/11 has a hyperfocal distance of 54.6 feet, whilst on DX a 50mm lens (which gives a 75mm FX equivalent angle of view) at f/11 has a hyperfocal distance of 36.4 feet. That's not really close.

>I used a CoC of 0.02 inches.

The CoC for the FX (35mm) format is usually approximated to 0.03mm, and for DX 0.02mm - either way it's not inches.

Brian
Welsh Nikonian

Patrick28

Port Jervis, US
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#20. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 19

Patrick28 Silver Member Nikonian since 22nd Apr 2010
Sat 17-Jul-10 08:12 PM

Yes, mm, of course, not inches. Thanks.

I didn't know that about the DX camera behaving differently for the hyperfocal distance. (Back in the early 50's we didn't have DX lenses ;>)

I think my numbers are 'close enough for government work' for the FX but certainly cannot be applied to the DX. That is a gross, nonworkable difference that you cite for 50(75)mm DX.

I think the idea of using 1.5 as an approximation of the log of 2 is usable, however. I would think that if one started with the given hyperbolic distance for a specific DX lens at f11, the numbers for the other f-stops could be derived in the same manner as for the FX lens suggested in the earlier post. Surely (??!!) the aperture settings on a DX lens are based on the power of two.

Before we had AF the hyperbolic distance was used far more frequently. The above numbers were useful for setting the focus manually using the scale on the lens where the tolerance far exceeded the precision of the numbers. I used to walk around with my Kine Exacta set at the hyperfocal distance for the f-stop that the light meter suggested with the shutter set at the reciprocal of the focal length. I never tried to use those numbers for macro work.

I've learned a lot from your posts. This is the best thing I've discovered since the D700. Thanks!

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briantilley

Paignton, UK
30235 posts

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#21. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 20

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
Sun 18-Jul-10 06:30 AM

>Surely (??!!) the aperture settings on a DX lens are
>based on the power of two.

Yes - nothing to worry about there

Brian
Welsh Nikonian

elec164

US
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#22. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 19

elec164 Silver Member Nikonian since 15th Jan 2009
Mon 19-Jul-10 12:37 PM

>Maybe I'm misunderstanding, but I don't think that works. For
>example... according to the standard tables, on FX a 75mm lens
>at f/11 has a hyperfocal distance of 54.6 feet, whilst on DX a
>50mm lens (which gives a 75mm FX equivalent angle of view) at
>f/11 has a hyperfocal distance of 36.4 feet. That's not
>really close.
>

Actually I found Patrick’s formulas interesting.

And if you used the same apertures for both formats it would work.

As mentioned in an earlier post, DOF is determined at the time of capture by the aperture and subject distance regardless of format size and focal length. I am not saying format size is not important, for you need to be cognizant of the amount of enlargement in post processing to insure you capture the proper CoC for your intended use of the capture.

Which brings to mind what Luis stated, “The DX <-> FX DOF difference doesn't change if camera <-> subject distance remains the same using the same lens and the final print magnification of the subject is the same” which pretty much mirrors what I have been saying and is proven out if you use Cambridge in Colour flexible DOF calculator.

But I disagree with Luis when he stated “Once you stray on any of these to yield the same FOV on the same size print (i.e. use different focal length lenses between the two shots…)”. This is where (for me at least) the old rote method used to learn DOF hampers understanding. In truth, if you keep the percentage of fill of the subject in the frame the same, the DOF will be the same regardless of the format for a given aperture.

We amateurs learned that higher f-numbers=narrower DOF and shorter focal lengths= greater DOF. When we put that in practice we experience what it says so we do not stop and think or question it. And it is further confused by the fact that we constantly refer to the f-number as aperture and confuse that with a physical size which it is not. The f-number is dimensionless and was meant for exposure calculation not DOF calculation. To use it for DOF calculation it needs to be used in conjunction with focal length. Using 50mm on a DX and 75mm on a FF camera at the same aperture will provide the same DOF and hyper-focal distance. The reason it works is because of the CoC difference between DX and FF. Yes the 75mm lens provides more magnification and a larger CoC at the time of capture. But the FF format tolerates a larger CoC so the lesser amount of enlargement post process equals out the larger CoC at the time of capture and both the FF and DX camera provide the same DOF (assuming no cropping and same print size and viewing distance).

