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So, I'm calibrating my lenses to a new camera body, a 5D Mark IV. When calibrating my 70-200 ƒ2.8 (version 1), I found that the focus was accurate at 70mm. However, when I calibrated it at the 200mm end, it was inaccurate.

The problem is, the inaccuracy is erratic. Rather than being just back focused or just front focused, which I can adjust for, the 5D would sometimes be back focused, sometimes front focused, and sometimes almost accurate.

Being that it is a long lens, I figured maybe camera shake might be causing it, so I retested using mirror lock-up to minimize shake. But I'm still seeing the same result.

I found the same behavior on my 17-40 ƒ4.0. Accurate at 17, erratic at 40.

I didn't see this behavior a couple years ago when I calibrated the same lenses using my 6D Mark II.

Ideas anyone?

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Ok, for future people searching for this issue, I think this is resolved.

The issue (for me, at least) was not enough light on the target. I was doing the testing indoors, beside a window mid-day (curtains open) and below a tungsten dining room light. But this was not enough light on the target for the auto focus to perform it's best. IIRC, my shutter was about 1.5 seconds in those conditions.

So today I lit it with a daylight balanced LED panel and re-tested. I always expect some variability, so I take 5 shots at each focal range I test, refocusing between each.

This time there was still some variability, but instead of + or - 12 units (on whatever scale they use), it was more like + or - 3. And there was only one shot that was 3 off, so I'll assume that it was an outlier.

So thanks everyone for your comments.

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In a way you could say that autofocus is less reliable when a lens is zoomed... if you do not correspondingly increase the focus distance.

More accurately; what happens is that the depth of focus decreases, which makes any focus errors more apparent (there is always an error tolerance; there has to be).

Also, many zoom lenses have more optical errors at the extremes (one or both). That certainly can impact autofocus accuracy/reliability.

But IME, the main issue with autofocus calibration is poor/improper methods.

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