Focal length doesn’t actually refer to the length from front to back of your lens; it is in fact the distance between the optical centre of a lens and the imaging sensor of the camera. The focal length of a lens changes the angle of view which impacts the image that is ultimately captured. A shorter focal length will have a wider angle of view allowing a greater area of the subject to be captured, whereas a longer focal length has a narrower field of view resulting in greater magnification.
It’s easiest to remember that the longer the focal length the smaller the field of view and the larger the subject appears to be.
A ‘standard’, or ‘normal’ lens (so called because it closely approximates the human field of view) is 50mm. Broadly speaking, anything over this becomes telephoto, while anything under becomes wide-angle.
The focal length of a lens is fixed for non-zoom lenses – also known as prime lenses. For example, the EF-35 F/1.4 L II
will always have a 35mm focal length. With zoom lenses, the focal length changes between a set range. For example, the EF 24-70 F2.8 L USM II
will have a focal length between 24mm and 70mm, depending on much the lens is zoomed in or out.
One important factor to bear in mind with focal length is that the size of the imaging sensor will also affect the angle of view. For example, all of Canon’s EF lens range are designed to be used on ‘Full Frame’ cameras, ie, a camera with a sensor that is the same size as the 35mm film format. When you put that lens on a camera with a smaller ‘APS-C’ sized sensor, such as Canon’s entry to mid level range EOS cameras, the image will be cropped slightly, giving a telephoto effect. Because of this, you’ll often hear photographers using the term ‘crop factor’ and ‘35mm equivalent’, to explain how the imaging sensor size has changed the ‘effective focal length’.