Optical Print Centering


mount – width:  
print – width:  
space inside mat:  
extra space at bottom:  
frame rabbet width:  
  area ratio =  
mat ratio =  

draw/save | rotate | restore



resize based on area ratio =    Φ2

and mount aspect ratio =      print


scale the display:  
print position – from the top:  
from the sides:  
mat widths – top:  


save as text


Optical centering is a method of postioning a print on a mount using a geometric formula to place it slightly above center.  This script calculates the print position using any of several formulas, and illustrates it graphically.

The units are arbitrary.  Fractions are optional.

Include the space between the mat and print edges if appropriate.  Extra space at the bottom is optional.

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.

Lock print aspect ratio allows you to easily adjust the size of the print by changing either of the dimensions.

Draw/save saves the entered dimensions, which you may later restore.

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.

The mat ratio, or the ratio of the top mat width to the sum of the top and bottom mat widths (0.5 when the print is vertically centered), is an alternative to optical centering.  I tend to like a value of about 0.45.  Custom top mat allows you to move the print up and down on the mount.  Click the arrowheads beside the fields to increment the values, or click and drag the print with the mouse.


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 (Φ ≈ 1.61803), 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 Φ2 ≈ 2.62.  The area ratio is displayed above when calculating the print and mat dimensions.  I have used the entire visible mount area, 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.

Note that scale influences the effect of the mount size.  The golden ratio works best for prints viewed from a distance significantly larger than the image diagonal.  Very large prints viewed close-up may need smaller area ratios and mat sizes.  The reverse is true for tiny prints on hand-sized mounts.

Use the resize functions to resize the print for the mount, or to create a mount for the print, based on the area ratio.  When resizing the print, its aspect ratio is always preserved.  You may adjust the mount’s aspect ratio by entering a number or clicking the arrowheads; or click “print” to use the print’s aspect ratio.  Equal all around resets the mount’s aspect ratio.

Click the arrowheads beside scale the display to adjust the preview size.

The centering and resizing functions in the upper panel (except for custom top mat) are based on the visible area inside the frame.  The print position and mat widths in the lower panel are relative to the mount itself.

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,  7/16 +  is close to  15/321/8 -  is close to  3/32.)

The inches to cm option is for those who prefer to use inches for the overall material dimensions (the upper panel), while at the same time measuring the mat widths and print position in centimeters (the lower panel).

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

For more information, please visit russellcottrell.com/photo

© 2017 by Russell Cottrell.  Updated 12/11/2017.