Have you ever had prints back from your photos with areas cropped and wondered why? Nine times out of ten, this is down to there being a difference between the aspect ratio of your file and the desired print size.
At it's simplest, aspect ratio refers to the shape of your image, and how the width corresponds to the height. A rectangular print such as 12x8" and square 10x10" have different aspect ratios.
You'll find that most modern cameras shoot at 3:2 aspect ratio which is how 35mm film was shot, so when you print your standard sizes will be 6x4, 12x8, 18x12, etc . This isn't always the case though as compact system cameras such as the recent Micro 4/3 cameras shoot by default at 4:3 so check in the settings what the aspect ratio of your image is.
The original image here is shot in 3:2 aspect ratio. You can see from the thumbnails below the effect that different print sizes have on the crop of an image.
To ensure your image will print comfortably to most sizes always try and leave plenty of room around the edge when framing your picture in camera. With the high resolution produced by modern digital cameras you should be able to adjust the crop afterwards without a noticeable loss in quality. Also, avoid cropping your image in editing software if you're unsure about print sizes as cropping general reduces the quality of the image as you're removing data. If you do need to crop, always keep an original, unedited copy of the file.
Common aspect ratio's and print sizes
3:2 - 6x4, 9x6, 12x8, 18x12, 24x16, 30x20, 36x24
4:3 - 8x6, 12x9
5:4 - 5x4, 10x8, 20x16
5:7 - 5x7....
1:1 (square) - 5x5, 10x10, 12x12, e.t.c