So remembering that the f-number is a dimensionless number, and to find the aperture you need to use it in conjunction with the focal length, we find that to be at the same aperture achieved with f/11 at 50mm you would need f/16.4 at 75mm.

So if you take Patrick’s calculation and shift it by one stop (to achieve similar aperture for the given focal lengths needed on each format respectively) it would be very useful (not precise but useful).

Example of hyper-focal distance calculation: Patrick’s numbers=xx DOF Master numbers =(xx)
50mm DX: f/11=38(36.4), f/8=57(51.4), f/5.6=76(72.7), f/4=114(102.7), f/2.8=152(145.2)
75mm FF: f/16=38(38.7), f/11=57(54.6), f/8=76(77.1), f/5.6=114(109), f/4=152(154)

Actually the FF numbers are closer to both Patrick’s and DOF Master calculations. There is a disparity when equating DX with FF when using the calculator at DOF Masters online calculator that I attribute to the inability to provide similar apertures. But in playing around with it when formulating and typing this reply, I found a set of apertures that are relatively close that minimizes the differences between DX and FF and they are f/4.8 for DX and f/7.1 for FF. Those f-numbers (while odd sizes) provide almost the same aperture on both formats. And it better enables one to see that in truth, regardless of the format and focal length need for FOV, aperture and subject distance determines DOF and hyper-focal distance in respect to, and regardless of focal length and format size.

In summing up, in that regard, I really do feel that if one wishes to get a better understanding of this one would be well served to use Cambridge in Colours tutorial on DOF and CoC. And if you really want to understand it will help to forget what you thought you new, and look at the tutorial with an open mind. But be forewarned, it will drive you a bit nuts for it seems at first to sort of go against the traditional rote learning. But once you realize that you misunderstood what you first learned, it will all come together in the end.

For most others, just print up a DOF chart for your camera, or download the DOF app for the Palm or iPhone and just go out and shoot and don’t think about, for if you do, it may very well drive you nuts!!!

And in fact, it really does not change what you put in practice with the rote method.

Pete

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LuisGonzalezLT

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#23. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 22

LuisGonzalezLT Silver Member Nikonian since 04th Nov 2005
Mon 19-Jul-10 05:50 PM

What I was getting at was that DOF is primarily camera <-> subject distance related, not focal length related, but totally screwed up my example. i.e. When you stand 50 feet away from a subject and shoot DX and 20mm, then switch to FX and 35mm, w/o changing distance and maintaining the same FOV and same aperture, the DOF is "similar".

I say "similar" because as you move toward wider and wider focal lengths the DOF is less and less evenly "centered" - the shorter the focal length the more depth you have behind the subject than in front ( while in a long telephoto it is almost evenly divided front to back. That does change the "focal length doesn't matter argument" a little bit.

Luis Gonzalez
Everlasting Photography, Inc.

briantilley

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#24. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 23

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
Mon 19-Jul-10 05:59 PM

>i.e. When you stand 50 feet away from a subject and shoot DX and
>20mm, then switch to FX and 35mm, w/o changing distance and
>maintaining the same FOV and same aperture, the DOF is "similar".

I'm afraid that's not actually true.

If you check the calculator that George linked to earlier (or any similar alternative), you'll find the DoF differs in your scenario...

DX, 20mm lens, 50 feet distance, f/5.6 - Hyperfocal Distance is 11.7 feet
FX, 30mm lens, 50 feet distance, f/5.6 - Hyperfocal Distance is 17.5 feet.

Brian
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elec164

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#25. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 23

elec164 Silver Member Nikonian since 15th Jan 2009
Mon 19-Jul-10 07:10 PM

>What I was getting at was that DOF is primarily camera
><-> subject distance related,

Actually you would be better served by stating that it is aperture <--> subject distance related. Focal length and FOV would be determined by the format size.


> i.e. When you stand 50 feet
>away from a subject and shoot DX and 20mm, then switch to FX
>and 35mm, w/o changing distance and maintaining the same FOV
>and same aperture, the DOF is "similar".
>
>I say "similar" because as you move toward wider and
>wider focal lengths the DOF is less and less evenly
>"centered" - the shorter the focal length the more
>depth you have behind the subject than in front ( while in a
>long telephoto it is almost evenly divided front to back. That
>does change the "focal length doesn't matter
>argument" a little bit.
>

That is an anomaly that Cambridge in Colour does bring up and discusses through this quote “For focal distances resulting in high magnification, or very near the hyperfocal distance, wide angle lenses may provide a greater DoF than telephoto lenses.” Your example of 20mm DX and 35mm FF is one of them, but at 25mm DX and 37.5mm FF using the same aperture, the anomaly for the most part is gone.

