The first thing to recognise here is that digital cameras take pictures at a different ratio to traditional film cameras. Digital cameras were orginally designed to capture images for computer monitors, and as you probably know, these have dimensions typically like 640 x 480 and 800 x 600. If you divide the height by the width, you will find that the ratio is 4:3.
 
A traditional camera takes pictures that fit perfectly into a 6x4" photo. If you again divide the height by the width, you find that this is a different ratio, 3:2. So whats the problem you ask? Nothing really, but you have a decision to make when you come to print your photos. The standard photo sizes, and frames were designed for film and the 3:2 ratio. See our article on cropping for a visual description of the issue.
 
The table below details which commonly available print sizes are best for your images. If you have a digital camera, most probably you will need to choose a print size from the first two columns.