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12-16-2011, 05:40 AM - 2 Likes   #1
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Dif Lim of DFA 100 macro test

Not sure if this is discussed elsewhere but for my own edification, I decided to run a diffraction limit test of my own for my DFA 100mm macro. My setup was a pair of AB 400 strobes placed close enough that the light adjustment went easily from f8 to f32. I used a Sekonic light meter to adjust the output for each f stop. I did a custom WB and used MF and manual everything else and turned off shake reduction. Yes tripod mounted. I focused using LV on the very top of this daisy type flower and did not move a thing after that. Starting at f8 and slowly closing the aperture, I move f9.5,11,13,16,19,22,27, and 32. Three shots each just to be sure I got it right.
Using LR in the library panel, I viewed the images side by side at 2:1, a quite high pixel peeping size. The resolution stayed the same while the DOF increased with each closing down of the iris. It maxed out at f19. At f22, diffraction limiting did step in and the image just started to lose detail. It became worse the further I went. f32 was completely unusable. Since I am going to be using this lens for my floral imaging, it was important to know just how it responded.
BTW, I used my wife's store bought flowers and she admonished me for using such old flowers. Then she saw the test images and was amazed at the beauty her old flowers still possessed. I am quite happy with both of my DFAs, the 50 and 100. I find the 50mm much better for hand held captures on my nature walks.
Hope this helps someone. Dave

12-16-2011, 06:09 AM   #2
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Was this on the K-5?
I'm asking because the sensor might factor in to the test.

Anyway, Photozone's test of the lens on the 10MP K10
Pentax SMC-D FA 100mm f/2.8 macro - Review / Test Report - Analysis
also shows a big drop in the 50% MTF resolution
between f/16 and f/22, which matches your results.
OTOH, their FA 100/2.8 macro test on the K10
doesn't show a big drop until the f/22-f/32 step.
12-16-2011, 01:20 PM   #3
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Yes, K-5 with ISO 80 and speed set to X sync 1/180 sec
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