Diffraction, depth of field and sensor size

[Piet]

Hi,

I joined CiC a month ago and I did introduced myself in the Common Room.

I am a bird photographer, but do also macro work. Since I am using a Nikon D300 camera, f11 is the smallest aperture that I use due to diffraction. I did considered to "upgrade" to a full frame camera (D700/D3) where you can go to F16 before diffraction gets really noticable.

It is not possible for me to get my hands on a full frame to compare it with the D300. So I did a few calculations by comparing diffrent sizes sensors for the same FoV at the same number of pixels (12mp) and at their diffraction limited aperture. The results are as follow:

Format size____Focal length____CoC______Aperture____DoF at 2m____DoF at 20m
36x54.............128...................0.025..... ..19................223mm.........36 069mm
24x36.............85....................0.017..... ..13................228mm.........36 229mm
20x30.............71....................0.014..... ..10.5.............229mm..........36 282mm
16x24.............57....................0.011..... ..8.4...............231mm..........36 336mm

Focal length is for the same FoV for the different sensor sizes.
CoC is taken for 2 pixels.
Aperture is at diffraction limit where CoC=airy disk.
Calcultions were done with Max Lyons photographic calculators http://www.tawbaware.com/maxlyons/calc.htm

According to these results DoF for different sensor sizes are practically exactly the same at their diffraction limited apertures. As far as DoF is considered, their is no advantage to use a smaller aperture on a larger size sensor with the same FoV and the same no of pixels. When the pixels are doubled on the same size sensor, DoF is cut by half at close range (2m) and nearly 3 times at 20m. At least one can crop down to less pixels to get the desired DoF and there by avoiding diffractions. Otherwise you have to stop down and loose anyhow not only resolution due to diffraction, but also contras and sharpness. (CiC has a very good article on diffraction).

The effect of diffraction is still acceptable at 1 stop smaller than the diffraction limited aperture. However at 2 stops smaller, resolution drops to the same as with a sensor with half the no of pixels at 1 stop smaller than its diffraction limited aperture. This can be seen with lens reviews on the D3 and D3x, where the resolution is the same on the D3x (24mp) at f16 as f11 on the D3 (12mp). (See DPReview's lens test). Furthermore, DoF is also the same then. Since diffraction doesn't only affect resolution, but also contras and sharpness, IQ could probably be beter at f11 on a D3 than f16 on a D3x where DoF will be the same.

I have re-invented the wheel now, but there is a preception that one will get more DoF with a full frame camera because diffraction kicks in at a smaller aperture. (This also depends on the pixel pitch). I am not a physicist and I did not do any side by side test to confirm this. Your c&c will be welcome since I want clarity for myself on this.

Piet

[Piet]

Hi,

Since no one reply to my thread, I must have written nonsense or it is totally ununderstandable

Anyhow, I have overlooked the second page on diffraction on CiC that adresses basically these issuses. There is also a very good discusion on Dpreview started by Sean McHugh. See http://forums.dpreview.com/forums/re...ssage=35709137

Piet

[Peter Ryan]

Hi Piet,

Welcome to CiC and I am sorry no one has responded to your post. I have just read this but it is above my understanding to give you an answer. This may be the case for others. I am pleased you managed to find some resolution here in the tutorials.

Please make sure you continue to post questions.

[arith]

Hi Piet

I didn't see your entry until just. They are great calculators by the way. Welcome. I only know the diffraction limit for my camera to be around f7.1 red/blue diffraction limited and didn't consider fov dof ect in different sensor sizes. It could be useful if you are looking to upgrade to know what if any improvement you will get, but I suppose the down side is you will need much more expensive lenses for the full frame to get the same speed I mean.

[Colin Southern]

Personally, I think people worry about diffraction a lot more than they need to. Case in point; this morning I was engaged by a client to shoot a new product - to get sufficient DoF I had to shoot at F32, and had to back the camera with a 200mm lens off to around 2.5m resulting in around 2800 pixels being available along the long axis of the shot. At F32 you might think that diffraction would be a huge problem, but it's not -- and with normal capture sharpening it was a total non-issue.

These days I just don't worry about diffraction; it's better to kill a few pixels but save the image than vice-versa.

[Donald]

Piet

My apologies also for not responding, but, like Peter, your post was way beyond my understanding and ability to answer.

As I've written in other threads, my photography is driven by emotion rather than technical knoweldge.

