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.