Thin lens: image distance and magnification

Formula and example

How it is calculated

1/f = 1/g + 1/b; magnification = −b/g

Worked example

f = 10 cm and g = 30 cm give b = 15 cm. A magnification of −0.5 describes an inverted image half the size of the object.

Things to check

• In this sign convention, a positive image distance indicates a real image and a negative distance indicates a virtual image.
• The approximation applies to thin lenses and paraxial rays. An object in the focal plane has its image at infinity.

Sources and references

• OpenStax: Thin lenses ↗