...Back to our challenge of getting sharp photos of the Tangs and Wrasses and Angelfish - since the fish are moving quickly, it's best to use shutter speeds around 1/125th of a second or faster, if possible. That usually means shooting "wide open," at the lens' maximum aperture, because even bright metal halides simply don't provide enough light to accommodate really fast shutter speeds. The maximum aperture setting allows the most possible light through the lens. To achieve this setting, put the camera in aperture priority mode and select the smallest available f-stop number (largest aperture), then take a few pictures. If the shutter speed was fast enough, and if the focus was accurate, the fish should be in sharp focus. There is a downside, however, to using the largest or larger apertures: the depth of field will be very shallow. Sometimes this is a good thing, such as when trying to isolate the subject against a background with a lot of distracting detail. When shooting fish it's best to focus on their eyes, as that's where the viewer's attention is drawn. As with portrait photography, it's imperative that the eyes of the subject be nice and sharp.