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06-08-2012, 05:08 AM   #1
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Is DOF at macro scales independent of focal length?

I have a 35 Ltd. and have just take delivery of a DFA 100 WR (yay me!).

More out of interest than anything else, I was wondering whether they had identical DOF at a given magnification. Obviously, at normal distances the 100mm will have shallower DOF than the 35mm for a given aperture. However, at 1:1 the 100mm focuses at 30.3cm while the 35mm focuses at 13.9cm. I was wondering whether the shallower DOF of the increased focal length is exactly matched by the shallower depth due to close focusing.

06-08-2012, 05:18 AM   #2
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You would have to calculate it in this particular case. DOf is influenced by distance to subject, aperture and focal length. You can use a website such as DOFmaster to evaluate DOF in your particular case but I'm guessing there will be differences.
06-08-2012, 05:56 AM   #3
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As I understand it, DOF is a function of magnification and independent of focal length. So, at the same f-stop and magnification (e.g. 1:1) you should get the same DOF with the DFA 100 WR that you get with your DA 35 Ltd. However, since the working distance will be much different, the rate of falloff of DOF should be greater with the 35. So your background will be blurrier with the 35 than with the 100 (at the same f-stop). On the other hand, the background in the field of view will be wider with the 35.
06-08-2012, 07:15 AM   #4
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I could wish that TopQuark post a side-by-side sample shot of the view of an object at 1:1 and the background with each lens. That would help some of the slower members (me) understand where the truth lies.

06-08-2012, 07:21 AM   #5
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QuoteOriginally posted by bdphoto Quote
As I understand it, DOF is a function of magnification and independent of focal length. So, at the same f-stop and magnification (e.g. 1:1) you should get the same DOF with the DFA 100 WR that you get with your DA 35 Ltd. However, since the working distance will be much different, the rate of falloff of DOF should be greater with the 35. So your background will be blurrier with the 35 than with the 100 (at the same f-stop). On the other hand, the background in the field of view will be wider with the 35.
Very correct.
So 1:1 magnification at f/2.8 gives you the same DOF regardless the focal length and distribution of the DOF will also be 1:1, some say the DOF is always 1:2 but that's almost never true.

However... pupil magnification does change the DOF with macro photography so between different macro lenses there will be a difference, so the DA35 and the DFA100 won't give you the same DOF
The 100mm give you more DOF then the DA35.
Sadly i don't know the pupil magnification of either lens so can't calculate it for you.
I could write a mail to Pentax to ask.


Here you have a calculator where you can precisely calculate the DOF.
VWDOF

And here is a read about depth of field and also explain the pupil magnification and his effect.
Depth of field


QuoteOriginally posted by imtheguy Quote
I could wish that TopQuark post a side-by-side sample shot of the view of an object at 1:1 and the background with each lens. That would help some of the slower members (me) understand where the truth lies.
Site above has samples... well calculated samples and diagrams.

Last edited by Anvh; 06-08-2012 at 07:28 AM.
06-08-2012, 07:53 AM   #6
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QuoteOriginally posted by top-quark Quote
I have a 35 Ltd. and have just take delivery of a DFA 100 WR (yay me!).

More out of interest than anything else, I was wondering whether they had identical DOF at a given magnification. Obviously, at normal distances the 100mm will have shallower DOF than the 35mm for a given aperture. However, at 1:1 the 100mm focuses at 30.3cm while the 35mm focuses at 13.9cm. I was wondering whether the shallower DOF of the increased focal length is exactly matched by the shallower depth due to close focusing.
I did a study for portrait lenses, using a DOF calculator and considering the magnification of each lens, I have since repeated it for telephoto lenses, with the same result. I have not considered macro, but I expect the results should hold true as well

what i found (to a first order approximation i.e. within less than 10%) was that if you compare any two different focal length lenses, AT THE SAME APERTURE, the depth of field, for a given magnification was constant.

that means at for example F4 on a 50 mm macro at 1:2 magnification, the depth of field would be the same as for a 100mm macro at F4 and also 1:2 magnification but shooting from twice as far away.
06-08-2012, 09:09 AM   #7
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QuoteOriginally posted by bdphoto Quote
As I understand it, DOF is a function of magnification and independent of focal length.
Magnification is a function of distance and focal length, thus DOF depends on focal length, among other factors.
06-08-2012, 09:18 AM   #8
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QuoteOriginally posted by Lowell Goudge Quote
I did a study for portrait lenses, using a DOF calculator and considering the magnification of each lens, I have since repeated it for telephoto lenses, with the same result. I have not considered macro, but I expect the results should hold true as well
Depends on which DOF calculator you use, you really need to put in the pupil magnification to get a useful number when working in the macro realm.

