Diffraction and Digital SLRs

[John]

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

John.

Originally Posted by Don Hoey

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

[Leif]

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.

Leif

[Don Hoey]

Originally Posted by Leif

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.

Leif

Leif,

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

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

[Leif]

Originally Posted by Don Hoey

Leif,

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

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

[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

[Don Hoey]

Originally Posted by 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

[Don Hoey]

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

[Don Hoey]

There are times when thoughts of diffraction limits are immaterial.

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.

Don

[John]

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

[Canis Vulpes]

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.