In lens focus microadjustment – is it possible?
Of course it is. In fact, each lens of each type is a little bit different from the averedge lens – so the camera will react different with the autofocus.
As You probably know, in the camera there are 3 optical systems, which have to be adjusted on to the other: the cmos, the AF-system and the optical viewfinder. They are adjusted mechanically in the factory, after then adjusted by a software too – but they would work precisely only with the master lens, with one single focal distance.
As we use lenses with different properties, we have to tell this to the camera, and the camera will make the recalculation (or compensation) for the given lens. This is why a manual lens with an emulator chip will work well only if the chip is programmed for the real focal length.
But lenses are different each of them, so there is a possibility for a second correction, which is stored in the lens and will be transmitted to the camera. By the older lenses the needed correction was determined by the factory and marked on the lens board very simple: in the form of some soldering pads, which can be shorted or not by a little soldering. Each pad reperesents a bit, and with a table You can understand, if the lens was programmed to a little bit backfocus or frontfocus.
You can ask me: ok, but why is this interesting, if it has been set by the factory. the answer is simple: at first, I like to know, how thinks work. At second, the users camera is not a master camera, so the camera itself can have a front or backfocus, and after a repair the lens could be other then before – so there is a need of adjustment.
What i know is, that the EF 50 mm F1.4 lens and the EF 85 mm F1.8 lens have this possibility, and works!
EF 50 mm F1.4
I made some experiments with the pads (You need just an 8 pol DIP switch ans some thin wires), and it seems, the here are used only the pads with the numbers: 3, 4, 5, 6.
The focus correction is given for the vertical (V) and the horizontal (H) direction. Can be in plus (backfocus) or in minus (frontfocus).
By the EF 85 mm F1.8 You have this:
In this case the setting to + or – has given a modification of ca. 0,5% of the distance to the object.