The angle-of-view is how wide an area is seen by a given lens.

Angle-of-view is determined by two factors, the focal length of the lens, and the area of light sensitive material used to capture your image. That means, when your lens becomes longer the angle of view is reducing and the final image will contain lesser area of the image plain. This is what gives the effect of zooming in or enlarging what is being seen to a particular area of interest.

As a general case when the focal length becomes twice the original the angle of view becomes half the original. Do remember that this can vary with the lens design. Taking another general case into account the aperture also reduces with the increase in focal length, that is there is reduction of total light coming into to the image sensor, this again is mostly dependent on the lens design but for any given design when you are reducing the angle of view there will be a decrease in the amount of light falling on the sensor. This is the reason why small angle of view lens use higher quality optics like the Canon L series.

The four photos below have been taken from the same distance from the object but at increasing focal length and thus smaller angle of views.

The above set of photographs should explain the why part, the primary reason for reducing angle of view is to attain higher zooming ability.

Different sensor formats also affect the angle-of-view. The 50mm standard lens on a 35mm camera would be considered to be a wide-angle lens on a larger format camera such as a 6×7 roll film camera, and on smaller formats 50mm is a telephoto.

This difference in the angle-of-view will also affect the depth-of- field. Smaller format cameras will have more in sharp focus for a particular angle-of-view than larger format cameras because the focal length of the lens used will be shorter.

The focal length table below gives the equivalent focal lengths of lenses on a compact digital camera with a 2/3inch digital sensor, through to a 5×4in large format camera.

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