Camera accessories: split prism circle focusing screen

For anyone who has use an old-school film SLR camera, you will find the manual focus aid a familiar sight.
The idea is that when something is out of focus, the center circle will appear disjoint, and the closer the image comes to in focus, the closer each half (upper and lower) will come to meeting exactly. This is a wonderful aid when using manual focus, as it makes it much easier to tell when focus is achieved, and allows for a more accurate judgment of focus.

A friend at work today told me that you can replace the viewfinder (the eyepiece that you look through on the back of the camera) with an after-market eyepiece that includes this split prism circle functionality. There are several drawbacks though.



One is that the prism becomes more difficult to use as the lens is stopped down to smaller apertures. Basically as less light is allowed to travel through the lens, the prism has less light to work with, and thus becomes very sensitive to the positioning of the eye relative to the viewfinder. In short, if your eye isn't perfectly "square" with the viewfinder, then either half of the circle, or both, may appear completely black. This is less of a problem with modern automatic lenses, as they remain wide open until the picture is taken, so viewing while composing the shot will likely not be affected by this in most cases.

Another is that it can affect the automatic exposure metering of the camera. This is only a concern for lenses that have a smaller maximum aperture, those with a maximum aperture of greater than f/4 (i.e. potentially a problem for variable aperture lenses at the f/5.6 end, or when using a teleconverter on a lens, effectively decreasing the maximum aperture). The only solution to this problem is to take a test shot, look at the histogram, and correct the exposure if necessary before taking another shot. This may not be possible in all situations (i.e. the bird has left or the play is now over in a sports game or whatever).

Lastly, the camera displays the auto focus points through the viewfinder. This gives the shooter a way to know where the camera is looking to focus, and when the camera achieves focus, it will blink a red light in the auto focus point to indicate to the shooter that focus has been achieved and which AF point it was achieved through. The indicators for the auto focus points are not included with this new eye piece. They can be added for an extra $40, but even still they will no light up when focus is achieved. This (small) functionality is lost entirely.

The new screens cost $98 to begin with, then it is another $55 if you want the brighter version, and then another $40 for the AF point indicators, resulting in a $198 piece of glass about 1/2 an inch square. Seems spendy for something that losses some functionalities, interferes with others, and gains you a very usable manual focus feature.

I will have to think about that one for a while. While I would like the split screen focus functionality, I don't want to lose the AF point blink, although admittedly this is a small issue, and practically is not useful for anything. I also don't want to have my camera make incorrect meter readings. Right now I don't have a lens that this would be an issue, and I only intend on getting a single lens where this may become an issue, one day way down the road. I do intend on getting a teleconverter or two, and they will probably be used on that one lens, thus exacerbating the issue for that particular lens.

One company that makes this item is Katz Eye, and the page for the focusing screen for my camera can be found here.

Another company that makes them is Haoda Fu, one review, two review, three review, both reviewed, Fu order.