Consistent backfocus with an SLR

[mfogiel]

I have been trying lately, to shoot some of my lenses at or close to wide open on my Nikon FM3A bodies, and to my annoyance, I've noticed that most of the shots had a visible back focus, even at a distance of several meters. I decided to test this at home - I took all my F mount lenses from 35mm onwards, and shot them against the focus test chart. I focused to the best of my ability, using the ground glass rather than the split image, and I always had my glasses on, just like I do with the rangefinders.
The results were quite astonishing: to a smaller or bigger degree ALL lenses exhibited a consistent back focus already wide open, say ranging from 1 to 5 cm at the 1m mark.
I wonder, whether this has to do with the default Nikon diopter correction interfering with my glasses, or is it the problem of the bodies - but I've tested 2 bodies and lenses both from Nikon and Zeiss, and the results were identical.
Any thoughts?

 

[Lflex]

The error would be with the mirror or matte screen placement, not the diopter adjustment. The screen is user replaceable. Check if it is installed correctly.

 

[yossarian123]

Yes, +1 on the mirror. I had an FM where one corner of the mirror was visibly raised (~1mm or so) higher than the others. That was enough to cause back focus with all of my lenses. I sent it to a tech for a CLA and they fixed it up perfectly.

 

[besk]

As was pointed out to me long ago when discussing rangefinder focus adjustments that SLRs are also able to get out of adjustment concerning agreement between the focus screen and the film. It doesn't seem to happen so frequently but is worth watching.

 

[Nikon Bob]

There may even be an adjustment screw in the mirror box to change the position of the mirror. That said, it is strange that you have had the same problem with two bodies. Has your prescription for your eye glasses changed recently? Nikon supplies cameras like the FM3a with a -1 eye piece diopter and is for normal vision to give a clear image a 1 meter.
I guess it depends on if your eye glass correct is for normal vision at one meter so that the factory supplied eye piece diopter works properly for you.

Bob