Macro and diffraction - A test that has surprised me

[jcuknz]

So the take-home message for me is that if pushed too far, which can easily happen with extension tubes without realizing it, high f-stop diffraction effects can become large enough to matter.

I wonder if that is why people reverse the lens when using considerable extension.

[chauncey]

f16, ISO400, 1/60th

f40, ISO400, 1/60th

Aah...are those settings a typo?
I just recently ran bokeh comparisons and found that when I increased f/stop, I had to decrease SS.

[revi]

Aah...are those settings a typo?
(..)

Nope, they are real values:
if you get close to a subject, the distance between lens and sensor increases. F-stop values are calculated as:
(lens-sensor distance) / (diaphragm diameter). The latter doubles between focusing at infinity and focusing for a
1:1 image, so f-stop value doubles. Apparently Nikon cameras take focusing distance into account (and thus report
the corrected f-stop), others just report the aperture value as if the lens is focused at infinity (see previous posts in
this thread).

[Arlen]

Chauncey, maybe Grahame will weigh in with the definitive word, but I don't think his exif data was a typo. In the setup he was using, it's the output of the flash that mainly controls the final exposure. So even though his aperture was smaller (and shutter speed the same) in his second example, the camera automatically attempts to increase flash output to get proper exposure. He did note however that his flash seemed to hit its output limit in that second shot, so that it may have been a bit underexposed.

I wonder if that is why people reverse the lens when using considerable extension.

I don't think so. Most of the time the main reason is that if you already have a normal lens that can be reversed, it's cheaper than buying an additional lens for its macro capability. Though there are some really high quality older lenses (usually hard to find) that are reported to give exceptional macro images when reversed, and there are a small number of dedicated aficionados who prefer them.

[chauncey]

Got it...thanks guys

[Stagecoach]

Arlen, Chauncey,

The Exif data given is not a typo but the effective aperture as indicated by the camera and contained within the image file Exif.

Regarding the exposure Arlens assumption is correct. I used my two small SBR-200s attached to the lens front fitted with their optional diffusers. These diffusers although very good cause a power loss due to the light being both re-directed from a straight direction and also then passing through the diffuser material. Very simply, the power is not sufficient for small apertures and drops off when getting above f29 whether used in Manual or TTL mode.

The reason for the diffuser design where the light path is re-directed twice is to allow you to place them in front of the lens, giving almost front lighting when you are using maximum magnification of 1:1 at closest camera to subject distance possible. This power loss causing underexposure can be corrected in post.

I have just this minute considered that there may be another option to overcome this although in the real world its not really a concern which would be to drop the ISO from its base value of 200 which of course will have other affects on IQ.