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| p.11 #3 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes |
I have known this but phase detection focus shifts with light temperature. A couple of years ago, I returned 3 camera bodies and after months of frustration testing 6 camera bodies and lenses, I finally realized that focus shifts depending on light temperature. Under warm incandescent light, cameras front-focuses (or move in that direction). Under whiter light such as LEDs and florescent lights, cameras back-focus. Even outdoors, there is a shift going from sunlight to shade, mid-day to dawn/dusk. I don't know why this can't be compensated in camera electronics since auto-balance sensor can measure color temperature.
Most cameras suffer from this, including the D800/E. AWB sensors probably aren't reliable enough to rely upon for integration into the PDAF logic.
I have to say the 5D3 focus accuracy in various conditions is much, much better than the 7D for example but if you examine closely, you can get frustrated with any camera. I am happy with my camera/lens combination in this case. The MFA average is exactly zero.
I agree, the 5DM3/1DX appear to be unique in their consistent ability to operate at a zero MFA. My theory is that they've tightened the manufacturing tolerances of these bodies.
The Dot-tune method seems to work well but I don't believe in the MFA since the value changes with light temperature, distance, and with focal length (in case of zoom lens). If you set the MFA to any value other than 0, it will make things worse in some other conditions. If my camera/lens need MFA, I would send them back and get them calibrated.
For bodies that need MFA I wouldn't suggest avoiding tuning just because PDAF operates differently in different lighting temperatures. I tune to my most used lighting temperature for a given lens and if I find myself in a lighting situation that differs significantly from what I tuned then I just tune again on location, which is fast and easy with DotTune but not so with other methods. I also have tungsten -> daylight deltas written down for my most used lenses (along with focus shift deltas) so mostly I just adjust the tune value by memory.
I do understand that it can be beneficial if your camera/lens is offset to one direction but in my experience, focus consistency gets worse when MFA is enabled and set to a non-zero value.
My experience is contrary, for reasons I explain here.