R5 back-focusing EF @ 400mm f/5.6?

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Bigger Regular Member • Posts: 231
R5 back-focusing EF @ 400mm f/5.6?
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I picked up the new RF 100-400mm f/5.6-8 lens as soon as it came out to have a lighter telephoto I could carry around for general use. I tested it against my EF 400mm f/5.6L on my R5 (using the Canon EF/RF converter). I found the RF had better center sharpness (but the EF corners were better). This was surprising, but maybe my old 400mm prime needed service, so I tested it against my EF 100-400mm f/4.5-5.6L II at 400mm, and found the center sharpness was still better on the RF, even with the EF at f/8. This seemed impossible, so I did some more digging....

I found that if I used focus bracketing on the R5 I could get sharper images from the EF 400mm lenses. But using DPAF, the RF generally produced sharper centers. So I did some more digging....

I started testing in my basement, but ended up setting siemens star targets at 0.5m intervals at 40m across my yard to test the camera and lens combos at 100:1 reproduction ratio outdoors under more realistic conditions. I would set up spot focus on different targets, and then check to see which ones were actually in focus by looking at the aliasing in the center, and the targets behind were more in focus than the ones in front.

Focus bracketing would produce sharper images on the 2nd or 3rd image because the R5 actually front-focuses before stepping back toward infinity. If I manually peaked the focus, then autofocused, the R5 would shift to back-focus defocus. The problem seemed to be worse with teleconverters and with low light, not surprisingly. I didn't see the problem on my EF 600m f/4L II though.

My EF100-400mm II @ 400mm typically had a little less than a meter of back-focus at 40m, and my EF 400mm f/5.6L had more than a meter (both on the R5 with DPAF). But on my 5DsR, with contrast autofocus in live view, the focus was consistently spot on with both EF lenses, even with teleconverters. I ran the test until dusk using the EF 400mm @ f/5.6 and ISO100, and the focus stayed spot on until the exposure went over 2s, and the live view became a snowstorm.

I don't think it's too much to ask the R5 to do what the 5DsR can do. Sure, the 5DsR live view focus is slower, but nobody wants an out of focus image faster! I searched for this problem in other threads and saw some comments on R5 DPAF problems with long lenses, but nothing too specific. Has anyone else had problems with this, especially with EF lenses at 400mm f/5.6?

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