why micro focus adjustment ?

Started Feb 25, 2013 | Questions thread
theswede
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Re: why micro focus adjustment ?
In reply to OldClicker, Feb 25, 2013

That is one reason for discrepancy in focus. Since the focus sensor relies on light coming from two pathways any alignment error will cause issues, and these issues may (will!) vary between lenses.

In addition the lense projection plane is not perfect. In the simplest lenses it is part of a sphere, and in more exotic zooms it can have very strange shapes, including a "wavy" pattern. I've never seen a lense with a flat projection plane though some teles come very close.

Any of these shapes can cause the apparent focus plane of the whole plane to differ from the focus plane as measured through two points - simply put, if the camera were to place focus where it appears to be it would be wrong - and this is compensated by the lense focus adjustment value (which has different names with different manufacturers). When a lense without mechanical issues has a focus problem this value is usually wrong, and it can be adjusted by a service centre.

All of this is complicated by focus plane offsets shifting when a lense is zoomed (or even stopped down, though that is seldom an issue to to DOF increase) and different focus points "looking" at different parts of the focus plane. It's a miracle and a testament to the engineering divisions of the camera companies focusing works as well as it does.

Jesper

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