Originally posted by NicoleAu I just wondered if people tend to shoot certain subjects at certain apertures or do you just wing it?
Depends on what your goals are. If you are merely taking snapshots and aren't overly concerned with the artistic quality of the photo, then winging may be just fine. But if you're seeking to control the look of the photo, then setting the aperture becomes crucially important. Aperture vitally affects depth and field and sharpness. There exist various trade-offs between depth of field, center sharpness, and corner/border sharpness at different aperatures. Depth of field consistently increases as one stops down. Sharpness initially increases as one stops down, reaches its highest point at around f4 or f5.6, and then begins decreasing. Border to border sharpness follows a different path. Wide open, most lens are moderately sharp toward the center but soft toward the corners. Corner/border sharpness usually improves until f8, after which it gradually worsens.
Now depending on what your goals are for your image determines your optimal focal length. For portraits, you want to shoot wide or close to wide open, because the narrow depth of field lets you isolate subject. There's a bit of a tradeoff between wide open and close to wide open. Wide open will allow you to get the most isolation (with a real fast lens, the background may be entirely blurred out), but your subject won't be tack sharp. Stopped down to f2.8, your subject will be sharper, but your background won't be as blurred out and you'll therefore have less dramatic isolation. So you have to choose what is most important to you: isolation or tack sharpness. BTW, there's really no reason to shoot a portrait at f8, unless you want to include the background (e.g., a portrait with the Grand Canyon as a backdrop).
Landscape photography, on the other hand, usually has different aims. With landscapes, border to border sharpness is often desires, so f8 becomes an optimal aperture in landscape work.
In macro photography, where depth of field, even stopped down, becomes exceedingly narrow, the optimal f-stop may be f16.