I have a Nikon D3100, and I am borrowing an old Nikkor 50mm SLR lens (http://www.destoutz.ch/lens_50mm_f1.8_1828564.html). I've had a lot of trouble with that lens, in that pictures that appear in-focus through the viewfinder are actually out-of-focus. Several practice shots (with the lens wide open or stopped way up) were ruined because the depth of field was off by a few feet -- though, in the viewfinder, it looked fine.
In researching this, I've uncovered a lot of interesting information about DoF-preview in DSLRs (which my D3100 doesn't appear to have) and how cameras will often keep the lens wide open to aid AF and to keep the viewfinder/liveview bright.
Live view on this camera seems to depict DoF accurately, and it doesn't even attempt to compensate for stopping down. However, it appears the viewfinder previews the shot at a fixed f-stop, regardless of whatever the lens is set to. I tested this by focusing on a subject in the viewfinder and turning the aperture ring; the image did not change at all. Both the bokeh and overall brightness were identical (if I had to guess, DoF looked to be at about F4-F5).
I can understand how "aperture-compensation" (I don't know the technical term) might work in a AF lens, but this ancient lens can only be controlled by me, not the camera. A DoF-preview should do nothing, were it available -- yet, somehow, the viewfinder in my humble little camera is magically crispifying the image.
How can this be? I want to accurately manual-focus through the viewfinder (the resolution on the LCD sucks)!
EDIT: In response to comments, I identified the lens. Everything matches except the focus range; on the site it says infinite but I can't get it to focus clearly in the viewfinder past 30 feet or so