This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
594. It is not enough to be able to make good negatives and good prints. The print should have a proper setting, to heighten its effect. The undesirable portions should be trimmed away, and a tasteful mount will greatly improve the finished photograph. Thousands of good photographic prints have been ruined by an unsuitable choice of mount, while indifferent second rate pictures have been greatly improved by discriminate and artistic taste in mounting. Indeed, taken together they have become quite an art in themselves. The best photographers, like the best artists, treat the mount like the frame of a picture, worthy of the most careful thought and study.
Where To Trim. As the purpose of all trimming is to improve the print by emphasizing and balancing the principal point of interest, it is evident that trimming is largely a matter of composition. If the picture is properly composed the essential parts of the view will reasonably fill the print. It is only necessary then to have due regard for the decorative effect of the principal lines and masses in the pictures. But sometimes an isolated bit of dark or light comes on the edge. Useless and undesirable details obscure the print and should be trimmed off. It is the concentration of the view that makes the picture. Make it a rule to trim mercilessly - i. e., trim down until the best proportions have been obtained. A square inch of interest is better than a square yard of monotony. A part is often of more value than the whole. Do not hesitate to sacrifice the half or more of the print, if it benefits or accentuates what remains.
596. In seascapes, take care to have the horizons level. In buildings trim so that the vertical lines are perfectly square. One expects them to be perpendicular, unless the original was out of plumb, as sometimes happens in old buildings or ruins. When the lines are not straight, through neglect to use the rising front in the camera, or the swing-back, tilt the print a little when cutting to compensate for this. Two pieces of cardboard shaped like a carpenter's square, laid on the print and moved about so as to get the most pleasing picture, will be a great help in judging how the print should be trimmed. In all this work be sure that the edges are parallel and the corners are square, regardless of the original size of the plate.
How To Trim. It goes without saying that the print should be thoroughly dried before trimming, otherwise there is a certainty either of tearing it or of leaving the edges ragged. The knife may be a common pocket knife, but it should be of good quality steel, with a slightly rounded point; and last, but not least, it must be kept sharp. Some trim with a pair of scissors, but this method does not insure accuracy or straight edges. We therefore recommend the regulation trimming board, with a measuring scale and cutting blade that does this work with neatness and despatch. They can be bought at the price almost of a pair of scissors or of a knife.