Also the tutorial mentions another exception with DOF in that extreme magnification can bring up another issue of pupil magnification (while I have read it I really do not fully understand that yet).

Pete

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LuisGonzalezLT

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#26. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 25

LuisGonzalezLT Silver Member Nikonian since 04th Nov 2005
Mon 19-Jul-10 09:37 PM

"aperture <--> subject distance "

Yes - that's it! My brain is really bleeding now.

Brian - my comparison was a 20mm vs a 35mm lens, not a 20 to a 30, but the details are moot as I went to do the calcs for the 20/35. I was eliminating the crop factor ( 20 * 1.5 ) to get the same FOV and kept the same subject distance. I thought that would be a wash but it is not. That's when I saw my first mistake. 35mm on FX and 20mm on DX do not give the same FOV. You actually need a 22.76mm on DX to equal a 35mm on FX. DX is just a tad more than a 1.5X crop. If you look at Bill's FOV tables you'll see that.

Width Height Diagonal
35mm on FX 54.4 37.8 63.4
20mm on DX 60.7 42.6 70.2
22.76mm on DX 54.4 37.8 63.4

Unfortunately, since the DX format isn't exactly a 1.5X crop using a generic 1.5 with the online calculators may add some error to their calculations.

Anyway - I ran the 35mm and a 22.8mm (closest I could get) through both of the DOF calcs mentioned above and below are the results. I had to select 1.5X on the one calculator since they did not provide manual entry for crop factor, and also a DX camera selection on the other ( which I don't know whether they use 1.5X or the accurate crop factor ) so those numbers are/may be off a bit. They did give slightly different results so a further examination of their code would be needed to quantify why. Their assumptions are probably a bit different.

Whatever the details the point of my exercise is to support the fact that keeping the same subject distance and only swapping DX/FX and lenses to keep the same FOV does change DOF as you corrected me on.

I also did a hyperfocal example between FX + 35mm vs DX + 22.8mm to show how that differs.

I also am not throwing print size differences into this particular exercise.

Luis Gonzalez
Everlasting Photography, Inc.



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elec164

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#27. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 26

elec164 Silver Member Nikonian since 15th Jan 2009
Mon 19-Jul-10 11:47 PM | edited Tue 20-Jul-10 12:19 AM by elec164

Luis, you feel into the rote trap. F-number is not a physical dimension and does not describe the aperture area in and of itself.

A 22.8mm lens at f/5.6 provides the same exposure as a 35mm lens at f/5.6, but the 22.8mm lens aperture is smaller and provides a narrower DOF.

The aperture of a 22.8mm lens at f/5.6 is about 13 square mm. The aperture of a 35mm lens at f/5.6 is about 30 square mm or close to 3 times the area. To achieve the same aperture you need to stop the 35mm lens down to about f/8.6, which will then provide an aperture area of 13 square mm and the same DOF at given focal distance and magnification.

Although Brain corrected me about this earlier, this is why I make the mistake of saying “using the actual aperture” for most people confuse the f-number as a physical dimension and think that f/5.6 on a 22.8mm lens and 35mm lens provides the same aperture. As I just showed, it does not. And when calculating DOF all that matters is aperture, subject distance and magnification.

Pete


Edited to add:

The last sentence should really be prefaced with in the majority of cases for as Luis pointed out, for really wide-angles there is an anomaly that is covered in the tutorial at Cambridge in Colour where higher magnifications and distances close to the hyper-focal distance wide-angle lenses will provide a greater DOF.

Pete

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LuisGonzalezLT

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#28. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 27

LuisGonzalezLT Silver Member Nikonian since 04th Nov 2005
Tue 20-Jul-10 12:13 AM

22.8mm focal length divided by F/ratio of 5.6 =
4.07mm diameter objective, which is a circular surface area of
13 sq.mm.

35mm focal length divided by F/ratio of 5.6 =
6.25mm diameter objective, which is a circular surface area of
30.7 sq.mm.

