Focus adjustment should be consistent between bodies, shouldn't it?

Vertigo · 14 Jun 2015 at 15:45

Firstly let me say that I'm aware of how focus microadjustment works and that you're not just "compensating for a poorly calibrated lens" but that there are tolerances in all lenses and bodies and you're just calibrating the combination of both.

That said, shouldn't the difference between two bodies always be consistent?

For example, I might calibrate a 100mm lens on one body and find it needs -3, then calibrate the same lens on another body and find it needs +1.

Surely, if I then proceed to calibrate other lenses on both bodies, the difference between the optimal settings should always be 4, so if a 50mm was +2 on the first body, I'd expect it to be +6 on the second.

Basically, in this scenario, the two bodies are +4 "out" in relation to each other.

That makes sense doesn't it? Basically I'm not seeing this - I've calibrated several lenses on two different bodies and, in some cases, the consistency just isn't there and the differences are all over the place. I can't figure out why and just wanted to check that there's no logic explanation for this and they should all differ by the same amount.

Phal · 14 Jun 2015 at 15:50

Are these the same model bodies? Different bodies could work differently etc so that could be an explanation.

If they're the same model bodys and still give you different results then I can't personally explain it but that's not to say there isn't a reason

JBuk · 14 Jun 2015 at 16:19

In an ideal case then you would expect the 50mm lens to be a difference of 4 but there's probably other tiny mechanical and physical variables between the mounts , such as how tight it locks , distortion on the connecting faces, probably only a difference of microns but enough to throw out the af across each set of lenses and bodies

Vertigo · 14 Jun 2015 at 16:50

Two different bodies. I just figured that the only tolerance at work with the body is the distance between the mount and the focal plane.

JBuk · 14 Jun 2015 at 17:00

Vertigo1 said:
Two different bodies. I just figured that the only tolerance at work with the body is the distance between the mount and the focal plane.

Yeah that's what I meant, the mounts on each body and each lens.

There will be slight discrepancies between each combination even though theoretically they should be the same differences

Vertigo · 14 Jun 2015 at 17:11

Think I need to start again in a controlled environment and test each lens on each body under exactly the same conditions.

a1ex2001 · 18 Jun 2015 at 08:33

Vertigo1 said:
Think I need to start again in a controlled environment and test each lens on each body under exactly the same conditions.

The question I always ask in these situations is are you having problems with real world images? If so test away in a controlled environment but don't get hung up on it get it good enough so your real world images are sharp and then stop. If your not having issues with your real world images then step away from the calibration kit you will waste hours and hours never be satisfied and probably end up making very little real difference to your photography.

D.P. · 18 Jun 2015 at 12:39

No, every body and every lens will have its own tolerance errors. You can't tune a lens so it works on different bodies, which is a big issue with sigma fancy USB Dock.

a1ex2001 · 18 Jun 2015 at 15:51

D.P. said:
No, every body and every lens will have its own tolerance errors. You can't tune a lens so it works on different bodies, which is a big issue with sigma fancy USB Dock.

I don't think that's what he's asking, what he is saying is that the two bodies should be consistently different ie the micro adjust for the two cameras will be different for every lens but different by the same amount.