One of the most significant features of a lens is its aperture. The aperture is the opening of the lens through which the camera sees and can be made larger or smaller to affect exposure (brightness) of an image and/or "depth of field" (see further below).

Like the pupil of an eye, the larger the opening, the more light it receives and the smaller the opening, the less light it receives. In daylight our pupils contract to limit the light we receive so we are not blinded. At night our pupils expand so we can gather more light to see objects in the dark. Similarly in bright circumstances we might set the lens aperture smaller so the images are not too bright, and at night we might set the aperture larger so the images are not too dark.

Apertures are usually labeled on a lens as something like "1:2.8" which reads as "1 to 2.8" and means the lens focal length is equal to 2.8 apertures, or "widths" of the lens opening (its diameter). For instance if a lens has a focal length of 50mm and an aperture of 1:2.8, the width of the lens opening is about 18mm (18mm x 2.8 = about 50mm).

Apertures are always related to "one whole focal length" of the lens and for this reason the first number is always "1" (one) no matter what the focal length actually is. It is not important to know or figure the width of the lens opening in millimeters. It is only important to understand the relative size of the second number to the first. The lower the second number is, the larger the opening is. The larger the second number is, the smaller the lens opening is.

Aperture numbers such as 1:2.8 are often abbreviated as just 2.8. Again the first number is always "1" (one) so we know when we speak of an aperture of 2.8 we mean 1:2.8. The aperture may also be listed as f/2.8. This is called an "f-stop" and means the same thing. A "stop" is a measure of exposure (brightness) which will be discussed in a later article. For now, it is enough to say the larger the aperture (smaller aperture number), the more light the lens and camera "see" allowing them to see better indoors, at night or in other low-light situations.

Lenses are capable of having a range of apertures. Most lenses have a series of "fins" or "blades" around the inside of the lens which can be controlled to make the aperture smaller, like making the pupil smaller. The aperture number listed on lenses and in advertisements is the maximum aperture for that lens; the largest possible lens opening size. Making the aperture smaller is called "stopping down" because a smaller opening sees less light and brings the exposure (the brightness or "stop") down. Smaller openings have larger f-numbers such as f/4 (or 1:4), f/5.6 (or 1:5.6)...f/16 (or 1:16), etc.

Aperture is also used as a means to control the "depth of field" in a photograph. Depth of field (sometimes abbreviated "DOF") is the degree to which objects in front and behind the main focus of an image are also generally in focus. If the depth-of-field in an image is "thin" or "narrow" only the main object is in focus. Objects in front or behind the main object quickly drop out of focus. In fact, the depth of field may be so "narrow" only part of the main object may be in focus. For instance the eyes of a person in a portrait may be sharply in focus while their ears and nose are softly out of focus if the depth of field is narrow enough.

Similarly a "deep" depth of field may be used to bring most or all of a photograph into focus. This is especially common for shooting landscapes:

Smaller apertures {larger f-stop numbers i.e. f/8) produce deeper depths of field while larger apertures (smaller f-stop numbers i.e. f/2.8) produce narrower depths of field. This is similar to the effect of squinting eyes to better focus our vision. At the same time there is a limit to how far we can squint, and how small the aperture can be, before the image becomes less sharp due to an effect called diffraction, where we are trying to "squeeze" the image we are trying to see into an aperture too small to take the image in clearly.

Lastly, because larger apertures bring in more light the camera takes less time to form the photograph. The difference between being able to take a photograph for 1/30th of a second versus taking the same photograph for 1/125th of a second may sound minimal, but it can be the difference between a blurry photo of something in fast motion versus a clean shot. Being able to bring in more light may also be the difference between getting a great wedding photo in a dimly lit chapel versus nothing usable at all. Large aperture lenses are often referred to as "fast" lenses, "fast" glass, or "low-light" lenses because of these abilities and are highly desirable for sports, weddings, wildlife and hyperactive children.