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 by Rand and Litschel.  Here is my equation that determines the print position mathematically, and a script that graphically illustrates the results.

From: Russell
Subject: Re: Automatic optical centering - WAS: Square Format Framing - 1 attachments
Newsgroups: rec.photo.darkroom
Date: 2000-12-20 23:57:23 PST

Wm = Width of mount
Hm = Height of mount
Wp = width of print
Hp = height of print

Distance between top of print and top of mount = (Wm + Wp) (Hm - Hp) / 4Wm


"Pierre Renault" wrote in message
news:9010E33FAcheungmomailandnewsc@ ...
> Here's how to automatically optically center any print on any board.
> Place the print to be mounted in the upper left-hand corner of the
> board.  If you are placing a print under a matte board, then place a
> piece of paper the same size as the desired cut-out.
> Bisect the vertical strip to the right of the print and draw a light
> line with a pencil from top to bottom.  Do the same thing for the strip
> below the print.  Draw a line from where the horizontal line crosses the
> vertical line you've drawn to the right edge of the board.
> Next, place a straightedge from the point on the right-hand edge of the
> board where the horizontal line ends to the bottom left-hand corner of
> the print.  Mark the spot where the line intersects the line bisects the
> vertical strip.
> Place the lower right-hand corner of the print at this intersection.
> This works first time, every time.
> Pierre

(Archived in Google Groups.)

The 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.  From Rand and Litschel:

An optical illusion occurs if a print is absolutely centered on a board.  This illusion makes the top border of the board appear larger than the bottom and gives too much visual weight to the top.  Optical centering is a technique to overcome this illusion.  It allows for more space at the bottom of the board, in a direct relation to the size of the print, and establishes a firm visual foundation for viewing the print.

From Lambrecht and Woodhouse, Way Beyond Monochrome 2e:  Advanced Techniques for Traditional Black & White Photography:

It is commonly agreed, and obvious even to the most untrained observer, that a print centered on the vertical axis appears to be too low on the mount.  This print placement creates an unfortunate optical illusion that the print is not equally spaced at top and bottom . . . .  In other words, the print seems to sag below the vertical center.

One accepted technique involves placing the print near the ‘optical center’ of the mount . . . .

See also The Photographer’s Eye:  Composition and Design for Better Digital Photos by Michael Freeman:

There is a natural tendency to apply our experience of gravity to images and image frames.  As we’ll see in Chapter 3, verticals express a gravitational pull downwards, while horizontal bases provide a supporting flatness.

More discussion at the largeformatphotography.info forum thread, Matting prints centered.


The script

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 ( = 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.  I have a print on my office wall that is scaled to exactly Φ2 on a 32 × 40 inch mount; the mat widths are almost 7 inches.  From close up, they look way too big; 5 ½ inches looks like it would have been plenty, consistent with an area ratio of about 2.  But from a distance, the impression changes and the wide mat starts to look more appropriate.  Conversely, tiny prints on hand-sized mounts may need larger area ratios.

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.

20x36.5 24x30

This page obviously started out as an illustration of the optical centering technique described at the beginning.  However, as Lambrecht and Woodhouse point out,

This technique is only a good starting point, and not an automatic substitute for accomplished design or personal preferences.  If placing the print at the optical center results in an unattractive, narrow border on top or bottom of the print, additional vertical adjustments have to be made.

Setting a peferred mat ratio may give more pleasing results, as well as providing a more consistent appearance from print to print.  The optical centering technique does not work well with this method.  For example, beginning with the default mount and print sizes for the script above, and using optical centering, resizing the mount (20 × 36.5) so that the area ratio is 2.62 and the mat ratio is 0.45 results in the tall, bandage-shaped configuration at right.  Resizing the image (18 × 13.35) rather than the mount results in an even more bizarre horizontal-on-vertical configuration.  The calculate functions above are all based on the area ratio with or without the mat ratio, and not optical centering.

Side note:  when optically centering, the mat ratio is a simple function of the widths of the mount and print, not the heights (mat ratio = (Wm + Wp) / 4Wm).  So to use optical centering with area and mat ratios, first adjust one of the widths to set the mat ratio, then adjust one of the heights to set the area ratio.

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



Text-only version for saving
Mobile version

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 fairly self-explanatory.  After entering a mount width, “optimize” calculates a height for which the optically-centered mat has equal top and sides.  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.  If you enter a rabbet width, the hidden edge of the mount is colored black.  Press “try” to display the dimensions before running the script.

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.

Optical_Centering ZekeBassoon2004Centered

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