Most micro four thirds digital camera sensors use a 4:3 aspect ratio. In simple terms this means your image can be four inches by three inches. It can also be four centimeters by three centimeters. Digital SLR cameras use a more favorable 3:2 aspect. Again, this gives us three inches by two inches. While that's not a realistic size with some simple math you can infer that a 3:2 aspect will yield a 6x4, 9x6, 12x8 or even a 7x4.6 or 10x6.6 photo. "So what?" I hear you ask further. Well, looking at some of those numbers we can deduce that they don't fit easily into standardized, ready-to-hang, off-the-shelf picture frames.
We are all familiar with the standard picture frames available; 4x6, 5x7, 8x10, 11x17 and (now that we have digital printing) 8.5x11. These dimensions have been around a very long time but they all (but one - 6x4) don't fit into the same aspect ratio of our camera's sensor. That means something has to be cropped out, either top, bottom or both or even the sides. Let's take a dSLR's 3:2 sensor and one of my favorite print sizes, the 8x10.
If I were to take a photo with a point and shoot that has the 4:3 aspect ratio and I wanted an 8x10 I would be closer to my goal. At this ratio I would get an 8x10.6 inch image size. I would only be losing a little over half an inch in this case. This tells me that if I shoot with my dSLR I have to allow for cropping on the short sides while with my point and shoot I will need to consider a bit on the long sides.
For a 5x7 print with my 4:3 point and shoot I'll only have to lose a quarter of an inch on the long sides. Not bad. I can easily live with that even if I forget to allow for it. With my dSLR I'll be losing a half inch on the short side. A rather larger margin of error. While not considerably an image killer, losing a part of your image can become significant when you are already cropping in tight with your original composition. So when you are out there shooting, consider your crop factor in relation to your sensor's aspect ratio. You don't want to end up having to sacrifice important image information because you didn't allow enough room in your original composition.