Figure shows a simple version of a zoom lens. The converging lens has focal length f1 and the diverging lens has focal length f2 = - |f2| .The two lenses are separated
Magnification of a Lens Calculator
Focal Length Calculator
Zoom Lenses
Lens Focal Length - Wavelength Opto-Electronic
Simple Formulas for the Telescope Owner - Sky & Telescope - Sky & Telescope