In some shots where I shoot wide open (f/5.6 on a 135 lens) I've noticed that the plane of focus on the negative/scanned image is consistently in front of where it was when I set the focus on the ground glass. Here is an example:
http://farm4.static.flickr.com/3443/...a7b4a8b3_o.jpg
I'm fairly certain that when I composed and focused on the ground glass, the plane of focus encompassed the window frame and the mask on it. I wasn't really doing any adjustments if I remember correctly. When I scanned the shot, it certainly looks like the plane of focus is about 1-2 feet in front of where I wanted it to be. I have another shot where the subject was about 30 feet away and the plane of focus is a few feet in front of it.
What are my possible culprits and how do I find out for certain?
It's likely to be user error, but which kind? Bumping the camera forward? Loose rear standard? Or could it be something with my holders or the way I put film in, or a mechanical misalignment of some sort? Something with the lens?
Usually I stop down a little so this isn't much of a problem, but it's certainly annoying. If I have to do some rounds of lens chart tests and suchlike, I certainly will.
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