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