focal length 
noun
1The distance between the centre of a lens or curved mirror and its focus.Yet Stork determined the probable focal length of a concave mirror made by inverting and silvering the convex mirror shown in the painting....
- This produces a rounded lens with a small focal length, so the drop can focus light only to a point near itself.
- This will give a negative focal length for a convex lens of air inside water.
1.1The equivalent distance in a compound lens or telescope.He developed a general rule for the focal length of lenses and described a reflecting telescope....
- Huygens later went back to the study of optics and developed long focal length lenses used in ‘aerial telescopes.’
- One evening he was observing with a rather crude refractor (a lens telescope), with a focal length of 24 feet.