Have you ever had an image printed only to find that some of it is missing? Cameras take pictures at a specific aspect ratio. Different camera models will have different aspect ratios. Aspect ratio is the ratio of the longest side, to the shortest side. So if your cameraís aspect ratio is 4:3 (4 to 3) that means that the longest side is 4 units wide and the shortest side is 3 units wide. The number before the colon refers to the width and the number after refers to the height. The numbers do not refer to a measurement, just the relationship between the width and height.
Most DSLR cameras have an aspect ratio of 4:3. If you wanted to print a photo from an image straight from your 4:3 aspect ratio camera on 6x4 inch photo paper, a portion of your image would be cropped, as the 4:3 aspect ratio is not right for the 6x4 photo paper. To get a perfect 6x4 photo with no cropping your image would need an aspect ratio of 3:2.
Aspect ratio can be a little confusing, so hopefully this will clear things up a little. Say you have an image that has a width of 300 in pixels and a height of 200. This image would have an aspect ratio of 3:2. If you had an image that was 900 pixels (width) by 600 pixels (height), that too would have an aspect ratio of 3:2. In regards to the aspect ratio, the size of your image is not important, as long as the ratio between the width and height are the same.
Now, if your image had a width and height of 300 pixels by 250 pixels (as oppose the previous example of 300x200) and you wanted to print this on photo paper with an aspect ratio of 3:2, 50 pixels of the image would be cropped off as the image would be too long on one side. Here I am talking about pixels, but you could use inches or centimetres.
Some cameras will allow you to shoot in different aspect ratios. Generally itís good practice to shoot your images a bit wider than the composition you want (extra space around your shot) when taking the photo. This way you can crop the image to your specifications for whatever size paper you want later on.
If you cannot resize your own images and do not have your own printer, then itís a good idea to find a good printing lab that will deal carefully with your images and attempt to get the best from them. Good printers can recommend the best size paper for your photo as they should understand about aspect ratios.
If money is no issue then you can print the whole image on larger paper or one with the same aspect ratio as your image and have a hand made frame made. This is the way to go if you need the whole image but standard sized papers (6x4, 10 x8 etc.) and frames wonít fit.
It's best not to get too hung up about aspect ratios and concentrate more on your creativity. Donít let the technical side of things get in the way. Taking photos is about having fun, so get out and shoot and think about the aspect ratio later!