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Focal Length

The focal length of the lens is the distance between the lens and the image sensor when the subject is in focus, usually stated in millimeters (e.g., 28 mm, 50 mm, or 100 mm). In the case of zoom lenses, both the minimum and maximum focal lengths are stated, for example 18–55 mm.

Definition of Focal Length

Lens focal length tell us the angle of view – How much of the scene will be captured —and the magnification—how large individual elements will be. The longer the focal length, the narrower the angle of view and the higher the magnification.

The Important Staff

Focal length describes the angle of view of lens.  That is, how much of a scene before us the lens captures. And in addition, how large subjects within the frame appear. The longer the focal length of a lens, the narrower its angle of view.

Field of View and Equivalent Focal Length

A fixed focal length lens, also known as a conventional or entocentric lens, is a lens with a fixed angular field of view (AFOV). By focusing the lens for different working distances (WDs), differently sized field of view (FOV) can be obtained, though the viewing angle is constant. AFOV is typically specified as the full angle (in degrees) associated with the horizontal dimension (width) of the sensor that the lens is to be used with.

Zooms vs Prime

The main difference between a prime lens and a zoom lens is of focal length. While prime lenses have a fixed focal length, zoom lenses, on the other hand, have a wider and variable focal length. Prime lenses tend to be smaller, lighter and offer better low light performance than zoom lenses.


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