Understanding Aspect Ratio
This is just a light dip into the most important aspects of aspect ratio (pun unintended) as it relates to photos on this website. For a more in-depth essay on the general topic, look up the term in Wikipedia - it can make your eyes glaze over.
Aspect Ratio defined
To start, aspect ratio refers to the relationship between one side of an image to the other side. We can express that whole number relationship, for example, as a 2x3 ratio or a 1x4 ratio. On the other hand, the size of an image is the actual measurement of the length of one side of an image and the length of its other side, such as 4" x 6" or 5" x 20".
What's useful about this is the relationship between aspect ratio and size. Basically, you can examine the size of a frame or the inside dimension of a mat, for instance 12"x16", and divide each length by the same number that will still produce whole numbers to reveal its ratio. In this case, dividing 12x16 inches (or centimeters, feet, etc.) by 4 will reveal that has a 2x3 aspect ratio.
This is very handy if you already know the aspect ratio of an image and wonder which frame or mat sizes will accommodate it. I've provided a page which already lists common aspect ratios and the photo print paper sizes that will exactly fit that ratio (so you can avoid doing the math over and over).
This relationship can also work the other way. Suppose you know that you want an 8" x 12" print; you either do the math (dividing each side by the same number - in this case 4) or go to the same page link, to see that print paper of that size will exactly accommodate any photo with a 2x3 aspect ratio.
What about other aspect ratios?
Here's a common problem: What if you want to print a photo at a size that matches many standard frame sizes, such as a 5" x 7" (5x7 ratio) or 8" x 10" or 16" x 20" (which have the same 4x5 aspect ratio)? Most photographs from digital cameras that emulate 35mm film produce photos in the 2x3 aspect ratio (2x3 is a very common aspect ratio for photos on this site and elsewhere).
The answer to the question posed above is that you will have to either crop the photo to the desired size or print the photo on slightly larger paper (which will produce white bands on the top and bottom or on the sides) and then trim it to fit the desired size. Cropping is usually the most straight forward way to proceed unless that will remove elements of the photo that you want to keep. Both of these approaches are supported by this website and are accomplished during the print purchase process. Take a look at how this works by viewing this SmugMug provided video or by pretending to buy a photo and trying it out (you can cancel out of that process without committing to a purchase).