Focal length

The focal length of an optical system is a measure of how strongly the system converges or diverges rays of light. For common optical systems, it is the distance over which collimated rays coming from infinity converge to a point. If collimated rays converge to a physical point, the lens is said to be positive (convex), whereas if rays diverge the focus point is virtual and the lens is said to be negative (concave cf. Fig. 9). All optics used in machine vision application are overall positive, i.e. they focus incoming light onto the sensor plane.

Positive and negative lenses.
Positive (left) and negative (right) lens.

For optical systems used in machine vision, in which rays reflected from a faraway object are focused onto the sensor plane, the focal length can be also seen as a measure of how much area is imaged on the sensor (Field of View): the longer the focal length, the smaller the FoV and vice versa (this is not completely true for some particular optical systems, e.g. in astronomy and microscopy).

Focal length and field of view.
Focal length and field of view.
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