Halving and doubling
One clic k here, on e clic k th ere
■ The standard scales for both aperture (stops) and shutter speed (seconds or fractions of a second) are organised the way they are so that each step or click in either case reduces or increases the amount of light by the same amount. So, increasing the shutter speed by a single click will let in less light, so you have to open the aperture by a single click to let in enough light to compensate for this. This approach makes it easy to keep the same exposure while changing the motion-freezing capability of the camera and the depth of field (below). I wanted some water streaking for softness in this shot, but was not certain exactly how much, and how much depth of field around the vase was necessary. In manual mode, and with the camera on a tripod, I tried it at three different shutter speeds,
each time halving the exposure time, which also meant I had to double the aperture to compensate.