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Constrained Cropping in Photoshop
Today is
Updated  Thursday, December 30, 1999
by Randy Glass - Copyright 1999 by the Feather River Canyon News - All rights reserved.
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Constrained Cropping in Photoshop
   One of the problems folks run into when enhancing digital images is when cropping. It's easy to crop an image- the hard part is cropping it and still maintaining the ratio of height to width as the original. Why does that matter? It doesn't, unless you are printing the images for inclusion in a photo album or for framing. In that case, you want the prints to match the desired output size.

   First you need to understand ratios. A ratio is a comparison of two numbers. In this case, we are comparing the height of an image to its width. They work like fractions, but we are goign to express them in decimals because that's how Photoshop works.

   Let's look at one specific example. Take a digital image that measures 1280x960 in pixels. To find the ratio, divide the smaller number by the larger number. So 1280/960 equals 1.33333... So we turn this around and say that the ratio of this image, comparing the Height (960) to the Width (1280) is 1:1.333 (read as "One to one point three three three"). That means that the width of this image is 1.333 times larger than the height.

   If you do the math, all the common digital image files are the same ratio:
1600x1200 = 1:1.333
1280x960 = 1:1.333
1024x768 = 1:1.333
800x600 = 1:1.333
640x460 = 1:1.333

   For photographic prints you will find that the ratios differ:
4x5" = 1:1.25
4.5x5" =1:1.111
4x6" = 1:1.5
4.5x6" = 1:1.333
5x6" = 1:1.2
5x7" = 1:1.4
8x10" = 1:1.25
11x17"= 1.5454

   Our problem is that the image file H:W ratio needs to match the output format H:W ratio.

   What do you do when the image ratio does not match the output ratio? Something has to give when the image is 1280x960 (input is 1:1.333) and the print is going to be 4.5" x 5" (output is 1:1.111). As you can see above, no amount of simple resizing of the image is going to make the smaller image represented by the blue area the same size as the larger image represented by the yellow area without some cropping or left-over space.

   The difficult part is cropping the image so that the final crop exactly matches the ratio of the desired output. Why is this important? In many printing applications, the image is resized until one dimension (height OR width) fills the desired size boundary. As you can see above, the blue 1280x960 image has been re-sized to now fit the 4.5x5" print size as closely as possible. If it were printed that size it would not properly fit in a frame or album- it would be smaller than the desired size or some protion of teh image would be cropped by the printing application.

   What to do? Photoshop has the ability to "Constrain" the cropping marquee. That is, the crop can be pre-set to any ratio the user desires so that it matches a desired output ratio. it is important to remember that we are dealing with a ratio of H:W (such as 1:1.25) and not an actual physical size (such as 4" x 5".

   Here's how to manage this. After selecting the rectangular marquee tool, click on the "Marquee Options" tab and select "Constrained Aspect Ratio." Then enter the correct numbers in the "Width" and "Height" windows. In the above case, we would want to set the crop marquee ratio to 1:1.111. Since the image is wider in the width, the larger number (1.111) has been entered in the width window. If this image was in portrait orientation (taller than it is wide) then the numbers would be reversed with 1.111 entered as the height and 1 entered for width.

   I have set the constrained cropping as discussed above, and as you can see above, the 1280x960 image has three cropping marquess superimposed. The image H:W ratio is 1:1.333 and the marquees are 1:1.111. The largest marquee represents the greatest amount of area of the original blue image that can be cropped and printed to fill a 4.5"x5" print. Of course, once the crop size has been established it can be moved about the image to select the best portion of the image to print.

   It is best to save the cropped image as a new file (with a new name- possibly in a folder named "Print" or some such thing). This way the original image has been left untouched and can be later used to print in other sizes.

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