Can I Set the Canon T4i Image Size
Rob Bernhard wrote:
This is something that is better done in a post-production environment. Shoot the full resolution in-camera and crop and export a files with the 8x10 aspect ratio afterward. Depending on how you work with your files, there are tools that can automate the cropping and still give you the flexibility of adjusting where the crop sits on your image prior to export.
That if you think you may be cropping to 8x10 in post, do not fill the frame with your subject. If you do you wont be able to get the ratio you want for the finished image. This is also another reason why you should keep the camera set to full resolution.