Marcus Hawkins

Angle of view is the maximum view a camera is capable of ‘seeing’ through a lens, expressed in degrees. The choice of focal length is obviously key here, with longer lenses having a narrower angle of view than shorter lenses. For instance, a 200mm lens has an angle of view of 12 degrees, while a 20mm lens offers a wider angle of view of 94 degrees on a full-frame camera. When it comes to zoom lenses, the angle of view changes according to the focal length the lens is zoomed at. However, the size of the imaging sensor inside the camera also affects the angle of view.

The majority of lenses are designed for full-frame digital SLRs. These are cameras where the sensor is approximately the size of a frame of standard 35mm film. So, with a full-frame camera, what you see is what you get (well, it is if you use Live View – more on that later). However, most digital SLRs have a sensor that’s smaller than a full-frame one, and similar in proportion to the old-school APS-C film format. Sensors this size see a smaller part of the image through the lens, and consequently everything looks bigger in the frame.

No, the focal length isn’t increased at all – a 50mm lens is a 50mm lens, regardless of the camera it’s attached to. It’s just the angle of view that gets narrower, so it appears as if you’re using a longer lens. To work out what this ‘equivalent focal length’ is – the one that would give a similar angle of view on a full-frame camera – you need to multiply the actual focal length by the ‘crop factor’ of the sensor. This crop factor varies

It provides a standardised way of comparing lenses across different types of camera. In fact, the only time you really need to take the equivalent focal length into account is when you’re choosing a lens. For wildlife and sports photography, where getting close to the subject is often impractical, cameras with smaller sensors can be a real benefit. A 400mm lens fitted to a Nikon D7100 gives you the equivalent field of view of a 600mm lens on a full-frame SLR. Attach a x1.4 teleconverter to the lens, and the effective focal length becomes a whopping 846mm ((400 x 1.4) x 1.5)! For landscape photographers using wide-angle lenses on smaller sensor cameras can be potentially