How tightly can I crop my prints? Why is the top of a head being cut off my photo?

This FAQ is somewhat technical, but explains concepts of cropping, printing, and how close photo elements should be the edge of a print.
dotphoto users sometimes ask, "Why doesn't the print look exactly like my cropping? I cropped my image perfectly to the aspect ratio of the print."
In the image below, the original image is on the left and the print is on the right. Why is the head on the right slightly closer to the top?
Three reasons:
  1. Paper moves. Even in your inkjet printer, paper moves slightly, so cropping very tightly to the edge can produce unexpected, imprecise results. 
  2. When printing to the edge of paper, printers expose or spray beyond the cut. If this were not the case, you would frequently see white lines around the edges. Printers call the area beyond the cut "the bleed." Most printers recommend that the safe area for important graphic elements is .125" (1/8 inch) and for textual elements is .25" (1/4").
  3. Paper may be cut slightly short by automated, high-speed cutters.
Math further complicates the issue. 
The original image was cropped to 1407 x 1005 -- the perfect aspect ratio for a 5x7:  1005/1407 = 7/5 = 1.4
However, dotphoto (and most printers) prints at 300 dots per inch, so the image is blown up to 1500 x 2100 -- that is: (5" x 300) by (7" x 300). With overspray, that becomes 1550 x 2170. A smaller image like this one (1005 dots versus 1500 dots for 5") magnifies the portion of the image that will be over-sprayed when the image is blown up to print.
There are two ways to handle near-edge cropping:
  1. Leave an eighth of an inch around your entire image for bleed. Generally, try not to put important photo elements against an edge.
  2. If you want an absolutely true uncropped image, consider printing it uncropped on the next size paper: for instance, print a 5x7 on an 8x10 uncropped, but first increase the size of your 5x7 to 1500 x 2100 and place it on a canvas of 2400 x 3000. (You can accomplish this with the dotphoto editing tools.) Of course, if you print on the larger size, you will have white space all around your image, and you will need to cut the image yourself -- but you will have a perfectly cropped image.

dotPhoto FAQs: how to do almost anything at dotPhoto