Aspect ratio is the relationship of the width of an image to its height, of its horizontal to vertical dimensions.
Common aspect ratios are 3:2, 4:3, 5:5 and 16:9 (HDTV). The number preceding the colon represents the width of the image and the number after the colon represents the height.
For example, both a 4×6″ and 20×30″ print have an aspect ratio of 3:2. A 3×4″ print has an aspect ratio of 4:3. Images with an aspect ratio of 4:5 can be printed 4×5, 8×10, 16×20 or 24×30.”
Some digital cameras let you select more than one aspect ratio within the settings menu. The ratios match standard print sizes. For digital cameras with only one setting, you can crop images to the desired aspect ratio when editing.
All photo editing programs have tools for cropping at various aspect ratios. You can choose either a standard or custom aspect ratio.
Why setting aspect ratio is important
If a photo doesn’t have the the same aspect ratio as the paper it’s printed on, one of two things happen. Prints will have an odd sized border on some sides or the printer will arbitrarily crop. To be in control of how a photo is cropped, it’s best to crop it yourself before printing.
To maintain high visual quality when cropping images, shoot at the highest resolution and quality settings. Digital cameras with high megapixels produce images with areas that can be cropped more without a reduction in image quality. Proper cropping is an important step when preparing images for printing.
Whenever possible, compose shots so that they require a minimum of amount cropping.