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Reader Question: Focusing with Sony Mirrorless

See my Sony mirrorless wish list at B&H Photo.

Shooting 36/42/50 megapixel cameras demands perfection in technical execution, which means the #1 execution error is focus error, though that is a general term whose cause is not always self evident. There are many issues that interact, including autofocus accuracy and precision, focus shift, field curvature and real depth of field.

Gerner W writes:

It came to my attention in a Sony forum that some users of the Sony GM 85/1.4 and GM 24-70/2.8 have faced some problems achieving consistent AF-C focus with these two be mentioned lenses.

We know Sony changed the way of AF after the 2nd or 3rd FW update so that the iris closes down to the set aperture in order to eliminate any issues with focus shift and in some cases field curvature? Not sure about the last one.

Time ago I acquired a GM 24-70 for my A7R2 and haven't faced an eventual problem, but will do some provocative tests today to determine if I have a problem or not.

You mention such an eventual problem in your review of the GM85, but have you also detected the eventual phenomenon on the GM 24-70 but not found it worth to mention in the review?

You also mentioned that eventual turning off "picture effect" would help out the problem ... but I find this solution most inappropriate since where's my preview of my exposure compensation etc etc gone then.

DIGLLOYD: I am unsure about Sony’s timeline, etc. And Sony’s Setting Effect has no bearing on the issue; that was/is a red herring.

There is only one significant issue I am aware of for autofocus with Sony cameras: Sony mirrorless cameras focus with the lens stopped down, that is, if the camera is set to f/8, then the lens diaphragm is at f/8 and the camera will focus at f/8.

Focusing fully stopped down is a serious algorithmic bug that leads to significant focus error, since there is no way for the camera to center the zone of focus on the desired point (even a human will have great difficulty due to the depth of field)—it will be sharp where focused 99.9% of the time in my experience, but how the total zone of sharpness is centered around that area can be quite variable—worse under some conditions like dim light and close range.

Sony could fix this problematic focusing behavior with firmware update by offering a user preference, but I doubt Sony even has it on the radar. Other vendors always focus with the lens wide open, which is better but still a problem with some lenses. The best solution would be a “focus stopped down no more than ___” option, lens specific.

The foregoing is why when I shoot for comparisons and focus stacking or anything on a tripod, I always focus wide open or one stop down (rarely 2 stops down). However, it is not quite that simple; see these articles which apply to any camera platform:

A major factor in total sharpness optimization is differential focus shift, that is, focus shift that is different in peripheral areas versus the center, particularly with the Sony 85mm f/1.4 GM; see for example Aperture Series: Running Creek. Even the Zeiss Otus 28mm f/1.4 APO-Distagon (and many other fine lenses) have such behaviors, well worth understanding for peak results. Such behaviors often derive (particularly in high performance lenses) from a balancing of oblique spherical aberration against other undesirable aberrations.

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