I see this question in the Sue Bryce Facebook group come up often. What mat size to chose: the one with the 7×10 or 8×10 opening? While the final decision is a matter of artistic preference, I can tell you what is the size that you should use, if you want to keep the aspect of your photos the same of how they were captured.
Aspect Ratio
It all boils down to simply look at the aspect ratio of your camera. I use a Canon 6D, which, like most DSLRs uses a 3:2 aspect ratio. There is a very simple way you can verify that with your camera. Look at the largest format that it can shoot, which is the native size of the camera’s sensor. In my case, it is 5472 x 3648. If we divide 5472 by 3648 we obtain 1.5. That means that the width is 1.5 times the height. So, 5472/3 = 1824. 1824×2 = 3684. That is exactly an aspect ratio 3:2. 1842 fits 3 times in the width and 2 times in the height and 2 x 1.5 = 3. So, an aspect ratio of 1.5 is the equivalent of the 3:2 notation. Let’s keep that 1.5 in mind.
If we divide 10 by 8, the 8×10 opening for the mat, we obtain 1.25, which is much more squarish than 1.5. If we divide 10 by 7 we obtain 1.42, which is much closer to the 1.5 native aspect ratio of our cameras.
Therefore, if we want to have an opening that is close to the natural aspect ratio of our photos we should use the mats with a 7×10 opening. If you select the 8×10 then the photo will need to be cropped to become closer to a square.
Hope this helps.