Focal length of a camera lens system is a measure of how strongly it converges (focuses) light. A photographic lens is used in a camera to make images of the objects on the film or other image sensors.
Some links…
- wiki: focal length
- wiki: photographic lens
- photoxels: Focal length
- [focal length](http://www.trustedreviews.com/digital-cameras/review/2007/01/23/Digital-
Measurement of focal length is done in mm. nowadays:
- Wide-angle lens 18mm, 20mm, 28mm, 35mm
- Normal lens 50mm, 55mm
- Telephoto lens 90mm, 135mm, 200mm, 300mm