Optical Print Centering


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.

You may click and drag the print with the mouse to manually adjust the vertical position.

mount – width:  
print – width:  
space inside mat:  
frame rabbet width:  
top mat width:    


draw | rotate     area ratio =   
  mat ratio =   

show advanced options
print position – from the top:  
from the sides:  
mat widths – top:  


Custom top mat width allows you to move the print up and down on the mount, should none of the other options look quite right.  Click the arrowheads beside the top mat width fields to increment the values.  The fields will have the same format (fractions vs. decimals) as the output values in the lower panel.  The mat ratio, top mat width / (top mat width + bottom mat width), is also displayed; I tend to like a value of about 0.45.

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, 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.

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  15/321/8 -  is  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).

Advanced options:

Preserve aspect ratio allows you to easily adjust the size of the print.

Use the calculate functions to create a mount for the print, or resize the print for the mount, with the displayed area ratio.  Mount size with mat ratio preserves the aspect ratio of the mount; without mat ratio, the print will be centered with equal mat widths all around.  Print size preserves all aspect ratios; without mat ratio, the print is vertically centered.  Mount size, equal top and sides is a special case of mount size; mat ratio is used but aspect ratio is not preserved.

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

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

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© 2015 by Russell Cottrell.  Updated 10/4/2015.