One of the most daunting aspects about stepping up from a smartphone or compact camera to a DSLR or mirrorless system camera is the move to interchangeable lenses. In fact even experienced photographers get a bit confused by the huge variety of camera lenses available. This guide to the most important features of camera lenses should help make choosing the right one easier.

Understanding lens mounts

The most important thing to know about camera lenses is that they have a specific mount so they can only be used with certain cameras. Third party manufacturers like Sigma, Tamron, Tokina and Zeiss make lenses with mounts to fit a range of cameras, but Canon, for example, only makes lenses to fit Canon cameras and Nikon only makes lenses that fit Nikon cameras.

The picture is a little more complicated by the fact that some lenses are designed for specific camera ranges. Canon EF-S mount lenses, for insatnce, are designed to go on APS-C format Canon cameras and cannot be used on full-frame cameras.

Sony also has two different camera lines, the SLT range that uses A-mount lenses and compact system cameras that have the Sony E-mount. Our article What Sony Lens Should I Buy: Full-Frame Vs APS-C and Camera Mounts Explained gives more detail.

Olympus and Panasonic both use the Micro Four Thirds mount on their cameras, so you can use an Olympus lens on a Panasonic camera or vide versa.

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Focal length explained

A lens’s focal length is the distance in millimetres between its optical centre when it is focused on infinity and the sensor or film inside the camera. The optical centre of a modern lens isn’t easy to identify because the barrel conceals several lens elements.

However, although there are few exceptions, short focal length lenses are usually shorter in length than long focal length lenses.

What is angle of view in photography?

A lens’ angle of view is a measure of the extent of a scene that it can project on to a camera’s sensor. A wide-angle lens is so called because it captures a wide view of a scene (ie all or most of it) and it has a large angle of view.

Angle of view can be measured horizontally, vertically and diagonally. The Canon EF 20mm f/2.8 USM lens, for example, has a horizontal angle of view of 84 degrees, vertical angle of view of 62 degrees and a diagonal angle of view of 94 degrees. In comparison, the EF 85mm f/1.8 angle of view measurements are 24, 16 and 28 degrees – much narrower.

On full-frame cameras lenses with a focal length of around 50mm are considered ‘standard’, those with shorter focal lengths are wide-angle and those with longer focal lengths are telephoto optics.

Wide-angle lenses are popular for shooting landscapes because they enable you to capture an expansive vista and telephoto lenses are useful for picking out distant details or tight framing of action.

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The difference between prime and zoom lenses

Prime lenses are optics that have a single focal length, but zoom lenses allow you to change focal length to get a wider or narrower view.

Zoom lenses are very popular because of their convenience, but prime lenses generally produce better image quality than a zoom lens of similar price.

What is focal length magnification?

Focal length is a physical measurement but the image framing that a lens produces depends upon the size of the sensor in the camera it is mounted on.

This is because a full-frame lens is designed to project an image across the whole sensor, which is the same size as a 35mm film frame. If this lens is then mounted on an APS-C format camera, the smaller sensor only occupies a part of the image produced by the lens.

As a result, the image captured by the camera looks cropped in comparison with the image captured using the same lenses on a full-frame camera, or like a longer focal length lens has been used.

The focal length magnification (also know as a ‘crop factor’) describes the increase in effective focal length that the sensor produces.

Nikon, Sony and Fuji APS-C format cameras have a focal length magnification factor of 1.5x, which means that a 50mm lens produces images comparable with a 75mm lens on a full-frame camera. Canon APS-C format camera have a 1.6x magnification which makes a 50mm lens look like an 80mm optic.

Front of a lens

Understanding your lens’s aperture

Lenses have an iris which can open and close inside their barrel to control how much light can reach the camera’s sensor. The iris is made up of a series of blades and the more blades there are, the more rounded iris hole or aperture.

Aperture value is calculated by dividing the focal length (f) by the aperture diameter in millimetres. This means that an aperture of f/8 lets in the same amount of light whatever lens it is set on.

What are fast and slow lenses?

Lenses with a wide maximum aperture (eg f/1.8) are called ‘fast’ because they let in lots of light and allow fast shutter speeds to be used.

The maximum aperture of many zoom lenses varies as its focal length is adjusted, but more expensive zooms have a fixed maximum aperture such as f/2.8 or f/4.

These lenses are generally more desirable because the exposure doesn’t alter as you zoom in or out, but they also tend to be bigger, heavier and more expensive than zooms with variable maximum apertures.

Stabilisation

Some lenses have giro inside that can detect movement and then shift an element to compensate. This enables the camera to capture sharp images at shutter speeds that would normally result in blurred photographs. The reduction in shutter speed that’s possible is measured in stops or exposure values (EV) and is commonly 3-4EV.

Sony, Olympus and Panasonic have started to introduce hybrid stabilisation systems that use in-body stabilisation as well as lens stabilisation to extend the range of movement compensation.

Understanding lens markings

Lens names often seem like a long and confusing list of numbers and letters but they are designed to spell out the lens focal length and maximum aperture along with some of the key technologies inside.

The Canon EF 85mm f/1.8 USM, for example, has the EF mount, a focal length of 85mm, a maximum aperture of f/1.8 and an Ultra Sonic Motor for autofocusing.

Meanwhile the Canon EF-S 15-85mm f/3.5-5.6 IS USM has Canon’s EF-S mount (so it’s for APS-C format SLRs) and is a zoom lens with a focal length of 15-85mm (it produces images equivalent to a 24-136mm lens on a full-frame camera).

It also has a variable maximum aperture which changes from f/3.5 at the wide-angle end to f/5.6 at the longest point. In addition to the IS indicates that there’s Image Stabilisation built-in along with an Ultra Sonic Motor.

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