I tell everyone they need to crop their photos to the correct aspect ratio. However, many people have NO IDEA what language I'm speaking and what I'm trying to tell them when I say this. I was thinking about this as I was editing a few of my most recent sessions this morning, and I decided to share exactly what I'm talking about.
Aspect ratio: the ratio of the width to the height of an image.
5:7, 4:5, 2:3, are all common aspect ratios for photo prints (print sizes of 5x7, 8x10, 4x6). An 8x10 is a common print size. The aspect ratio for an 8x10 is 4:5. (Remember back to grade school and think of fractions... 8 and 10 can be reduced to 4 and 5 by dividing both by 2.)
WHY DO YOU CARE?
You care because those print sizes cut off part of your photo. It will change for each aspect ratio. And you want to DECIDE which parts are cut off!
This photo is the same aspect ratio as the camera takes: 4x6. This is the image I gave to the parents. It allows them the most flexibility when printing. However, I cringe when I see this photo on your wall, as is. It needs to be cropped!
When Grandma wants a 4x6 of this to put in her brag book, it should be cropped, using the same aspect ratio (4x6) to focus on the image, like this:
If Grandma wants an 8x10, it will look slightly different, because an 8x10 has a different aspect ratio than a 4x6. An 8x10 is squattier - it's shorter and wider than a 4x6. This is what this image would look like as an 8x10:
There are many other "rules" for cropping, such as the rule of thirds, placing your subject off-center, leaving breathing room in front of a face, use leading lines, etc.
With many photos, there are multiple options that will look great! Bottom line, play with your photo until you find a crop that looks good. You may not know why, but you'll know what looks good when you see it. Don't be afraid to experiment!