Help judging performance of my FE55z, FE35f2.8z horizon/infinity

Started Jul 5, 2016 | Questions thread
JimKasson
JimKasson Forum Pro • Posts: 26,718
How much depth tolerance at infinity focus?
1

Here is a plot of on-axis MTF50, measured in cycles/picture height, for a simulated Otus 55mm f/1.4 (actually, it's a 55mm lens with the aberrations of an Otus 85/1.4), mounted on an a7RII, focused at 10 km, with various object distances:

You can use this chart to find safe distances for distant objects in the field of your lens testing, assuming that the lens that you're testing isn't better than the Otus. If you do have lenses that are better than the Otus, let me know; I can re-run the curves for a diffraction-limited lens.

Here's how to use the chart. Pick the aperture you're going to use, and find the curve o for that aperture. Start at the left hand side of the graph, and run your finger along the curve until you find the place where it starts to bend down. For the f/2.8 curve, that's about 500 meters. Your lens testing will be unaffected by object distance as long as objects that you use to judge sharpness are all more distant than 500 meters.

To get the distances for other focal lengths, do the above, and then multiply the distance by:

(f/55)^2

where f is the focal length in mm of the lens that you want to test.

Jim

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