Question CG fundamentals: camera question, how to emulate a crop (camera calculations/settings)

So I want to produce a 1080x1920 crop of an image that is 1080x2280. (2nd image)

Right now, I render to a 1080x1920 render texture. This does not do the same things as cropping. I think it may change the fov or other camera settings. (or does it?) This is preferable over rendering the full 1080x2280 image then cropping for performance reasons (we want to minimize the amount of rendering since we are exporting a video from these images.)

Any help with this would be great! Thanks.