by Rand and Litschel.
Here is my equation that determines the position mathematically, and a graphic illustration of the results.Optical Print Centering
Pierre Renault in rec.photo.darkroom posted a very interesting method of optically centering a print on a mount.
The original source seems to be
Black & White Photography
"Pierre Renault" wrote in messageThe optically-centered print is slightly above center. This may be more visually pleasing, because of an illusion in which a vertically-centered print seems to “sink” a little on the mount.
The following script calculates the 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.
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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 ( = the white area in the illustration at right), 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.)
| 1/16 | 1/8 | 3/16 | 1/4 | 5/16 | 3/8 | 7/16 | 1/2 | 9/16 | 5/8 | 11/16 | 3/4 | 13/16 | 7/8 | 15/16 |
| .0625 | .125 | .1875 | .25 | .3125 | .375 | .4375 | .5 | .5625 | .625 | .6875 | .75 | .8125 | .875 | .9375 |
Downloads
Run this document as a smart phone app! Specific instructions on how to do it on Android here. 
Photoshop script
Some members of the Photoshop community have tried the optical centering concept, by expanding the canvas then centering the original image on a virtual mount. See Photoshop User TV: Episode 253 for a demonstration. Here is a photoshop script, Optical_Centering.jsx, that should do the job. It should be self-explanatory. If you enter a rabbet width, the hidden edge of the mount is colored black. If you enter a mat space, the mat is colored gray so you can see the space. You can recolor or uncolor it as needed.
Tested in CS2 and CS5. You may need to flatten the image first. Settings are saved to an .ini file in your default user folder. Updated 2/1/2011.
Just to point out, if you are wanting to print an image optically-centered on a mount-sized piece of paper, you might try calculating the print position, then using it as the top margin in the print dialog.