ChrisR wrote:Ah that's interesting! Is this about 20x? (It gets harder to tell at 20x, doesn't it!?)
I think the magnification is around 20X. Yes, I think it gets harder to determine the exact magnification at higher magn's. You need to have some reference points or rulers to do that. I don't have that, so I have to put that on my "to do/to find list".
I found myself seeing some smearing of detail so dropped the image into PS and put red marks on top, while peering at high magnification. One gets lost in the detail so is not aware of where in the field one is looking, which helps with objectivity.
The fall-off seems fairly gradual once it's visible. My circle of just-about-visibleness is about 13mm, here
I hope you don't mind me molesting your image - happy to delete it. Perhaps it would appear better with more bellows extension - if the circle grew to cover a 1.6x crop sensor and retain good performance? I can't see any obvious bad things in the circle
It's OK, I'm glad to hear your input and I think others are too.
That image you are referring to, I wanted to use bellows extension near the limit when I saw vignetting started to occur on the FF. I have done some more tests with the 7D and I will upload them as they are finished. (I have old and slow computers).
I share your lust(!) for a perfect test target made with easily found materials. That's a "work in progress"! Fine particles of brittle material have good edges:- Pencil lead is OK but black, crayons are colourful but too soft, I can't buy carborundum grit, glass has been disappointing. Rubbing 600 grit SiC paper together makes a lot of ugly dust from the adhesive. The marked side of a microscope scale is useful, but often not as sharply marked as would be nice.
Yes, it would be nice to have perfect test targets to have as references for testing. I think they need to be different given the magnification, one that's fine at 1X probably don't fits for 20X.
"Sometimes you see more than what you want"
Chris: I edit my post here while I done some thinking......
Maybe the nature gives us the best test subjects?
I think we seen good test shoots when butterflies and moths wings are used as test subjects. They may also be good for different magnifications.