I know these things from building telescopes, but we don't have to worry about depth of field on those since we're always focused at infinity I'm usually more worried about keeping the exit pupil size smaller than the lasik corrected area of my corneas

Luis Gonzalez
Everlasting Photography, Inc.

elec164

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#29. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 28

elec164 Silver Member Nikonian since 15th Jan 2009
Tue 20-Jul-10 01:36 AM

A bit of topic but yeah, you never really restrict the aperture of a telescope for light collection and usually the size of the mirror or lens aperture is the f-number of the scope (although the opening of the enclosure could certainly become the restriction). Take my little Astroscan for instance. It has a 4.25-inch mirror but an effective aperture of 4.125 inches and focal length of 17.5 inches giving an f-number of 4.24.

But I must admit I am a bit ignorant on whether the eyepiece focal length will change the f-number. I know that I learned to calculate the effective f-number of the afocal projection system, but don’t know if that applies to the eyepiece restriction as well. But I know what you mean about the exit pupil with my one and only attempt at afocal projection with my camera. Another confusing point (for me at least) is that with eyepieces, shorter focal lengths provide greater magnification.

But aperture is the heart of understanding this issue of DX and FF capture. For the most part it is not the focal length that creates the difference in DOF but the difference in aperture at a given f-number.

Example at similar aperture (not same f-number)

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Examples showing the issue with wider angle lenses and nearing hyper-focal distance.

Click on image to view larger version


Again this is academic and you really do not need to have this level of understanding for affective use of DOF. But I believe this does fulfill the original request at a better understanding. And yes wider angle lenses will give greater DOF when near hyper-focal, but will be about the same DOF from near focal distance throughout most of the focal range. A complex issue to say the least. But every time I enter one of these discussions it becomes a bit clear for me.

Pete








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LuisGonzalezLT

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#30. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 29

LuisGonzalezLT Silver Member Nikonian since 04th Nov 2005
Tue 20-Jul-10 09:52 AM

Yeah - off topic a bit... sorry
"with eyepieces, shorter focal lengths provide greater magnification"

The magnification (power) is simply the focal length of your "lens" divided by the focal length of your "eyepiece"

Those DOF tables do drive the aperture ( not the F/stop ) point home.

Luis Gonzalez
Everlasting Photography, Inc.

elec164

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#31. "RE: Question about DOF with D700 (and other full frame cameras)" | In response to Reply # 30

elec164 Silver Member Nikonian since 15th Jan 2009
Tue 20-Jul-10 12:37 PM

I hope this discussion was helpful for all involved.

My search for a better understanding of this began before I became a Nikonian and before my present DSLR. I belong to a Yahoo Digital Photography group in which the P&S versus DSLR argument comes up often. Among the many vocal opponents there are two members who are particularly knowledgeable. One works for Carl Zeiss Sports Optics and the other has a doctorate in planetary science who regularly works on teams for the design of satellite imaging systems as well as data analysis for terrestrial as well as space exploration. You think this discussion was tough and made your brain bleed ?? Trying to follow their discussions confused the heck out of me so I was determined to get a better handle on this so I could follow them.

I would not say I fully understand all this yet, but realized much of what I thought I knew was not fully accurate.

I believe one thing that has not been brought up here is the fact that the CoC is tenuous at best. If you really want to over think this then there is a lot to be considered.

A CoC of .03mm for a FF camera is only for a full frame capture enlarged to 6.7x10 viewed at 10 inches. If you intend to make a 20x30 that will appear as apparently sharp as that 6.7x10 viewed at 10 inches, then you need a much smaller CoC at the time of capture. So in that scenario you would need to use a CoC of .009(diffraction affects aside but must also be considered). But if you are going to view the 20x30 at 30 inch distance, then you are back to the .03 CoC.

Or if you intended to crop out 50% of the capture and enlarge that to 6.7x10 you would need a CoC .019. Then again if you wish to make an 8x10 you would need a CoC of .024. And I think that is why some disagree on the FF CoC in the calculators. A full frame capture being 3:2 aspect ratio would need about an 8.5 times enlargement with a 2 inch crop on the long side giving a needed CoC of about .024. Where an enlargement of the long side to 10 inches would need a 7 times enlargement but fall short with only 6.7 inches on the short side but resulting in a CoC of .028 (of which some just round up to .03).

Personally while I understand this better now, in reality I put very little of it into practice. Being just an amateur hack, I never over think the whole DOF thing and similar to Bridget, I download the app for my Palm device and it is handy to have. But for me and the purpose I use it, a hand grenade is good enough.

Pete

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G