lens - Why do I see focus less accurately in my DSLR's viewfinder and screen with old lenses?

I've had an old completely analog Praktika SLR with a set of lenses for a few years now. The camera and lenses are still quite good but I wanted to go digital.

Having recently acquired a Nikon D5200, I was curious if it was possible to use the old lenses on my new body.

Of course I realized manual mode would be the only available mode, but I'm used to shooting manually anyway (the Praktika is from the '70s, so fully manual). I did find out that I would need an adapter with a correction lens, and I bought one with and one without the correcting lens.

It sort of works, except I've no way of knowing the lighting conditions beforehand (the camera doesn't know the aperture, so it can't estimate the lighting, apparently).

However there is one very big problem. When looking through the viewfinder, an item seems to be in focus. When I look in the LCD screen, it's very much out of focus (and not by a tiny margin, a very large margin). The actual photo matches the LCD screen.

I tried fiddling with the little wheel next to the viewfinder, but it's pretty much impossible to get it workable.

I tested the same situation with the kit lens, and it is perfectly in focus on both the viewfinder and the screen.

Extra note: The old lens I'm using is a 50mm (so, 75mm now, I guess) with an aperture of 2.8. No zoom.

Also, when using the adapter without the correction lens, the same applies but I have a much smaller viewing distance (almost macro distance) wich is the reason I also bought that adapter.

Does anyone have a clue what the problem is and if it's fixable ? It I can get it to work properly I'd suddenly have an enormous list of available high quality lenses for under 100 euros. So you can image why I'd like to make this work.

Answer

This isn't actually to do with the old lenses per se, but with the design of the viewfinder in modern DSLRs. In short, they're optimized for brightness, and assume automatic focusing. The downside is that they're not very accurate for manual focusing — not really showing differences in depth of field below what you get at f/2.8 or so, if even that. You'll see the same behavior with fast modern lenses.

There are third-party replacement viewfinder screens which are made for manual focusing. One of the most popular brands is KatzEye — I've heard nothing but good things about them (except for the occasional gripe about price, because they're not cheap). They have a model specifically meant for your Nikon D5200, and lots of others as well (including Canon, Pentax, and other brands too). These can be ordered in a variety of configurations, and usually include a split-prism focusing aid to help with manual focus.

Of course, there's an inherent problem that most consumer DSLRs have a viewfinder much smaller than that on even consumer-level SLRs (let alone higher end models). That's an artifact of the sensor size (along with price consciousness) and can't really be helped.

Some other questions and answers on this site provide more information: