Focal length can create image distortion.
Focal length is an attribute of a camera lens that controls an image's magnification and angle of view. Camera manufacturers list focal length in millimeters and usually print it on the outside of the lens. It is different from the number written in millimeters around the glass of the lens that is followed by a circle with a slash through it; this is the filter diameter of your lens. Focal length can affect the depth of focus in your image and even create distortion.
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.
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