SLR Viewfinder Optics

by Peter Rowlands & Peter Scoones

Reproduced from in focus 52 (August 1994)

The old saying that "The Good Lord giveth and taketh away" was probably inspired by designers of SLR viewfinder optics, for, in trying to provide the best viewing image underwater, certain compromises, both optically and physically, have to be accepted.

The problem is caused by our eyes being further away from the viewfinder than would be the case on land. The thickness of the housing plus the distance from your mask to your eye increases the viewing distance by at least 1 " with a compact housing and a low volume mask. Use a larger housing and a bigger mask and the viewing distance could increase to up to T'. Underwater, the small amount of water between your mask and the camera will give a useful slight enlargement. So for absolute certainty you would be advised to try the viewfinder underwater to establish its true performance.

Some top of the range SLR camera such as the Nikon F, F2, F3, and F4, Canon FI and Pentax LX, have optional viewfinders, known as Action or Sports finders. These are much larger prisms which replace the standard viewfinder and allow the full frame to be viewed from up to T' away. This offers the best possible viewing underwater, but at a price (of both the camera and the viewfinder) and increased bulk (of both carnera system and housing).

Until the introduction of the Nikon F801, most standard SLR viewfinders in housings resulted in the photographer not being able to see the whole frame. The advantage of the F801 is that it has a "high eypoinC viewfinder prism, which allows the whole frame to be viewed from up to 19min away on land. This, when combined with a housing whose viewfinder port is touching the camera viewfinder, results in virtually full frame viewing of the fully magnified image. A quick roll of the eye or slight movement of the head will let you scan the whole image area including the viewfinder readout. The high-eyepoint design gives the largest practical viewing image and represents a design which provides optimum viewfinder performance.

"The good Lord " comes in where housing design results in increased viewing distance and the need for additional optical devices, which allow the full frame to be viewed. In providing full frame viewing, the apparent image in the viewfinder has to be placed further away from the eye. This results in a significant reduction in image size (not often pointed out by manufacturers of such optical devices) and a slight loss of light transmission. This loss of light increases as the image size increases, hence there is a trade off between size and brightness. The image size reduction will make manual focusing more difficult, but if you use autofcwus then of cousre focussing is no problem. The reduction in image size can also cause shortsighted people problems when trying to read the viewfinder information panel.

So there you have it. Action/Sports finders provide a no-compromise optical solution; high eye point viewfinders represent the most efficient design which, in well designed housings, offer the fullest image size but without the corners being visible; and viewfinder enhancers provide full frame viewing with the corners visible but with a reduction of image size.

The best way to choose the system you require is to look through each type and see which suits you best.