When choosing focal length, consider the following:

Cost: a lens with a shorter focal length is less expensive.

Depth of field capability: A longer focal length provides less depth of field; a shorter focal length lens provides more depth of field.

Distortion: A longer focal length lens provides a less distorted image. Gauging and pattern matching tools are sensitive to distortion. In the image to the right, notice the image distortion from the 4 mm lens.