Camera College
If you want to know more about lenses and how ‘focal length’ affects your pictures, read on to find out. Don’t miss next month’s instalment either, as we’ll be looking at how to play with perspective…
How does focal length affect pictures?
One of the main advantages of taking picture with an SLR or a mirrorless camera is that you can change the lens to suit a particular photo opportunity. The focal length you choose can have a huge impact on your pictures, allowing you to squeeze more into the shot or narrow the view so that small objects are larger in the frame. The focal length isn’t the physical length of a lens: rather, it’s the distance from its optical centre to the camera’s imaging sensor when the lens is focused at infinity. It’s measured in millimetres, with the range between 35-50mm being considered the ‘standard’ focal length, as it roughly matches your field of vision. Wide-angle lenses have shorter focal lengths, with long or ‘telephoto’ lenses having focal lengths of 70mm and upwards.
When we talk about focal length, it’s really the angle of view that we’re concerned with. This is a way of describing how much of a scene a lens can ‘see’. As its name suggests, a wideangle lens is capable of taking in a great deal. An ultra-wide ‘fisheye’ lens, for example, may be able to swallow a full 180°. As a result, wide-angle lenses are the natural choice when you want to squeeze a lot in but are unable to physically move farther away to do so. They allow you to show a subject and its surroundings, and they are the go-to lens for landscapes, architecture and interiors.
Although they aren’t referred to as ‘narrow-angle’
lenses, telephotos offer progressively narrower angles of view – sometimes of just a few degrees. This has a magnifying effect, making distant subjects appear much bigger in the frame, and it’s the reason why lenses of 200mm and upwards are used for wildlife and sports photography. The drawback to having such a narrow angle to work with is that it’s easy to lose track of the subject if it moves: a tiny shift in the position of the camera can have a huge effect on which part of the scene is picked out.
One thing to bear in mind is that the angle of view displayed in the camera’s viewfinder and rear screen (and recorded in the final picture) is also affected by the size of the camera’s imaging sensor. Some cameras have a ‘full-frame’ sensor (measuring 36 x 24mm) which captures the full angle of view, but the majority use smaller sensors that measure around 24 x 16mm (‘APS-C’) or 17 x 13mm (‘Four Thirds’). A small sensor sees less of the image projected by the lens, so the same lens gives a narrower view when it’s attached to an APS-C camera than when it’s on a full-frame model. In effect, the focal length is magnified – by 1.5x
A small sensor sees less of the image projected by the lens, so the same lens gives a narrower view when it’s attached to an APS-C camera than when it’s on a full-frame model
for APS-C cameras (or x1.6 for Canon EOS APS-C cameras), and by x2 for Four Thirds cameras. This is why the actual focal length and the ‘effective’ or ‘equivalent’ focal length are often quoted alongside each other in a lens’s specifications.
The ‘crop factor’ of an APS-C or Four Thirds sensor can be an advantage when it comes to telephoto photography. For instance, it gives a 300mm lens an effective focal length of approximately 450mm when fitted to an APS-C camera. Conversely, the crop factor reduces how much you can fit into the picture when you use a wide-angle lens. A 24mm lens on a Four Thirds camera gives a similar angle of view to 48mm on a full-frame camera – much closer to the view of a standard lens. Broadly speaking, there are two different types of lenses available: primes and zooms. Prime lenses have fixed focal lengths, while zooms give you a range of different focal lengths in one lens. Zoom lenses are more convenient than prime lenses, as you can try a range of different looks without having to change the lens. Not only does this save time, it also reduces the risk of dust getting inside the camera.
The main advantage of prime lenses is that they are usually smaller and easier to handle than the equivalent zooms. They also tend to have larger maximum apertures for a given focal length, which can be an advantage if you want to separate the subject from the background or when you’re shooting in low light.
Zoom lenses are more convenient, as you can try a range of different looks without having to change the lens. This saves time and can reduce the risk of dust getting inside the camera