The Thin Lens Equation

Key Formula

	f	focal length of lens	m
	do	object distance	m
	di	image distance	m

Tips to Remember

• The object distance d_o and the image distance d_i are both measured from the lens. In your standard case with a convex lens, the lens will be between the object and the image, and both d_o and d_i will be considered positive.
• Be careful with the algebra in the fractions. If d_o and d_i are both 8 cm, that doesn’t make the focal length 16 cm. See the example at the right for the correct algebra.