When you go shopping for a picture frame, what's the most "common" size you'll find? 8" x 10". It's been like that for... well, longer than I've been around. I remember when I first got into photography, (the film years) and took my film to the drugstore to get processed. I had 4"x6" prints made, and one of them looked GREAT! I thought.."hmm... I'm going to blow this up and put it on my wall" so I ordered an 8x10. When I got the print back from the lab, part of the person's head was cut off!!
I checked my smaller 4x6 print, and it was fine... the negative looked okay... what happened? I was furious.
I didn't get it back then, and a lot of people still don't get it. The cameras have a little rectangular sensor. The size ratio for the small end compared to the long end of the rectangle is 2:3. I could go into a long post about math... but I'm a visual person. I was going to try and create a diagram that explains this, when I found a picture on Facebook that explains it very well.
This was created by photographer Jessica Downey from Chandler Arizona. Same photograph printed in 5 different aspect ratios. If you want to create a print that doesn't have one of the sides chopped off, then you should print at one of the 2:3 ratio sizes, which would be 4x6, 16x24, or 20x30.