example.
Format - 24x36 mm - 24x36 mm
COC - 0.030 mm - 0.030 mm
Focal length - 60 mm - 100 mm
Pupil factor - 1.0 - 0.7
F-number - 8 - 8
Magnification - 1 - 1
------------------output--------------------
Depth of field - 0.960 mm - 1.17 mm

Conclusion, 100mm has more DOF then 60mm lens in this example.
Saying you get less DOF with longer lenses simply isn't correct.

06-08-2012, 09:22 AM   #9
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QuoteOriginally posted by RioRico Quote
Magnification is a function of distance and focal length, thus DOF depends on focal length, among other factors.
So if i leave magnification and f-number the same and i change the focal length then the DOF will be different?
06-08-2012, 09:42 AM   #10
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QuoteOriginally posted by Anvh Quote
So if i leave magnification and f-number the same and i change the focal length then the DOF will be different?
To keep the same magnification with a different focal length, the focus distance is different. Plug in the numbers and see what happens.
06-08-2012, 09:45 AM   #11
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QuoteOriginally posted by RioRico Quote
To keep the same magnification with a different focal length, the focus distance is different. Plug in the numbers and see what happens.
But if i only change focal length then it would change my magnification and that's what it's all about with Macro photography.
06-08-2012, 10:22 AM   #12
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Is DOF at macro scales independent of focal length?

The useful answer is yes. DOF is independent of focal length for macros
==================================================================

The precise answer is...

Focal length always affects DOF due to hyperfocal effects but sometimes not-so-much.

Total DoF = (stuff independent of focal length)/ (1- (distance/hyperfocal.distance)^2)

Hyperfocal distance = F^2/cN where c is circle of confusion & N is f-number

When F increases, Hyperfocal distance increases, therefore the denominator increases, therefore total DoF always decreases with increasing F.

But it is only important when distance is more than about one third of the Hyperfocal.distance.

Dave in Iowa
06-08-2012, 10:22 AM   #13
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QuoteOriginally posted by imtheguy Quote
I could wish that TopQuark post a side-by-side sample shot of the view of an object at 1:1 and the background with each lens. That would help some of the slower members (me) understand where the truth lies.
I agree completely. We can theorize, but there's no substitute for actually going out and putting theory to the test.

These are all photos of the same damned flower, so apologies in advance for the repetition. By the way, if anyone knows what this is I'd be interested to know. They're growing all over my garden (Aylesbury, UK).

First up, 35mm, 1:1, F16



100% crop:



Next up, 100mm, 1:1, F16



100% crop:



If anything, I'd say the 100mm has very slightly deeper DOF, but that's just two pictures. The vagaries of manual focusing are more likely to be the reason for any difference here.

35mm, 1:3, F4



100mm, 1:3, F4


(It had started raining quite heavily when I took this one. Hooray for WR!)

35mm, 1:5, F2.8



100mm, 1:5, F2.8



Hard to say, but I can't see any significant differences in DOF in any of these pictures for a given magnification level and F-stop. Interestingly, taking into account differences in my position and the effect of the wind blowing stuff around the frame, I can't see any significant difference in the AOV either, which is not something I expected. There are slight differences in perspective due to the different working distances, but that is certainly to be expected.

So, the conclusion I'm going to draw here is that if you focus for a given magnification, DOF and AOV are independent of focal length. For macro photography, a difference in focal length would appear only to change the working distance.
06-08-2012, 10:30 AM   #14
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QuoteOriginally posted by Anvh Quote
Originally posted by imtheguy Quote
I could wish that TopQuark post a side-by-side sample shot of the view of an object at 1:1 and the background with each lens. That would help some of the slower members (me) understand where the truth lies.
Here's such a comparison shot with a cracker box in the background. 50mm lens vs 90mm lens, same DoF, 1:2 mag, focus on silk rose.


Dave in Iowa
06-08-2012, 10:47 PM   #15
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Those interested in portraiture should note that the same principle applies to this kind of photography: if you want a head and shoulders shot, DOF is dependent only on aperture. And, assuming you want all your subject's features in focus for this kind of shot, you'll be needing to stop down a fair bit if you're using a fast lens...

Last edited by m42man; 06-08-2012 at 10:58 PM.
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