Does an offset split-focus screen exist?

Question

I shoot a Nikon F3 SLR with a split-focus screen, and I do a lot of portraits with very shallow depth of fields (e.g. 50mm f2.8 at 0.5m).

When I shoot a portrait I focus on the eye first, then recompose so the head is near the top of the frame. When I do this, I guess the slight movement of the camera as I've tilted it down has thrown the focus off and I often see hairs 4/5cm behind the eyes in focus instead of the eyes.

With digital cameras this is a non-issue as you can adjust the focus point. But can you do the same with an SLR?

Does a split-focus screen exist where the split is two thirds up the screen (when shooting portrait orientation)? If not, could I make one by cutting up a standard focus screen and moving it across?

Answer 1

When I do this, I guess the slight movement of the camera as I've tilted it down has thrown the focus off and I often see hairs 4/5cm behind the eyes in focus instead of the eyes.

If you're careful to keep the camera in the same place, then the distance from the camera to the subject isn't changing. What is changing is the location of the subject in the frame. Most lenses are designed to minimize field curvature -- that is, they designed so that if you focus on a flat surface, as much of that surface as possible remains in focus. When you pivot the camera to recompose, the radial distance to the subject stays the same, but the subject moves out of the relatively flat field of focus.

One way to fix the problem is to change the way you move the camera. This is what Horitsu's comment that you should try not to tilt and instead try to shift, so distance keeps the same means -- if you shift the camera in the same plane rather than changing the angle, it's more likely that the subject will remain in the "plane" of focus.

Another way to handle it is to find a lens that has a more spherical field of focus, and then continue to recompose by pivoting. The Wikipedia link above tells us that wide angle lenses generally have more field curvature, and telephoto lenses have relatively little. 50mm is right between those extremes, so it may be that a 50mm lens with the truly spherical field that you'd need just doesn't exist.

Does a split-focus screen exist where the split is two thirds up the screen (when shooting portrait orientation)?

I've never seen a screen with multiple split prisms, or with the split prism anywhere other than the center. I think the best you can do here is to get one of the ones with a microprism ring so that the distance you need to move the camera to reframe after checking focus is as small as possible.

Answer 2

The basic factors of the problem:

• Although modern focus systems allow you to choose from several points to tell the camera what spot or area you want the camera to focus on, this has not always been the case. Early AF systems only had one or sometimes three points. Before then, manual only focus systems usually provided the greatest focusing detail only in the center of the frame.
• When shooting a subject not centered in the frame, a photographer would often center the subject behind the focusing prisms in the middle of the viewfinder, focus the subject, and then recompose to shoot the picture. The danger with focusing and then recomposing is that the subject distance to the lens' entrance pupil may change slightly and result in slightly missed focusing, especially when using wide apertures that result in narrow depth of field.

Knowing which way and how much to move the focus ring to compensate in such a situation was a valuable skill gained by practice and experience. So was knowing to rotate the camera around its own optical center, rather than to rotate around the center of the photographer's body.

Does a split-focus screen exist where the split is two thirds up the screen (when shooting portrait orientation)? If not, could I make one by cutting up a standard focus screen and moving it across?

I've never seen or heard of such a thing. But it is very hard to say something has never existed.

Accurate focus the way we expect it today was not seen to be as critical to the same degree we expect it today back in the film era for a variety of reasons:

• Most photos from 35mm cameras were displayed (printed) at 4x6 inches or smaller. The largest 'standard' size that most depth of field charts assumed was 8x10 inches. In contrast, when we pixel peep a 24MP image on a 24" HD monitor, we are zoomed in on the equivalent of a piece of a 60x40 inch print!
• The size of a film's chemical grains, which are a factor in determining the film's "speed," limited 135 film's resolution to about the equivalent of a 20-24MP digital sensor for 100ASA (100 ISO) film. Yes, you can use a drum scanner to get much larger files from 135 negatives, but all you are getting is higher resolution views of the grains in the film's emulsion, not more detail of the virtual image projected onto the film.
• Roll film is not near as close to being perfectly flat as digital sensors are. This is particularly true of 135 film rolled onto a thin spool in a small canister for months or even years before being unrolled just before exposure. Anyone who has ever tried to focus a projector on a flexible screen waving in the breeze understands how slight differences in the distance of various areas can affect focus. There were a few advanced cameras that actually used a type of vacuum to pull the film flatter against the back plate.
• Focus with color film was also limited by the varying depth of the three color layers in film. If you were perfectly focused for one color, the other two layers were ever so slightly out of focus.

Digital sensors, in contrast, are so near perfectly flat that now we have to coat the back surfaces of lens elements to prevent unwanted reflections from bouncing off the layers of the sensor stack. The theoretical limits of best focus are now much smaller. Even with newer and much sharper lenses available today the limiting factor in the highest resolution systems is fast becoming the resolving power of the lens rather than the resolving power and flatness of the recording medium.

Answer 3

High end SLR cameras featured an interchange focusing screen. I acquired several for my Nikon F. The most popular featured a centrally located “cross wedges” which were molded into the surface. I never saw or heard of wedges placed in any other location. As you know, these wedges produced a split offset appearance that comingled when focus was achieved. This method is focal length dependent, it does not work well if a long lens is mounted. For this reason and as an additional focusing aid, the cross wedges are surrounded by a ring of microprisms. These form a shimmering grid pattern that becomes uniform when focus is achieved. Surrounding this is another ring of ordinary ground glass. This area also aids in the focusing task. All this “stuff” is concentrated in the center of the screen. The remaining boundaries are not as suitable because superimposed is a Fresnel Len (concentric circles). Its job is to reduce the naturally occurring vignette that plagues the SLR induced by the reflex mirror and the roof pentaprism. All are needed to present to the eye an upright and right-reading image.

I think, if you look again at the various aids (areas) of the focusing screen, you will find that most all situations are well handled.