Understanding focal length in photography

Using the right lens is arguably the most critical part of your photographic setup. It's the optical quality of the lens, not the camera's resolution, that determines how sharp your images are. It's the lens that governs how much of the scene you're shooting is in focus, because it's primarily the lens aperture that dictates the depth of field. Crucially, it is also the lens's focal length that determines whether you capture a wide vista or a close-up of a distant subject.

Here we'll explain what focal length is and how it determines what part of the scene is captured by the camera, and explore focal length related terms such as prime, zoom and telephoto.

• What is focal length?
• Understanding prime and zoom lenses
• How crop factor affects focal length
• What is the focal length of the human eye?
• Wide-angle, standard and telephoto lenses explained
• Gaining extra reach with lens extenders and teleconverters

In a very simple lens containing just one element, the focal length is the distance in millimetres between the focal plane and the centre of the element when the lens is focused at infinity. In a film camera, the focal plane is the film; in a digital camera, it's the light-receptive surface of the sensor.

Modern lenses are much more complex than a single element, but they still have an optical centre known as the nodal point. That's the spot through which all light rays pass, converging to a point on their way to the sensor. The focal length is the distance between the focal plane and the lens's nodal point.

This partly explains how two lenses can have different dimensions and yet the same focal length – it's the optical centre that matters, not the physical length of the casing. The maximum aperture also has an impact. The Canon RF 50mm F1.8 STM and RF 50mm F1.2L USM, for example, which have the same focal length but different apertures, measure 60.3mm vs. 115.1mm in length respectively (fully extended). Their maximum diameters are 69.2mm for the former, as compared to 89.8mm for the latter.

Focal length is crucial because it determines the lens's field of view. The longer the focal length, the narrower the area of the scene captured by the lens. This means that a lens with a short focal length such as the Canon RF 16mm F2.8 STM captures a much broader view than a telephoto lens such as the RF 1200mm F8L IS USM.

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. A distant subject will appear smaller in the frame than it does when viewed through a lens with a narrow angle of view (that is, a longer focal length).

Because a camera's sensor and image frame are rectangular, you will sometimes see three measurements given for a lens's angle of view – horizontal, vertical and diagonal (corner-to-corner). For the RF 16mm F2.8 STM, the angles of view are 98°, 74°10' and 108°10' (horizontal, vertical and diagonal), while for the RF 1200mm F8L IS USM they are 1°45', 1°10' and 2°05'. Often, however, just one angle of view is quoted, usually the diagonal.

Lenses can be divided into two types: prime and zoom lenses. Prime lenses are those with a fixed focal length, such as the Canon RF 35mm F1.8 Macro IS STM, RF 85mm F1.2L USM and RF 100mm F2.8L Macro IS USM. A fixed focal length means that the perspective of the image can be changed only by physically moving the camera closer towards the subject or further away.

In contrast, zoom lenses have variable focal lengths. The Canon RF 14-35mm F4L IS USM, for instance, offers any focal length from 14mm to 35mm, while the popular RF 24-105mm F4L IS USM offers focal lengths from 24mm to 105mm, a broad range which makes it an excellent choice for everyday photography. Meanwhile, the RF 100-500mm F4.5-7.1L IS USM is a favourite lens for wildlife photography when the distance between the camera and the subject can vary dramatically.

This versatility means zoom lenses are more convenient because you can carry just one lens to be prepared for a range of shooting situations. However, prime lenses also have great benefits, such as being smaller and lighter or offering better optical quality and larger apertures. Find out more about choosing between prime and zoom lenses.

There is another factor that affects the field of view in a given image: the camera's sensor size. APS-C sensors are physically smaller than full-frame sensors, which means APS-C cameras won't utilise the full field of view of a full-frame lens. Instead, the image will be cropped to the sensor's smaller active area. The effect of this reduced field of view is the same as zooming in, making the subject larger in the frame. This change in the framing can therefore be described in two ways: you can say the APS-C sensor introduces a crop factor or a focal length multiplier. The two are actually the same thing.

For Canon APS-C cameras, the focal length multiplier (or crop factor) is 1.6x. This means that using a 50mm lens on a Canon APS-C camera gives a field of view equivalent to that of an 80mm lens on a full-frame camera (50 x 1.6 = 80). Hence, if you use the RF 50mm F1.8 STM lens, for example, on an APS-C camera such as the EOS R7, the lens is said to have an effective focal length of 80mm. The formula is the same when you use a zoom lens, but of course the calculation starts with the focal length to which you have set the lens.

To be clear, all this also applies to RF-S lenses. These lenses are designed for use with APS-C cameras and therefore project a smaller image circle than full-frame lenses, but the focal lengths given in their names describe their optical construction, as explained above. When you fit them on APS-C cameras, you still require the same calculation to determine their effective focal lengths – so the Canon RF-S 18-45mm F4.5-6.3 IS STM lens, for example, has an effective focal length of 28.8-72mm when fitted on an APS-C camera.