Photographers stop down their lenses by increasing f-stop, say from f/2 to f/4, which decreases the aperture of the lens thereby limiting the amount of light allowed through the lens and onto the film or sensor.
A stop is a doubling or halving of the amount of light let in when taking a photo. For example, a photographer who says they’re going to increase his exposure by 1 stop is simply saying he’s going to capture twice as much light as they did before.
Most people stop down a lens so they can see more details and get more depth of field. In any case, it’s hard to know when you’ve stopped down a lens too far because of diminishing returns.
Changing your shutter speed, aperture, or ISO will result in the same change in exposure as using stops.
Let’s do a simple example with shutter speed. Let’s say you want to make your exposure 1 stop brighter at 1/100 shutter speed. That means you have to cut your shutter speed in half, from 1/100 to 1/50. At 1/50, your exposure gets a stop brighter.
Conversely, let’s say you want to darken your photo by one stop, starting with 1/100 shutter speed. You have to shoot at 1/200 shutter speed now instead of 1/100. Your exposure is now 1 stop darker.
Stopping down depends on the conditions in which a photographer is shooting. For example, in low light situations, wide apertures are needed, and stopping down could cause underexposure. Adding supplemental lighting, such as a fill flash or lamp, can bring the light levels up to a point where the photographer can stop down the lens to make it sharper.