Printing photos of non-standard ratios
I rarely print photos, and when I do, they're usually small prints. But I'm considering printing some photos that I've cropped in non-standard ratios like 2.12:1, 1.92:1, etc. They're still fairly large (like 4700 by 2400 pixels), so I suspect there's enough pixels to get clean medium sized images. But my question is: how will these cropped images appear on standard dimensioned prints like 12x18 or 20x30? I suspect that these will be cropped further in one dimension or the other to fit the paper. How can I determine how it will look on paper? (I'll be using an online facility like elcocolor or snapfish.) Is my best option to simply crop the original image using the ratio of my desired print size?
Canon 50D, Canon 100 f2, Canon 100mm 2.8 macro, Canon 100-400 IS, Canon 50 1.4, Canon 10-22, Sigma 18-200 OS, Tamron Pro 1.4TC, Speedlite 430EX, Feisol 3402 + Giotto ballhead.