Why the difference? Well, digital cameras were first envisioned as devices to produce images for display on computer monitors, which have a 4:3 aspect ratio (as do many television screens). So matching the monitor aspect ratio made sense.
Today, however, digital cameras offer resolutions high enough to produce excellent prints. But there's a hitch: When you print a digital photo at traditional print sizes - 4 x 6, 5 x 7, 8 x 10, and so on - you lose some of the original image area because of the aspect ratio difference. (If you've ever had your digital files output at a retail lab, you've probably already noticed this change.)
Figures 1 and 2 show the percentage of image area you can expect to lose when you print your picture at 4 x 6, 5 x 7, or 8 x 10 inches. The left image in Figure 1 shows you the digital original; the darkened areas in the three remaining examples represent the lost portion of each picture.
Figure 1: A digital image (left) loses a portion of its original self when printed as a 4-x-6-inch photo (right).
Figure 2: You also lose some picture area when the photo is printed at 5 x 7 inches (left) or 8 x 10 inches (right).
This issue isn't a digital-only phenomenon, by the way. Your print lab must similarly crop away part of your 35mm film photo when making a 5 x 7 or 8 x 10 print, too. The only print size where you don't lose part of your 35mm original is 4 x 6, which has the same aspect ratio as the film negative (3:2).
Whether you're shooting digital or film, the problem has an easy solution. Just remember to include a small margin of background when you compose your pictures, as in Figure 1. That way, you won't lose any vital picture elements if you decide to print your photos at the traditional sizes.
If you're printing your own pictures, you can alternatively choose to reduce the size of your images so that they fit entirely within the 4-x-6-inch, 5-x-7-inch, or 8-x-10-inch boundary. You'll have a blank border along two sides of the photo. For this image, the patterned areas along the left and right sides of the picture represent the percentage of border needed to fill out an 8-x-10-inch print.
http://tech.yahoo.com/gd/accounting-for-aspect-ratio-in-digital-photography/2526;_ylt=AhR0KDiQAnioLvEJxs5gub8vLZA5
