Optical Print Centering
(SVG version; updated 12/18/2022)

First, enter mount and print dimensions, and press Enter (or click draw).  Select lock print size for a work that cannot be resized.  Select a mode (optically centered, mat ratio, or equal top and sides).  Then you may:

• left-click and drag on the display to move the print vertically;
• right-click and drag to resize the print;
• hold down the Ctrl key and do the same to resize the mount;


• click the arrowheads beside mat ratio, or enter a value and press Enter, to move the print vertically;


• press Alt + Arrow Up or Down to move the print vertically, or Alt + Arrow Left or Right to resize it.

mount – width:     by
height:     rotate
print – width:     draw
mat thickness:  
space inside mat:  
extra space at bottom:  
frame rabbet width:  
area ratio:     2 ≈ 2.62)
mat ratio:     (ideal ≈ 0.44)



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


save as text

The following guides appear when resizing or moving the print or mount:

mat window follows this guide for equal top and sides golden ratio lines for the print itself (area ratio) mat window follows this guide for optical centering green zone for mat ratio 0.4 - 0.5 excess space at the bottom when mat ratio < 0.5

The area at the top or bottom turns red when the top or bottom mat width is less than that of the sides.
Excess space at the top appears in red when the print is below center.

When resizing, the print’s vertical position is determined by the selected mode (optically centered, mat ratio, or equal top and sides); when moving the print vertically, the mode switches to mat ratio.

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; it also applies when resizing the print with the mouse.

1 √Φ Φ

The mat ratio, or the ratio of the top visible mat width to the sum of the top and bottom visible mat widths (0.5 when the print is vertically centered), is an alternative to optical centering.  An optimal value is maybe 0.44 (at which the bottom mat to top mat ratio is the square root of the golden ratio, or the ratio of the sides of the Kepler triangle, fwiw).

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 ≈ 1.62), 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.

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.

You may press R to rotate the display, G to show the guides, or O, M, or E to select optically centered, mat ratio, or equal top and sides.

The dimensions in the upper panel 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.

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).  16ths+ is my way of reading 32nds (half a 16th above the indicated value).

Sometimes optical centering results in the top mat width being narrower than that of the sides; equal top and sides sometimes results in the top mat width being narrower than that of the bottom.  In cases like these, you will probably want to use one of the other centering options, or make further adjustments.

For more information, please visit russellcottrell.com.

© 2022 by Russell Cottrell.  Updated 11/17/2022.