When is it useful to set your aperture settings to a higher number; for example, above 15?

Answer 1

Aperture and shutter speed together affect exposure.

Aperture alone affects depth of field, i.e. how much of a photograph appears in focus, and has nothing to do with whether it is a dark day or not. With a constant shutter speed increasing or decreasing the aperture will lighten or darken the image.

When a lens is wide open you have the least depth of field. As you stop down the lens depth of field increases through all f-stops until you reach the maximum when the lens is closed as much as possible.

Least amount of depth of field = lowest f-stop
Largest amount of depth of field = highest f-stop

To control excessive brightness both shutter speed and aperture can be used. Depending on many other factors you choose the settings of each appropriately.

That's the longer answer.

Short answer is - use higher number when you want more to appear to be in focus from near to far in your photo (regardless of exposure considerations). Both StevenW and William are partially correct.

Answer 2

Depending on the focal length of the lens (wide angle tends to have deeper Depth in field) any aperture above f11 should give you a wide depth in field. The higher the f-stop, the further back the picture is in focus.

I tend to use it when I want a large amount of the picture in focus, and when I need to have less light because of excessive brightness.

Remember, the higher the f-stop number, the smaller the opening in the lens.

Answer 3

Good answers on depth of field. Another reason would be if you had a specific need to get the slowest possible shutter speed, such as waterfall shots where the water gives a smooth appearance. Or if you wanted to intentionally blur action. Of course, if you're shooting in direct sunlight you might find yourself exceeding f16 just to get a proper exposure.

Answer 4

On a very dark cloudy day or when you want depth.