Weird focusing problem with a manual-focusing lens

I've got two Sony a850 cameras. No problems with them -- and I assume that this problem is not with them, but who knows. I get the same results with both using manual focusing lenses.

If I manually set the lenses to infinity, the photos come out fine -- so I know the lenses are working fine -- but if I use the viewfinder to focus on infinity, the correct focusing turns out to be much closer than infinity. This is true even if I use a 2X magnifier.

The viewfinder diopter is set for my eyesight, and even the magnifier (Minolta Vn 2X) is adjusted correctly. The image will appear correctly focused in the viewfinder, but the distance scale on the lens is focused closer than infitiy, and the resulting image is always off.

If I set the lens distance to infinity to get the photo correct, it's not sharp in the viewfinder -- even with correct diopter adjustment on two cameras.

Any ideas?

Here's what I think:

- 2 a850s that behave identically, suggests they are not the problem ie it's not the lens, or the lens to viewfinder path, mirror adjustment, etc

- the a850 is an SLR so what you see through the eyepiece is the viewing screen.

- so we can split the optical path subject -> eye into two independent parts

- the first part runs subject -> mirror -> viewing screen and is probably OK as you have 2 cameras (presumably without some common history of adjustments etc)

- The viewing screen is the "subject" for the second part of the optical path running: viewfinder- dioptre - your eye.

- dioptres should be adjusting your view of the viewing screen, so they oughtn't affect the lens focus on the subject

- so the conclusion from this would be that the problem is most likely because you have an inadvertent or particular technique for manual focus that is leading you awry, possibly exacerbated by the focusing screen not being ideal - eg designed for manual focusing.

I see that the A850 has a few different focusing screens (this from the web):

• Type G - The default screen, a good trade-off between brightness and focus discrimination
  
  
• Type M - A "Super Spherical Acute Matte" screen. This screen provides much sharper discrimination between in-focus and out-of-focus conditions, but at the cost of some brightness. It's only intended for use with lenses having maximum apertures of f/2.8 or greater.
  
  
• Type L - This screen is the same brightness as the default Type G type, but provides a grid to help with horizontal/vertical alignment or rule-of-thirds composition.

Doesn't sound as though any have particular aids to manual focusing unfortunately.

Of course, others may have a more informed analysis!