Understanding Camera Lenses: Numbers And Their Meanings

The numbers on a camera lens are not just random figures but rather a set of specifications that describe the lens's capabilities and compatibility. The first set of numbers you'll often see are the focal length and aperture range, usually presented as 18-55mm f/3.5-5.6. The focal length, measured in millimetres, indicates the lens's field of view, with smaller numbers offering a wider angle and larger numbers providing a more magnified view. The aperture, or f-stop, is the speed of the lens, determining how much light is let in and the depth of the image. A smaller f-stop number indicates a wider aperture, while a larger number signifies a narrower one.

Focal length

The focal length of a lens tells us the angle of view, or how much of the scene will be captured, and the magnification, or how large individual elements will be. The longer the focal length, the narrower the angle of view and the higher the magnification. Conversely, the shorter the focal length, the wider the angle of view and the lower the magnification.

There are two types of lenses: prime and zoom. Prime lenses have a fixed focal length, meaning there is no zoom function. Zoom lenses, on the other hand, have variable focal lengths. The focal length of your lens will tell you its field of view. A lens with a 20mm focal length gives you a wide-angle view, while a 50mm lens is considered standard, similar to the human eye. A 100mm focal length gives a tighter, more magnified view.

The focal length of a lens also determines whether you capture a wide vista or a close-up of a distant subject. A lens with a short focal length captures a much broader view than a telephoto lens. This is often expressed as a lens's angle of view, which is the angle between two lines drawn out from the nodal point to the outer edge of the lens's field of view. A shorter focal length, such as 24mm, produces a wide angle of view, while a longer focal length results in a narrower angle of view.

Understanding focal length is key to understanding how your camera works. It determines what your camera can focus on and how your images will turn out.

Aperture

The maximum aperture, or the widest aperture, is usually marked on the lens and tells you if the lens is "fast" or "slow". Faster lenses with wider apertures let in more light and give you better depth-of-field control but tend to be more expensive. The maximum aperture will be the first number in a sequence of two numbers, e.g. f/1.8/55 or 1:1.8 means the lens's maximum aperture is f/1.8.

The minimum aperture, or the smallest aperture, does not depend on the focal length but is limited by how narrowly the aperture closes. Very small apertures offer little benefit to shooting as they provide less sharpness due to diffraction at the edges of the aperture, while the added depth-of-field is also not generally useful.

Lens mount

A lens mount is an interface between a camera body and a lens. It is a feature of camera systems that allow for interchangeable lenses, such as rangefinder cameras, single-lens reflex types, and single-lens mirrorless types. Lens mounts are also used to connect optical components in instrumentation, such as modular components in optical laboratory prototyping, which use C-mount or T-mount elements.

Due to the proprietary nature of camera mounts, each one differs in mounting direction, mount size, flange distance, and the number and placement of electronic contacts. The direction of mounting can vary between brands, with most lens mounts requiring lenses to be attached by twisting them clockwise, while some brands like Nikon attach lenses in the reverse direction.

The mount size of a lens is determined by its throat size, inner diameter, and outer diameter. Throat size refers to the inner diameter of the mount minus the tabs used to mount the lens. Inner diameter represents the size of the lens opening, ignoring the tabs on the mount. Outer diameter refers to the full diameter of the bayonet mount.

Flange distance, or flange focal distance, is the distance between the mounting flange (the outer part of the lens mount) and the film or sensor plane. A larger mount size and shorter flange distance typically provide more flexibility for lens design, allowing for larger lenses that can provide more light to the sensor.

Lens diameter

The symbol that looks like a circle with a line through it (Ø) tells us the diameter of the lens. This is the measurement, in millimetres, of the outside circle of your lens. For example, Ø72 means your lens is 72mm in diameter.

This is important if you need a new lens cap or want to use lens filters. Screw-on filters come in different sizes, so you need to match the filter size to the lens diameter.

You can also use the lens diameter to work out the size of lens hood you need.

Focusing distance

The closest focusing distance, also known as the minimum shooting distance, refers to the shortest distance that must be between the subject and the surface of the image sensor (the focal plane) for the lens to focus. It is not affected by the length of the lens and does not change if you zoom the lens. The closest focusing distance is different from the working distance, which refers to the distance between the front end of the lens and the subject.

The minimum focusing distance is the shortest distance from the focusing plane to the subject at which it can be sharply rendered, or "in focus". If the camera is moved closer to the subject than the minimum focusing distance, it will not be possible to focus on it. Lens makers usually indicate the minimum focusing distance in the lens properties.

On a camera lens, the minimum focusing distance may be expressed in meters and feet (e.g. #m / #ft). This number represents the minimum distance at which the lens can focus. For example, a lens with a minimum focusing distance of 0.9m / 2.96ft can focus on anything closer than that distance. In some cases, the focusing distance may be given as a range, indicating that the minimum focusing distance increases as the lens zooms in.

Frequently asked questions