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focal length

This is some basic information about focal length of lenses.

The Focal Length is the magnifying power your lens. It is measured in millimetres (mm). A 50mm lens is called a "standard" lens because it has about the same magification as the human eye.

Here are some example images of the Grand Trunk Railway bridge in Prince George, taken at different focal lengths.

24mm (wide angle)
50mm (standard)
100mm (telephoto)
400mm (telephoto)

As you can see, there is a dramatic decrease in field of view as the focal length (magnification) increases.