When taking a picture, you press the shutter release and the shutter opens to allow light from the scene to be focused onto the image sensor. To get the ideal exposure, just the right amount of light must strike the image sensor. If there is too much or too little light, you'll need to make an adjustment. One way to do so is by opening or closing the lens's aperture, an adjustable opening that regulates how much light passes through the lens. “Stopping” down the aperture makes it smaller so it lets in less light. Opening it up lets in more light.

The size of the aperture is measured in f-stops. Each f-stop lets in half as much light as the next larger opening and twice as much light as the next smaller one. From largest opening to smallest, standard f-stops are as follows: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, and f/45.

F-stops are a little confusing because the larger the f-stop, the smaller the amount of light that is let into the camera. The easiest way to think of f-stops is in terms of fractions: just as 1/16 is less than 1/8, an f-stop of f/16 is smaller than — and lets in less light than — f/8.

The word ‘stop’ is also often used in photography to indicate half or double the light in a scene.

The maximum aperture of a lens determines how much you can open it. The maximum aperture is also referred to as the maximum iris, or the speed of a lens. Although lenses are referred to by their focal length, the specifications of a lens also carries a second number, such as 2.0 or 3.5, which indicates the maximum aperture of the lens. The maximum aperture is usually inscribed on the front of the lens after the notation “1:.” So, for example, maximum aperture of 2.8 will usually be noted on the lens as 1:2.8. Larger maximum apertures, such as f/1.8, let in more light than smaller apertures, such as f/3.2, allowing you to take better shots in low-light situations.