What is focal length, many new photographers ask? Focal length is just how long a lens is, right? There’s more to understanding focal length than just knowing the range of numbers. In this tutorial we’ll answer some of the common questions photographers have about focal length, as well as compare the effects different lenses can produce.
Contrary to common belief, focal length isn’t a measure of how long or short a lens is physically, but the distance in millimetres from the optical centre of a lens to the imaging sensor when the lens is focused at infinity.
Rather than being fazed by the physics, it’s easier to think of the way in which focal length affects image size.
For a camera with a full-frame sensor, for example, a standard lens (one that gives a similar perspective to the human eye) is 50mm.
Lenses with focal lengths less than 50mm are referred to as wide-angles – simply because they have a wider angle of view. Lenses with focal lengths greater than 50mm are known as telephotos, and these offer greater magnification thanks to their much narrower angle of view.
Angle of view? What’s that?
Essentially, the angle of view is the amount of a scene that a lens can take in, measured in degrees. For instance, a fisheye lens may offer an extremely wide 180° angle of view, meaning that it can capture everything in front of it (and to each side). A 200mm lens, on the other hand, will offer a much narrower angle of view of 12.3°. This allows you to fill the frame with a considerably smaller amount of the scene that you’re trying to photograph.
You mentioned a ‘full frame’ sensor earlier. Why is that relevant?
Full-frame sensors get their name because, at 36x24mm, they have similar dimensions to a frame of 35mm film. This means that they capture the full angle of view offered by a lens that’s been designed for a film or full-frame camera.
So a 75-300mm zoom lens mounted on a full-frame DSLR like the Canon EOS 5D Mark III or Nikon D800 offers a true focal length of 75-300mm.
However, the majority of cameras have sensors that are significantly smaller than full-frame.
Consequently, they’re exposed to a smaller area of the image projected by the lens, and it’s for this reason that they’re known as ‘cropped’ sensors – although they’re not really cropping the image, they’re just capturing a smaller area of the scene at the centre the lens.
Does this make a difference to how I take pictures?
Yes it does. Using the same lens at the same distance from the subject, a cropped sensor camera will capture a narrower angle of view than a full-frame camera.
This can be a problem when photographing landscapes with a wide-angle lens, as you won’t be able to get as much of the scene in the picture (at least, not without moving further away and making everything smaller in the picture).
On the other hand, it’s good news for wildlife photographers, with animals and birds appearing larger in the frame thanks to the increased effective focal length.
What do you mean by the effective focal length?
You’ll see this term, or the more frequently used ‘35mm equivalent focal length’, listed in a lens’s specs.
It provides a standard measure by which different lens and camera combinations can be compared, and it’s calculated by taking a lens’s focal length and multiplying it by the crop factor of your camera’s sensor.
For instance, the micro four thirds sensor used in an Olympus PEN camera is around half the size of a full-frame sensor. This means that a subject will appear twice as big in the frame when shot on a PEN.
To get the same magnification for a subject using a 35mm full-frame camera, you would need a lens with double the focal length.
The APS-C sized sensors found in most SLRs are slightly bigger than micro four thirds, but they still capture a smaller area than full-frame; Canon DSLRs have a crop factor of x1.6, while Nikon camera bodies are closer to x1.5.
So, a 75-300mm lens becomes a 120-480mm lens when it’s used on a camera like the Canon EOS 650D?
In terms of effective focal length, yes. But a 75-300mm lens is still a 75-300mm lens, whether it’s attached to a cropped-sensor camera or a full-frame one.
The perspective is constant, as is the image magnification – all that changes is the angle of view. To get around this problem, manufacturers also make a range of dedicated ‘digital only’ lenses.
What are digital lenses?
These are lenses that have been designed to work on cropped-sensor cameras. A crop factor still has to be applied to arrive at their effective focal length, but they’re smaller and (usually) wider than 35mm full-frame lenses.
So a 10-20mm digital lens gives an effective focal length of around 16-35mm (10-20mm x 1.6 or 1.5, depending on the camera model). Digital lenses are not compatible with full-frame bodies, as they can’t produce an image big enough to fill the larger sensor.