Optical Print Centering
The following script calculates the optically-centered print position and illustrates it graphically.
The units are arbitrary. Fractions are optional.
Include the space between the mat and print edges if appropriate. The preferred method is to center the mat window, but you may choose to center the print itself.
Rabbet width refers to the inner edge of the frame that overlaps the edge of the mount. In the illustration, the frame stock is 2x the width of the rabbet.
Sometimes optical centering results in the top margin being narrower than the sides. In that case, you will probably want to use one of the other centering options.
How big should the mount be? It is ultimately a matter of taste. I myself dislike mounts that seem overly large; there is something pretentious about them. But a good image deserves a little “personal space.” One option is to use the golden ratio ( ~ 1.6180), which dates back to Pythagoras and is considered an aesthetic ideal. If the ratio of the mount dimensions to the print dimensions is the golden ratio, then the ratio of the mount area to the print area is the golden ratio squared, or ~ 2.62. The area ratio is displayed above when calculating the print and mat dimensions. I have used the visible mount area including the space inside the mat, minus any excess space at the bottom of the mat, as this seems to determine the overall impression. A good range is maybe 2.3 – 2.9.
When using fractions, the rounding error may have undesirable effects such as making the vertical mat space visibly smaller than the horizontal. If the true value is closer to the 1/32nd position between two 1/16ths, the fraction is marked with a “+” (above the fraction) or “-” (below the fraction), so 1/32 accuracy may be used. (For example, 5 7/16 + is 5 15/32; 4 1/8 - is 4 3/32.)