Diffraction and Digital SLRs
 

As the time is now right in the northern hemisphere for close up shots of insects, large depth of field landscapes etc. I thought I would raise the subject of diffraction for the benefit of those members who are unaware of it. Despite posting a link to cambridgeincolour tutorials a while ago I only really looked at this subject when considering a camera upgrade. Here is the link http://www.cambridgeincolour.com/tut...hotography.htm

I did some checks with the D100 and saw the impact when using standard lenses on a nice sunny day at Well harbour and I was easily able to get to f22. D100 has a diffraction limit f11 and sure enough images started to noticeably degrade at f16. ( Combination of lens going soft and diffraction effect ) As I am checking out my old AIS lenses I have done some tests with the 55 micro. The D2X has a diffraction limit of f8. From these tests I have concluded that I will not go below f13 but shoot at less magnification and then crop the image when greater dof is required for a suject containg fine detail. I will also look up a link that Wolfie posted somewhere to a program that allows images taken at varying points of focus to be combined.

I have attached full frame and crops to show the effect. These have not had any sharpening applied.

The major benefit of this knowledge as I see it, is that where very fine detail is involved, reasonably staying within your diffraction boundries limits the amount of sharpening required in post processing. Which can only be a good thing for overall image quality.

Don

I theoretically knew about the diffraction effect but have never seen it so well demonstrated before. Will change my close up technique accordingly. Many thanks.

Don

Why is the defraction limit camera specific rather than lens specific :confused:

Clive,

To me it appears to be pixel pitch rather than camera specific. Hence the D100 6mp sensor responded as per the D70. I would expect the D50 to fall at the same point. My experience of this so far has been that it is from that point on apperture specific rather than lens specific. Macro lens does a bit better over the diffraction limit than a standard zoom but I guess that somewhere along the line it is better corrected for small apertures. ie my 50mm f1.4 only goes to f16, and a soft one at that, and the 55 f2.8 micro goes to f32.

No doubt some techie will come up with an answer but in the mean time I have just done the simple thing and taken test pics to establish my own limits. I don't know if quality glass behaves any differently, bank balance could not take the strain of that experiment. :D

Don

Clive it is more sensor size specific then camera specific, on film f22 was considered as small as you could go till diffraction adversley set in, now you have to take the sensor size into account eg on the Olympus E system its a doubling x2 of focal lengh which also means a doubling of DOF and dof is a cause of diffraction so f16 has the equivalent DOF as f32 on film and the same amount of diffraction, you just need to apply the x 1.5 factor for the D100 which would make it about f20. The very reason compact and prosumer cameras rarely have a aperture larger than f8 or f11.

crafty

Hi Crafty,

You probably have a greater understanding of these things.

When I did f16 things were starting to show obvious deterioration with the D100 and 18-35 zoom and by f22 fine detail, ie fine details on sailing boats was starting to dissapear altogether. With that camera/sensor/lens combination f13 is the max comfort point for me. D2X with that lens f11 tops and better at f8.

Lots here beyond my understanding - circles of confusion etc.

Perhaps further investigation as you are right about film, f22 and diffraction, so perhaps pixel pitch adds another dimension to that.

Don

A very interesting thread you have started here Don. I never realised that sensor size was a factor, apart from the degredation that arises from the extra enlargement needed with small sensors. As I understood it, degradaion due to defraction arises from bending of the light rays close to the perimeter of the iris. The smaller the iris the larger the ratio circumference/area of the iris, therefore, a larger proportion of the light rays get bent (defracted) with small apertures. The theoretical maximum resolution of a lens in lines/mm is 1600/f number. In practice this resolution is not achieved, but with a well corrected less it is approached after stopping down 2 stops.

John

Hi John,

I guessed this one would get you going. :D

A little bit of reading for you. http://micro.magnet.fsu.edu/primer/l...ctionhome.html

http://micro.magnet.fsu.edu/primer/j...icdiffraction/

This is something I knew nothing about a few weeks ago, but obviously it is something equipment reviewers are aware of. I have been through DPR camera reviews looking at the sample pics and noticed that all have been taken within that particular models diffraction limit.

I think here we may be seeing something of the differences between digital and film. Film grains having a radom spread where as digital sensor is a grid with micro lenses directing light into wells. I will do a graphic later of what I mean. Could well be wrong though. :D

Don

Hi Don, I will go and have a look at that reference now, thank you and I will look forward to seeing your graphic.

John

Well John it has taken all evening to do the graphics. I now wait to be pulled apart, but it is how I view what is happening.

I hope it all makes some sense as my brain is now in need of rest. Note the posting time.

Don

Well done Don! Yes Iv'e noted the posting time; two hours before I got up.

John.

Don: very interesting and informative posts. You seem to be one of the few people who consider diffraction in relation to sensor pit size. I know that Thom Hogan - a respected American commentator and Nikon user - mentions that the D200 hits the diffraction limit at about F13 and the D2x at about F11 (if my memory serves me). It is curious that many people comment that the advantage of the DX sensor is the removal of the nasty image edges where aberrations are worst. And yet, as you clearly show, a D2x can outresolve a lens even at the centre. Another aspect is that a lens usually improves as you stop it down, reaching a peak somewhere between F5.6 and F11. So if you use the lens wide open, the DX sensor may well crop soft edges, but it will also produce a softer image on axis compared to the so-called full frame camera.

Anyway, my own rule of thumb is to use a lens at F8 or F11 if possible, and F16 only if I need extreme DOF. For macro work the slight loss of sharpness in the plane of focus at F16 is countered by the increased sharpness over a wide depth.

