Table of Contents
The Thin Lens Equation
This equation relates focal length to object and image distances for thin lenses. Converging lenses (positive f) produce real inverted images when objects are beyond f, and virtual upright magnified images inside f. Diverging lenses (negative f) always produce virtual, upright, reduced images. This is fundamental to cameras, eyeglasses, microscopes, and telescopes.
The thin lens approximation works when lens thickness is small compared to focal length and distances. For thick lenses, principal planes must be considered. Aberrations in real lenses (spherical, chromatic, coma) are not captured by this equation.
Formula
Image Properties
| Object at | Image | Properties |
|---|---|---|
| Beyond 2f | Between f-2f | Real, inverted, reduced |
| At 2f | At 2f | Real, inverted, same size |
| f to 2f | Beyond 2f | Real, inverted, magnified |
| Inside f | Same side | Virtual, upright, magnified |
FAQ
What is lens power?
P = 1/f (meters), measured in diopters. +2D means f=0.5m. Eyeglass prescriptions use diopters. Myopia needs negative (diverging); hyperopia needs positive (converging).