Focus problem with 645

[mistral]

I bought a 2nd 645 kit from EBay and was experiencing focus trouble, when I relied on sharpness of focus screen image.

No amount of diopter adjustment would remedy.

If I relied on auto focus it was better, but AF with this kit is not my preference as it's slow and drains battery to boot.
Also, when AF was locked in, the screen image was OOF.
I had to refocus longer to focus screen image, that resulted in back focus.
Every time.

RATS!
I thought the camera might have been dropped or sthng.
Side by side comparison of bodies revealed prev owner had put three thin brass shims between the body and focus screen support rails.
Can anyone tell me why?
Was Prev O shooting IR perhaps?

I've removed shims and we have focus at film plane when the image appears focussed on screen.
(Oh what a difference!)

So I post here, in case anyone cares to comment, or in case they have similar experience and can save some anguish, time.
(it's lonely tho)

 

[fredstock]

A reason for a Prev O to change the length of the light path from lens to screen (that is what the shims do) is that possibly he/she makes use of a (digital?) back which has the sensor not at film plane but slightly behind it. This requires a compensation for the view plane. Of course in that situation autofocus is not an option, except when manual adjustment is performed after autofocus.
Your guess about shooting IR is difficult to judge because it cracks my brain to consider what happens when shooting IR. It is known that for IR one should focus before the actual focal point, i.e. focus the lens at sharp view image and than turn the distance point on the lens rim at the red line. In order to prevent this adjustment the Prev O may have added the shims to be sure that at manual focus on the screen the IR correction is always incorporated. You can check that by fixing the camera on a tripod, focus on an object, remove the viewer and the screen, add the shims, screen and viewer, adjust the lens to the IR mark and check now whether the object is sharp or not on the screen. When the object is sharp keep the shims for future IR shoots!

 

[mistral]

Thanks Fredstock. One or other explanations works for me.
Prev O modified screen height to ensure correct focus for whichever was the case, Digi or IR. My removing the shims has restored the situation.