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Anonymous
I have noticed this type of issue as well.
Try this: get an M42 lens with a distance scale (preferably a macro or a telephoto, since their scales are more accurate), then focus on something at a known distance. Compare the distance to what it says on the lens. Use the optical center of the lens (guess halfway down the barrel if you don't know) rather than the film plane.
If the distance scale on the lens is pretty close to accurate, then something about your focusing screen is out of alignment. If it's not, and the distance scale says something other than what you've measured, then your adapter ring is wrong. Not sure which way, though.
Another test: focus on something at infinity. Check the scale. Are you currently reading something like 30 feet or so? And if you set the lens to infinity, you overshoot? This means that your adapter is too thin (a common way of making sure the lens can focus at infinity, at the expense of some close-focus ability). This millimeter or so that they shave off might have an effect on your focusing, especially wide open. I'm not an optical technician, so I don't know if it actually changes the optical path to the sensor to a different extent than the path to the ground glass (if they both change the same amount, then it shouldn't make a difference).
I have two M42 adapters. One works properly with the distance scale, and one does not. Look at the flange around the outside edge, where it mounts to the outermost part of the EF mount -- it should be fairly beefy looking, about as thick as a piece of matboard. I measured both of mine with vernier calipers...the thicker, accurate one is 1.42mm thick, while the thinner one is 0.91mm. That makes enough of a difference that the scale is significantly off on the thinner one.
Another thing to consider might just be that it's hard to get critical focus on the crap screens in the xxxD series of cameras. I know that I really, really miss the split-prism in my Spotmatic.
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