Many people claim that mirrorless cameras don’t need AF fine tuning because the image sensor is used for focusing. On a DSLR there are specific sensors used for focusing when the mirror is down. There might be a small difference in exact position of these sensors compared to the live view sensor. This can be adjusted with the default value in the fine tune settings. Let’s assume the default value is “0”...I.e. perfect. Still different lenses may need calibration. Why? My understanding (correct me if I’m wrong) is that perfect focus is where the focus plane is same as the focal length of the lens. The lens mount could be slightly out of specification...the focus plane does not reach the image sensor (front focus) or will be further away (back focus). If this didn’t happen all you would need to do is to set the default value! All lenses would then focus correctly! This is not the case! So, are mirrorless lenses perfect? I guess not. All lenses have tolerances...sometimes they are good...sometimes not so good. To me, this means that mirrorless cameras need AF fine tuning as well!
Opinions please!
/Bosse

Many people claim that mirrorless cameras don’t need AF fine tuning because the image sensor is used for focusing. On a DSLR there are specific sensors used for focusing when the mirror is down. There might be a small difference in exact position of these sensors compared to the live view sensor. This can be adjusted with the default value in the fine tune settings. Let’s assume the default value is “0”...I.e. perfect. Still different lenses may need calibration. Why? My understanding (correct me if I’m wrong) is that perfect focus is where the focus plane is same as the focal length of the lens. The lens mount could be slightly out of specification...the focus plane does not reach the image sensor (front focus) or will be further away (back focus). If this didn’t happen all you would need to do is to set the default value! All lenses would then focus correctly! This is not the case! So, are mirrorless lenses perfect? I guess not. All lenses have tolerances...sometimes they are good...sometimes not so good. To me, this means that mirrorless cameras need AF fine tuning as well!
Opinions please!
/Bosse

No truly mirrorless camera that's using AF sensors embedded in the image sensor itself needs AF fine tuning. Think about it... Since the AF sensors and the imaging sensors are in exactly the same plane, actually are both part of the same surface, there's no way that they can be out of phase with each other. As a result, now AF fine tuning will ever be needed and the mirrorless design automatically compensates for variation in the lens or it's mounting. (Note: Lens or camera mount may need calibration to ensure they are perfectly parallel, so that the entire image area is focusing equally, but if needed that's usually accomplished during manufacture by shimming the bayonet mounts themselves.)

The Sony A7III has an AF fine tuning feature that's only necessary when using the LA-EA2 or LA-EA4 adapter, which introduces a translucent mirror to the light path (much like a DSLR) to redirect part of the image for focusing purposes, in order to allow DSLR A-mount lenses to be used on the mirrorless E-mount cameras. Ability to fine tune focus with that adapter is necessary to correct for minute variations in the adapter itself.

From an actual user, who has actually performed
AF micro adjustment on a7-XX series cameras:

Search the manual even further and you will find
that menu options may be "greyed out" whenever
they do not apply to the total outfit in use [body,
lens, flash, adapters, etc].

To "un-grey" the AF micro adjustment option, just
attach an LAEA-2 or -4, which use SLR-type AF to
AF A-Mount [Maxxum, SLT, etc] lenses. Mounting
either of these adapters will automatically disable
the sensor-based AF of the camera body.



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There is an entirely different misunderstanding, or perhaps
just a knowledge gap, per the OP. Focus errors from phase
detection AF are only secondarily related to assembly of the
precision parts. PD focus is, by its nature, only approximate
even if the devices involved were built 100% "to blueprint".

Even if, by sheer luck, a lens and a body happen to get built
at 100% accuracy, and even if you put the two together, it is
the nature of optical images, and of phase detection AF, that
some micro adjustment may be necessary.

The micro adjust is a phase detection adjustment, and not an
adjustment for an error in flange depth tolerance. So a single
micro adjustment cannot compensate for all lenses, cuz you
are not compensating for the body, but for the lenses as well.
FWIW the body is likely to be waaaaay closer to perfect build
than most lenses.

If you don't understand phase detection, I can't fix it in this
post. But each lens's unique curvature of field will "fool" the
PD AF. SLR AF has only PD sensors.

Live view cameras ... and live view mode on SLRs ... use CD
[contrast detection] AF which is slower, but cannot be fooled.
Late model live view systems have both PD and CD systems.
The PD is virtually instantaneous, but is error-prone. So, as
soon as PD is "satisfied", CD "leaps in" to perfect the focus.

The closest I will come to explaining the inner life of PD and
CD AF systems is to say that PD is like an optical rangefinder
operating at nearly pixel level, while CD is like direct optical
focusing via a ground glass panel, like a view camera. If you
have at least a crude grasp of that analogy then you can see
why PD is faster, while CD is slower but more accurate !

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Some lenses have discernible backlash within their focusing
mechanism. This can cause error with either AF system, but
is still likely to be partially compensated by CD AF.adjustment


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