[Eric M]

I shoot at f16 with the 50D, and f22 with the 5D2 when needed. Like Colin said, most folks worry more about diffraction than they need to.

Your post is more technical than I have time to sort out. Like most here, I know from experience that DLA is interesting theory that has limited impact on actual, everyday photography.

[benm]

>> I have re-invented the wheel now, but there is a preception that one will get more DoF with a full frame camera because diffraction kicks in at a smaller aperture.

This is the opposite reasoning of why most people buy a full frame camera instead of a crop camera. A full frame camera is better able to give you LESS DoF, sometimes a very desirable property. If you use a P&S (say with a sensor of 1/2.8") you can nearly always get everything in focus, whether you want to or not.

[Piet]

Hi everyone,

Thanks for the reaction.

Yes, may be diffraction is not such a big issue for macro work etc, probably because there aren't so much detail as one might think. It seems that the landscape photographers are a bit more concern about it due to all the fine detail they like to capture. As I have mentioned in the OP, resolution drops significantly 2 stops pass the diffraction limited aperture (f16 for 24mp) to about the same level as a sensor with half the no of mp (12mp) at the diffraction limited aperture (f11) of the 12 mp sensor. (DoF is then the same for both these sensors at these settings). From this it seems diffraction is probably only an issue when printing very large and the subject has a lot of fine detail (and you rub your nose against the print.) Then one must also asks oneself do I really need these high mp cameras? Certainly that will depends on your needs.

Anyhow it is good to know what the underlying principles are - even if it is mostly a non-issue

Piet

[Piet]

Hi benm,

Yes that is true if the same aperture is been used at the same FoV, DoF will be different, or otherwise DoF will be the same at the same aperture with the same magnification (FoV will then be different). Furtermore, diffraction kicks in at a smaller aperture with a larger sensor ea ~f11 for a 12mp fullframe vs ~f8 for a 12 mp aps-c due to different pixel pitch. DoF will be different if the magnification is the same, but as soon as you compensate for FoV the DoF is the same at these settings. I read and some people told me if diffraction gets a problem at apertures smaller than f11 on a D300, you can get a FF where f16 is still acceptable in terms of diffractions. That is true, but I don't gain anything as far as DoF is concern. Therefore there is no reason for me to upgrade to a FF for this purpose and even a higher mp camera will have no advantage when DoF is considered passed the diffraction limit. At least that is what the calculations tell us, but practically it seems I don't have to worry. So I am staying with my present equipment

Piet

[Colin Southern]

From this it seems diffraction is probably only an issue when printing very large and the subject has a lot of fine detail (and you rub your nose against the print.)

Exactly!

I primarily shoot landscape - I shoot F22 / F32 a LOT (as it aids in getting long exposures -- up to around 20 minutes WITH up to 8 stops of ND filter) - the smallest I ever print is 22 x 11 inches (normal size is 44 x 22 inches, and I go up to 22 x 88 inches) and I can tell you that diffraction just isn't a problem ... why? Because when people are looking at a 22 x 44 inch print they stand back a few feet, look at the whole thing, and utter "wow - what a great photo" ... what they DON'T do is go up to some part of it with a magnifying glass and say "darn - I see you were diffraction limited - what a pity the shot is ruined".

On the flip side, if I were to avoid small apertures then my exposure may only be perhaps a quarter of what it was and this can have a major effect on cloud motion, water smoothing etc - or in other words, I would have ruined the image trying to save the pixels.

If we were landlords, do you think it would be fair of us to kick out a tennant for not keeping the carpets clean because we went over every inch with a microscope, and found dust mites? (or would we just stand at the door and think "looks clean to me"?)

[Peter Ryan]

-- and with normal capture sharpening it was a total non-issue.

Colin, you have mentioned this in a number of posts recently.

Just wondering if you would like to elaborate on ‘normal capture sharpening’. I have my processes but would like to hear you comments on this subject.

[timo2]

At smaller apertures, dust spots are a bigger problem than diffraction.

[Colin Southern]

Colin, you have mentioned this in a number of posts recently.

Just wondering if you would like to elaborate on ‘normal capture sharpening’. I have my processes but would like to hear you comments on this subject.

Hi Peter,

Have a read of this thread, and perhaps come back to use with more questions if this doesn't explain everything you need to know?

When/How to Best Sharpen a Digital Photograph

[Colin Southern]

At smaller apertures, dust spots are a bigger problem than diffraction.

Oh Brother, ain't that the truth!