Images Proportions in Photography
The picture above is illustrates the full format of most DSLR cameras. As the picture is taken, it is ideally proportioned to produce a 6″x4″ print that contains the ENTIRE image. In ratio terms, that’s a 2 to 3 relationship between the height and width of the print.
Like 35mm film, the Canon DSLRs record a rectangle that is in 3:2 format. It fits the entire image perfectly into 4×6, 8×12, etc. prints that are also 3:2. If you want the print in any other proportion, like 8×10 or 5×7, then you must crop. The only alternative is to digitally “stretch” the image to fit, but that introduces distortions that will ruin your image.
There are a number of different ratios available for print, some of which are more common.
4×6 prints are 3:2, which means out of a DSLR they will print without cropping.
7.5×5, 10×7, 12×8, 15×10, 18×12, 30×20 prints are 3:2, so same thing.
4×5, 8×10, 16×20 are all 4:5 ratios. An image shot with anything that does not shoot at 4:5 will have some cropping.
5×7 is a 1:1.4 ratio, so an even different cropping.
You have a few different options. You can crop however you see fit (whatever works best for the shot), and print at whatever size will not crop further, and then cut the print. For example, say you have cropped your picture to 8×6. Print it on 8×10, and cut off the extra paper.