An aspect ratio is simply the width of an image divided by the height of the image. Aspect ratios work the same on a camera as they do on a TV or computer monitor. When you’re just taking pictures the aspect ratio isn’t that big of a deal. It’s later when you go to print your images, that you can run into problems.
Aspect ratios can be expressed as either an aspect like 4:3 or a decimal like 1.33. 4:3 and 1.33 both mean the same thing; the length of the image is 1.33 times longer than the height.
Your camera uses one of two different aspect ratios. Most point-and-shoot, four-thirds, and medium format cameras all use the same aspect ratio as a standard TV, the 4:3 aspect ratio. DSLRs, however, use a totally different ratio based on the size of traditional 35mm film. The 3:2 or 1.5 aspect ratio that DSLRs use, produces images slightly wider than most other cameras.
Different sizes of prints use different aspect ratios. The standard 4×6 print size uses the 3:2 aspect ratio, so images from a DSLR will fit perfectly and require no cropping. When you use a standard camera that uses the 4:3 aspect ratio, the image just doesn’t quite fit right. So, what do you do?
When you send your images to the photo lab, they will automatically crop the image to make it fit into the 4×6 print size. While it may not be a big deal in some of your photos, others may lose some important parts of the image. If you’re prepared for it ahead of time, you can compose the shot differently to account for the cropping. You can also crop the images yourself with software to make sure that the crop is done the way you want.
In general, cropping the image is the best solution, although there are other possibilities. You could fit the image inside of the 4×6 print, but the ends of the print would be left blank. You could also stretch the image so that it fits to the print size, but the image would get stretched and distorted in the process.
Below is a cheat sheet to help you easily determine the aspect ratio of most standard print sizes.
I shot the image below with a DSLR yesterday afternoon. The image is in the 3:2 aspect ratio. If I printed it as a 4×6 image it would fit perfectly, but what about other sizes? If you look at the image below, it shows how much of the image would get cropped if it was printed as a 5×7.
As you can see above, the sides of the image will get cropped slightly because a 5×7 has a smaller aspect ratio than the print. If I change my mind and want to print it as an 8×10 instead, what happens?
As you can see above, the 8×10 has an even smaller ratio and even more of the image gets cropped out than did in the 5×7 print.
What you should do
The first thing you should do is find out what aspect ratio your camera uses. Decide what your favorite print size is so that you know ahead of time how much your images will get cropped when you take them to get printed.
Probably the most important thing you can do when shooting is leave some room around the outside of the frame. You never know what size prints you may want to make in the future, so leave room to account for possible cropping.
If you know that part of your image is going to get cropped when its printed, apply the crop yourself to make sure that nothing important gets lost.