If you want to know where an 8x10 will crop in your camera, view an 8x10 print through your viewfinder. Put the two long edges of the print to the long edges of the frame. Make a note of what focus points the other sides are near and use those for your guide. Then you can get a permanent marker and make your own marks

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Due to the popularity of 8x10 being a common asked for size, I always compose a image so I can make 8x10 prints without losing necessary elements from the image.
When I can't manage, I drop the entire image on an 8x10 canvas, create a background and presto I have an 8x10.
