Albert, you need to think in terms of the aspect ratio - the length of your image divided by its width. It doesn't really matter whether you think in terms of pixels, centimetres, inches, or whatever (so long as you use the same unit for both dimensions
).
Pictures come out of your 20D at 3504x2336 pixels, so the native aspect ratio is 3504/2336 = 1.50. If you want to print at 6x4 or 12x8 then that's the same aspect ratio and so you can print the whole image.
But if you've cropped your picture then the aspect ratio might not be 1.50 any more. And other common print sizes (e.g. 7x5, 10x8, ...) don't have the 1.50 ratio either. I know you can do the sums yourself, but just as an illustration:
- 1.25 = 10x8
- 1.33 = standard PC monitor
- 1.40 = 7x5
- 1.41 = A5, A4, A3 etc. paper
- 1.44 = 11.5x8
- 1.50 = 12x8, 6x4, standard uncropped photo
Note that the 1.50 ratio is the largest, which means that you'll always need to crop the long dimension of your picture (i.e. make it squarer) to fit on these other shapes. You can work out how much to crop by dividing the aspect rations. For example, if you want to crop a picture from a ratio 0f 1.50 to 1.25: divide 1.25 by 1.50 and the answer is 0.833. So you keep 83.3% of your picture and crop out the remaining 16.7%.
Hope that makes sense!
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