Focusing issues with DSLR ... manual focus not accurate compared to AF.

[Keith]

I spent some time the other day doing micro AF adjustments for my three lenses on my Sigma SD1M ... you can fine tune the AF for each lens and the camera remembers that individual lens setting of course. They were all front focusing slightly but are now extremely accurate with AF. However this is not the case when focusing manually ... what I'm seeing in the viewfinder is obviously not what the sensor is seeing because when I review the manually focused images I have taken on my computer, they are showing perceptible front focus. This has to be a problem with the camera's focusing screen I gather and some pretty average quality control on Sigma's part. I wouldn't care quite so much if the camera's AF didn't tend to hunt a bit in less than ideal conditions when manual focusing can be preferable. I gather this problem is directly related to the height of the focusing screen being slightly out so probably not a lot that can be done?

 

[Zuiko-logist]

Yes it could be the shim for the focusing screen or may also be a misaligned mirror. Both are usually adjustable or a new shim/shim material might work, not sure how the Sigma is set up. From a QC point of view, I would guess the camera would/should have been set up to be within tolerance. How far out are AF and manual focus?

 

[Keith]

Yes it could be the shim for the focusing screen or may also be a misaligned mirror. Both are usually adjustable or a new shim/shim material might work, not sure how the Sigma is set up. From a QC point of view, I would guess the camera would/should have been set up to be within tolerance. How far out are AF and manual focus?

At a distance of just over a metre there is probably a couple of centimetres difference between AF and what appears to be correct manual focus. The problem is of course that after checking Dr Google correcting back focus is done by shimming the screen away from the prism ... for front focus what the hell are you supposed to do? I may just have to suffer it! 😶

I'll add that nothing has ever been disturbed in this camera ... I bought it new and it probably only has a couple of thousand shutter actuations on it at most!

 

[Zuiko-logist]

there may already be a shim in there. You would need a thinner one. I remember one focusing screen that came with a selection of shims (can’t remember which camera brand).

 

[Keith]

there may already be a shim in there. You would need a thinner one. I remember one focusing screen that came with a selection of shims (can’t remember which camera brand).

I'm hoping that if I do decide to delve into it!

 

[Zuiko-logist]

I wonder what Sigma thought was “within tolerance”.