Physical Description

• Technically, focal length describes the distance between the film or sensor plane and the rear nodal point of the lens when it is focused on infinity. In a zoom lens, this distance physically changes as you turn the zoom ring. A fixed lens has a set focal length and the distance is constant.
  
  

Magnification

• Focal length affects the magnification of the image. For a 35 mm camera, a long focal length, such as 105 mm, greatly increases the scale of the image in comparison to what the eye sees. A short focal length like 28 mm widens the angle of view captured in the image. Wide-to-long zoom lenses are capable of capturing images in both manners, while fixed lenses are wide, long, or "normal" (approximating what the human eye sees).
  
  

Angle of View

• Focal length also affects the angle of view in an image. Extremely wide-angle lenses are known as fisheyes and may have a focal length of 14 mm or less. They can create spherical images capable of capturing 180 degrees of view. Long lenses such as the 300-mm narrow down the angle of view as they magnify a scene. Because tiny camera movements greatly change the composition at such a small angle of view, it can be difficult to keep a long lens still to make a sharp image.
  
  

Camera-to-Lens Relationship

• For a basic 35-mm camera, a fixed lens that approximates what the human eye sees has a focal length of about 50 mm. As film size increases, so does the normal focal length of a lens. Therefore, the normal lens for a large format camera that uses 4 by 5 inch pieces of film is about 150 mm. This same lens is considered a long lens for a 35-mm camera, so focal length effects are relative to camera format.
  
  

Advantages/Disadvantages

• Long lenses that have higher focal length numbers create a shallow depth of field. For this reason, they are often used in portraiture to make subjects stand out from their backgrounds. Wide-angle lenses of lower focal length numbers tend to be easier to focus, but they can cause distortions when used to create a close-up on a subject. They curve straight lines and make the objects closest to the lens appear larger, and are therefore not ideal for facial portraits.