Wednesday, May 14, 2008

SLR Focusing Screens and Split Prisms

A couple of years ago, I upgraded from a 35mm film SLR (Nikon FM) to a DSLR with an APS-C sized sensor. It has been a good experience; I've been very happy with with output from the camera, and recently, I sold my film camera equipment.

One thing that does bug me about my new DSLR (a Nikon D70s) is the viewfinder. You can read all about it on the Internet. Everyone complains that the D70 viewfinder is small, dim, and "the worst viewfinder Nikon ever made". I'm ok with that part of it. I have decent vision, and I can see into the viewfinder with my glasses on... The problem I do have is that the focusing screen shows everything in focus all the time, even if it's actually out of focus!

This is most noticeable with a fast lens like an f1.4, but did notice something funny even with an f2.8. I thought I was going crazy, but when I looked through the D70s viewfinder, I had none of the defocus I saw in my FM's viewfinder. Then I learned that some focus screens make a compromise between brightness and the ability to manually focus (which means you can see when it's in focus and when it's out of focus). It looks like the D70s focus screen chooses brightness rather than viewing accuracy. You can confirm this by opening up lens aperture, looking in the viewfinder, taking a shot, and comparing the viewfinder image with the photo. You'll see a big difference in the defocus. I don't believe this is specific to the D70s, I think many new cameras make this trade-off, but I think it's wrong.

I think that half the point of using an SLR is to actually SEE what's going to hit the film. If the focus screen is just going to show me some distorted, version of it, then why wouldn't I just use a rangefinder? Smaller, lighter, quieter, less shake... Plus, I think it does a disservice to beginners who don't know how to use defocus. They won't know what they're missing, because they'll never see it in the viewfinder.

Anyway, now that I know what's going on, I'm going to change the focus screen to one that comes from a manual focusing film body. It has the added bonus of manual focusing aids which I also like.

In my investigation of this problem, I stumbled upon this article
Principle of the Split Image Focusing Aid and the Phase Comparison Autofocus Detector in Single Lens Reflex Cameras, written by Douglas Kerr about split prism focusing aids and AF techniques. It's pretty interesting and explains how these focusing techniques work.