I suppose if I had the uber-glass that some forum users own, then I would not bother with anything so vulgar as stopping down. :D

Leif

Leif,

You would be right there. Those lenses probably perform at their best between f4 and f5.6. :D

BTW I am reading your macro lens post with interest. Just wondering how limited the dof of a 200 would be compared to the 105 VR. After all you certainly have the sensor resolution to crop. I will check out a review and may well post in that thread.

Don

Theoretically the DOF is a function of F stop and image magnification alone assuming a fixed value for the size of the circle of confusion. The difference between the various macro/micro lenses is the perspective and the working distance. One of the nice aspects of a long lens is the ability to isolate a subject and create a uniform background.

However, when I compared images of a Bee Orchid using a 60mm macro at F11, and a 75-150 zoom + 3T diopter at 150mm and F11, I am sure the latter had reduced DOF. I suppose it might have been field curvature creating that illusion. Or alternatively the 60mm lens might just have been sharper, creatig the illusion of more DOF. More detailed tests might be worthwhile.

Leif

Don: As a result of your thread, it has struck me that an APS sensor is not as good for macro work as a so-called full size one. I used to often stop down to F22 for DOF, and perusing John Shaw's book Close Ups In Nature, I see he did the same. But, as you have pointed out, at F22 diffraction is significantly impacting the sharpness, and APS will only exacerbate that effect. So while F22 is useable on a full frame sensor (slight image degradation), F22 on an APS sensor is pushing the boundaries a bit too much. I will try to do some tests to see if F22 is useable (my criteria being the quality of an A4 print).

Leif

Leif,

Agree totally provided the full frame has large pixels ( say total 8mp ). Hence my earlier reference to Harry's image stacking program. Only really usable in very controlled environment.

This was one of the reasons I considered the D2Hs. 4mp but with large pixel well and a diffraction limit of f16. I finally decided that I would go with the higher sensor resolution and opt for slightly less magnification - therefore greater dof. Use the higher sensor resolution to allow the image to be cropped as a better option. For this to be a viable option one is really relying on TOP GLASS.

I have not really had much time to try this theory. I will dig out and post the full frame and crop of my Hoverfly as that was at f9 as an indication of the degree of crop.

I think it will be a worthwhile exercise taking one of your most detailed shots and seeing how much of a crop would still give a good print.

Don

Leif,

My 2 pics. This was not a scientific test but just a play. First outing of the 55 macro on the D2X. Breezy conditions ISO 400 - 1/400sec - F9 with no noise reduction.

For a proper test I really need a dead fly or similar to do a proper comparison between 6mp at f16 and 12mp at f13. ( no subject movement guaranteed )

My theory is that as the 12mp pixels are smaller then they will have the ability with the same lens to record finer detail. So provided f13 is the smallest aperture used on the 12mp sensor its cropped image will have greater detail than the 6mp sensor. I also think that a cropped f13 image will appear to have more sharp detail than one taken at f22 that has not been so highly magnified, but suffers from diffraction softness.

First suitable subject I find and I will do this test.

Don

There are times when thoughts of diffraction limits are immaterial. :rolleyes:

The coal used here is NOT Welsh finest Steam, but an imported product. With Welsh Steam Coal you have to chuck a bit of sawdust on the fire to get this effect. :D

Don

Very emotive picture Don. I was on holiday in Pickering last week and I took a few shots of steam locos. None as good as this though.

John

I have really enjoyed reading this thread and I have one question.

What effect will a teleconverter have on diffraction? If the diffraction limit of an APS size sensor with 12Mpx is f11 what happens when the effective aperture is at f11? i.e. camera indicating f11 for exposure but using a 2X converter the primary lens will be f5.6.

Stephen,

A very good question. My understanding is that it all down to the aperture of the lens itself. So if I am right, the converter will not have an additional effect. I don't have a TC so have not checked this out for myself.

Don

FWIW I agree with Don. Leif

Stephen,

Having just been through your gallery I cannot imagine you being much troubled by this. I notice you have only gone below f8 on your closeups. Always a f8 or wider on aircraft shots.

Just wondering if you are honing your reactions on Hoverflys as per Christines ( Sapphires ) pic. http://www.worldphotographyforum.com...ppuser=57&sl=s :D

Don

For close-ups I went smaller than f8 for depth and with aeroplanes my technique makes shutter speed king. If light allows a smaller aperture then the ISO gets lowered. f8 is about the cut off point to lower ISO. I have never been worried about f11 diffraction limit but I did know of it. Now I know why!!

When I get back home I'll go a test with TC and effective aperture (currently in Glasgow!)

I have found the last couple of days that I get the best with my 75-300mm lens and any combination of extension tubes that f8 is best with or without flash. I have also found if I can get a 1000s speed its even better.

I thought I'd add this in case anyone is interested.

Anyway, I recently realised that my comment on smaller sensor not being so good for macro work is in fact incorrect.

It is often said that smaller sensors have more depth of field, but I have never understood this. Well DOF is a function of aperture and image magnification only. Reducing the aperture increases the DOF. Increase the image magnification reduces the DOF. That is why we usually use very small apertures for insects and other small objects.

That second point is I believe the reason why a smaller sensor has larger DOF. Consider a full frame camera and an APS frame camera both using the same lens at the same aperture. Let's assume that we are taking the same picture, thus the APS camera must be further away, and the image magnification will be lower. By that I mean that the ratio of the subject size to the image size, where image size is measured on the sensor, is less. And so DOF in the DX image is greater.

I would guess that the difference is about 1 stop. So a lens working at F16 on a DX sensor would give similar DOF to the same lens working at F22 on a full frame sensor. Again this assumes the 'same' image.

So my conclusion is that APS cameras are just as good as full frame ones for macro.

However, although for macro more DOF sounds good, many people like to use shallow DOF for artistic effect e.g. to highlight part of a scene. That I think is an important reason why some people do not like APS cameras.

Leif