How do different aperture and shutter settings affect a photo with the same overall exposure?

Ok, here's my doubt. Suppose I want to get a certain exposure for an image. Keeping the ISO constant, there are two tentative possibilities here:

1. Put the camera at f 1.4 (say) and at 1/1600 shutter speed (suppose).


2. Put aperture at f 8.0 (say) and shutter speed at 1/50 (suppose).

Both would get me the exact same exposure. Yes, for the latter, I may have to use a tripod. But the question being, what would be the difference in the image thus obtained? I can fairly interchange the settings for the same shot (theoretically). Or can't I?

Answer

Different shutter speeds have an obvious different effect: more motion-blur. (That includes both subject motion and blur from any motion of the camera itself.)

Different apertures also produce different results; most notably that depth of field increases as you stop down. So, f/8 gives you a much deeper in-focus area than f/1.8. This effect is lessened on a smaller sensor (or, if for some reason you just crop out the center part of the image and blow it up), to the point where it's basically a non-factor on most point & shoot cameras.

But there are other effects of changing as well: lens sharpness, contrast, and vignetting characteristics change, usually improving significantly when you stop down a bit. (For a certain look, though, that technical improvement may not, in fact, be what you want.)

That's assuming perfect reciprocity — the idea that aperture and shutter speed really are perfectly interchangeable for exposure. With film that's not strictly true as you get to extremes — see this question on reciprocity failure. But for digital, it's not meaningfully a factor.