Lens Focal Length Explained

The focal length of a given lens determines the size of the recorded image on your digital sensor.  Lens focal length was confusing to me personally, until it was explained to me in basically this way.

Generally, the longer the focal length of the given lens, the larger the subject will be in the image.  A lens with a focal length of 100mm, will produce an image with the subject twice as high and twice as wide as the subject produced in an image with a 50mm lens.  Likewise, a lens with a focal length of 400mm will produce an image with the subject four times as high and wide as the subject produced in an image with a 100mm lens.

While varying the focal length of a lens also varies the amount of light which reaches the sensor, the f-number combines both the focal length and aperture into one simple number.  For example, an aperture of f/8, which represents a specific amount of light reaching the film, already takes into account the lens focal length.  So, any lens set at an aperture of f/8 lets the same amount of light in as any other lens set to f/8.  This is true for both film and digital cameras.

A 50mm lens, when used on a 35 mm film camera, is called a “normal” lens, since it captures the scene in front of you in a way that appears normal to the human eye.  A wide angle lens has a smaller focal length, which therefore takes in more of the scene, and makes things appear further away than they really are.  A telephoto lens has a longer focal length which takes in less of the scene and makes things appear closer than they really are.  A zoom lens lets you vary the lens focal length.

Digital cameras are different in that the image sensor is usually much smaller than the imaging area of 35mm film.  So, the focal length which produces an image which can be considered “normal” is actually quite a bit smaller than 50mm.  To make things much easier for consumers who were already familiar with 35mm film focal lengths, digital camera manufacturers describe the focal length of their cameras by referencing the 35mm film equivalent.