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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

Tom Corelis
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The aperture ring is a red herring here. The normal state when looking through the viewfinder is wide open — the aperture is only stopped down to the selected setting at the time of the exposure, via a mechanical coupling. The normally-open view gives less depth of field and therefore better shows the plane of focus, making it easier to focus correctly — at least in theory.

The problem is the viewfinder screen. On modern DSLRs (and recent SLRs, for that matter), the screen is designed to be as bright as possible at the expense of focus precision. Usually, you can't really accurately judge focus at f/4 or below, as you observed. So, you have to trust the AF system, which can often indicate focus even in manual-focus mode.

(A DOF preview feature, by the way, works by activating and holding the aperture stop-down coupling without tripping the shutter, so you can see the effect of a smaller aperture. For new users, the immediate obvious effect is that everything gets dark; the increased DOF is more subtle.)

One possible solution is to replace the viewfinder screen with one designed for manual focus, possibly with a spilt-prism focus aid. Katz Eye is one popular brand, and there are a number of cheap Chinese import options as well.

mattdm
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  • How is it possible that the viewfinder is capturing light at a different aperture setting than what the lens is currently set to? There's no CPU to dynamically adjust the aperture. Your answer makes sense to me assuming a modern lens, but this is an old lens. – Tom Corelis Sep 11 '12 at 21:33
  • It's not. The aperture is wide open and stopped down to the selected setting at the time of the exposure, via a mechanical coupling. (This is what a DOF preview feature does — it activates and holds that coupling without tripping the shutter, so you can see the effect of a smaller aperture.) – mattdm Sep 11 '12 at 22:28
  • Thank you. If you add that to the base answer I will mark it as the answer :-) – Tom Corelis Sep 11 '12 at 23:06
  • @mattdm is right. More, the focus screens on camera bodies designed for manual focusing did a lot to make the focus easy. They would often have two or even three types of focusing aids. For example, my Nikon F had a ground glass screen for the corners, a micro prism around the center and then a split prism in the very center. You could focus using any part, but it was much easier to get the focus spot on with the split prism. Less easy was the micro-prism.

    My current Canon 50D's screen is terrible for manual focus. I've had a new screen on my wish list for years.

     – Pat Farrell Sep 14 '12 at 03:29
  • @TomCorelis "How is it possible that the viewfinder is capturing light at a different aperture setting than what the lens is currently set to?" The microscopic textured shape of the surface of the focus screen prevents the edge rays allowed through the wide open aperture (of lenses with an aperture larger that about f/2-f/28) from passing through the focusing screen. The same texture prevents lenses with maximum apertures narrower than around f/2.8 from being as dark as they would be with a flat piece of ground glass. – Michael C Feb 16 '16 at 04:21