Aspect Ratios for ... Ventriloquists
By Mike Pasini, EditorImaging Resource Newsletter

No yellow Dummies book for this audience. We respect your intelligence. You
are, after all, reading what we wrote.
But if you're as smart as we think you are, you may have been confused by all this talk about aspect ratios at your friendly online photofinisher. We can clear that up right here.
An aspect ratio is just an awkward way of describing the shape of your image. Rather than use colorful imagery (as we do in discussing the "bouquet" of wines or the "temperature" of color), we use, uh, numbers. Like 1:1.3. Ugh.
We currently sit on the ANSI committee to standardize descriptive terms for the shapes of digital images. And at the moment, we are very deep in boxes of all kinds looking for just the right shapes. Because, believe it or not, not all prints are the same shape.
They're rectangles, yes, but a 4x6 is quite a different shape than an 8x10.
Rather than divulge our colorful language for describing rectangular shapes, we'll offer simple, mathematical proof that print sizes are different shapes. Just for fun, call the short side 1.
How many short sides are in the long side of a 4x6?
That would be one and a half (it takes one 4 with 2 left over, which is half of 4, to make 6). To make this easy, just divide the long side (6) by the short side (4) to get 1.5. The mathematical description of the shape of this 4x6 print is, therefore, 1 to 1.5 or 1:1.5. That's its aspect ratio, in fact.
How about the 8x10? The short side is the 8, so 1 is 8 inches here. How many short sides in 10? Well, it takes one 8 plus 2, which is a quarter of 8, to make 10. Or (more quickly) 10 divided by 8. So it's 1:1.25.
You can see by this what a square would be. The short side is ... well, the four sides of a square are all the same length. By definition. So a square's aspect ratio is 1:1.
Those are all not just different ratios but different shapes. The 8x10, for example, is closer to a square than the 4x6.

"Fine, fine, fine, what's this got to do with my 480 x 640pixel image,"
you ventriloquists ask, throwing your voice where we can't find it. Well, what
works for inches works for pixels. 480 is the short side. There is one 480 in
640 and 160 left over (which is a third of 480), so it's 1:1.33.
Which is not a perfect fit for either a 4x6 (1:1.5) or an 8x10 (1:1.25). Something has to go. Either you fit the short side or the long side but you can't match both.
Ofoto uses a "zoomandtrim technique similar to what a traditional photo lab would use, which automatically adjusts the image's dimensions to fit the desired print size." Basically that means they print the image large enough so that it spills over all four sides and then they trim to size. Exactly 4x6, 5x7 or 8x10 (which conveniently slip right into inexpensive prebuilt frames).
So to minimize cropping on a 4x6 print, make sure you have a 1:1.5 aspect ratio (and expect a little trimming). If you want a 5x7 print, make sure you have a 1:1.4 aspect ratio. And if you want an 8x10 print, make sure you have a 1:1.25 aspect ratio.
We thought we just heard a little voice somewhere ask, "How do you make sure?" Ventriloquists can be a tough audience.
Select your Crop or Rectangular Marquee Tool and enable the Fixed Size option, entering the pixels or inches of your target print size. That restricts the tool to a crop of that size.
If you'd like to see at a glance the exact number of pixels your image requires to match the aspect ratio of the four most popular print sizes, use the following Javascript Aspect Ratio Calculator.
This article is reprinted from The Imaging Resource Digital Photography Newsletter,
Beginner's Flash Column, published Decmeber 1, 2000