If you're interested in preserving composition using automatic methods, you may want to check out the field of image retargeting. Instead of cropping, it attemp

If you're interested in preserving composition using automatic methods, you may want to check out the field of image retargeting. Instead of cropping, it attempts to warp 'non-important' parts of the image to reach the desired aspect ratio. An implementation of it in photoshop is called 'content aware resize', but I haven't seen an evaluation of it against the state of the art research.