Lights, Camera, Action! Welcome to the Digital Camera World Community. Please introduce yourself here. You could also be in with a chance to win a Skylum Prize Bundle, visit our competition thread here.
QuestionHow do I know what the right aperture setting is?
Have you learned about the exposure triangle and the three main ways to control exposure? How your aperture and shutter speed and ISO all work together?
Besides helping to control exposure, your aperture also controls the depth of field in your image. The smaller the F number (the larger the opening) then the shallower your depth of field will be. The larger your F number (the smaller the opening) the greater your depth of field will be.
Example, you're photographing a portrait and you want them to stand out, you want the background to be out of focus. Then you might use something around F2.8 for your aperture.
Another example, you're photographing a cityscape, and you want as much as possible in focus. Then you might use something around F22 for your aperture.
So how do you know you've set your aperture correctly? I say it's when you get the exposure and depth of field you want for your image. How do you learn all of this? Some people read, some take classes, some play around with their camera settings until they understand how they all work together. With a digital camera, you'll get instant results and can experiment. If you have Live View it's even more instant!
Generally speaking you should avoid using a such a small aperture. Diffraction will start noticeably softening the image around f13 on crop, f16 on full frame.
Best practice for a standard shot is to avoid going over f11. If you happen to have something close to the camera that falls outside the depth of field then focus stacking is the way to go.
Generally speaking there are a couple of rules of thumb. Avoid shooting wide open, because your depth of field will be ridiculously shallow and few lenses are at their best that way - stopping down a little gives sharper results while maintaining a shallow depth of field that's useable.
Avoid closing past f16 for the reasons mentioned above, unless you're doing something specialised like a long exposure. (Even then I would only use the f16+ shot for the blurred part and composite in a shot taken at a sharper aperture).
Otherwise it's just a matter of remembering that smaller f-number = smaller depth of field and vice versa.