This series of articles is excerpted from Rocky Nook’s Enthusiast’s Guide to Exposure by John Greengo covers the controls — shutter, aperture and ISO. The week, John gives an overview of the camera’s aperture.

The aperture

An aperture is an opening through which light travels. In a camera, it also describes the mechanical device that adjusts the size of the opening in the lens. This device comprises several lightweight metal blades that can reduce the size of the lens opening to a variety of smaller sizes. Like shutter speeds, this is another way you can control the amount of light being let into your camera.


Each different lens opening is labeled with an f-stop number that reflects how much light is being transmitted through the lens. F-stop is derived from the term focal stop. The f-stop can be calculated by the focal length of the lens divided by its opening. A lens with a 50mm focal length and with a 50mm opening for the light to pass through would have a maximum f-stop of 1.0 (f/1.0). If that same lens had a 25mm opening, the maximum f-stop would be f/2.

Maximum Aperture

The maximum aperture is a key indicator of a lens’s performance. You can find this labeled quite clearly on most lenses (e.g., 1:2.8 or 1:3.5- 5.6). The first number and the colon indicate that the number is a fraction. The number before the colon is the top number in the fraction; the number after the colon is the bottom number in the fraction. It’s the second number, or set of numbers, that are key. In this example: 2.8 or 3.5-5.6. The lower the number, the more light the lens can let in.

Fast Apertures

Lenses that let in a lot of light are said to be “fast” lenses. This term is in reference to that lens’s ability to gather light quickly, giving you the option of pairing it with fast shutter speeds. We could just as correctly call them “low ISO” lenses because they also allow for low ISO settings. A

Fast Aperture Is Relative

The concept of what a fast aperture depends on what focal length you’re talking about. A 50mm f/1.4 lens is considered fast, but with a 300mm lens, f/2.8 is considered fast. Lenses in the normal focal length range (35mm, 50mm, 85mm) are usually the fastest available. As focal lengths move further away from 50mm, the slower they tend to be.

Opening photo: ©Kevin Ames