How do (did) you focus slow zooms on an MF SLR?

[Sarcophilus Harrisii]

The split prism blacking out on slow zooms bothered me for a long time (and like others, I resorted to using the matte part of the screen to focus on the few zooms I had that weren't AF). But one day, quite by accident, I discovered that if I moved my eye around a little bit in the viewfinder, shazam, the blacked out split prism would go clear. It was a little tricky, but it happened. I also discovered that nearly every time I had already pre focused the lens using the matte part of the screen, the split prism showed that I had missed the focus. Maybe not by enough to make any difference, but it was indeed a little off.

This is regarding film SLRs, which usually (or so I hear) have much better and brighter focus screens than DSLRs. The only DSLR I ever owned, briefly, was a Nikon D50, and it certainly had a darker and squintier viewfinder than my N8008s, which was like looking through a large picture window with lots of eye relief..

And let's not forget focus confirmation lights. Some of my Nikon film cameras had nifty little green lights in the corner of the viewfinders to tell you when you had correct focus.

It's worth being aware of the fact that whilst a split wedge rangefinder incorporated into a single lens reflex focus screen enables you to quickly and easily get your focus on your subject accurately, it may not always necessarily be the most reliable option for achieving precise focus. It's something I've experienced myself in the past.

A split RF can usually be perfectly accurate but it's good to be aware of the possibility of error. If focusing a particular lens with a screen incorporating a split gives different points of focus with the plain glass or microprism, the latter two are more likely to be correct than the split. This article in the March 1965 issue of Modern Photography by Herbert Keppler discusses the reasons why inaccuracies can occur when using a split. It's well worth being aware of the point if an SLR is being used to auto-collimate another camera. Where possible a plain ground glass screen should be used to assess the focus target at the film plane of the camera being adjusted. Using a split RF for the purpose may yield perfect adjustments but it's also possible to induce calibration errors: a screen without a split avoids this problem.
Cheers,
Brett

 

[sevo]

I also discovered that nearly every time I had already pre focused the lens using the matte part of the screen, the split prism showed that I had missed the focus.

The edges of that lens (which are what a split prism selects) miss the focus - a matter of spherical aberration...

 

[MaxElmar]

It's called (in Nikon speak) a Type B (or D) screen, Dante. It's the solution that was offered at the time (for interchangeable screen cameras) and there is no better solution now (well, live view in the digital realm). If you have an FM or FE, you're kind of limited to F3.5 or faster... like the 28-50/3.5 and the 50-135/3.5 zoom-Nikkors. Even then they darken the RF spot a bit...

 

[Sarcophilus Harrisii]

The edges of that lens (which are what a split prism selects) miss the focus - a matter of spherical aberration...

Indeed, the above nicely sums up the explanation discussed in the article I linked in my previous post.

Some additional discussion of the challenges involved in achieving accurate focus both with a rangefinder and a single lens reflex (including the advantages and limitations of split RF reflex focus screens with various focal lengths) may be found in this article by Bennett Sherman in the May 1965 issue of Modern Photography.
Cheers,
Brett