How do I calculate image distance with a thin lens?

Use + and solve for di. For cm and cm, the image distance is 15 cm.

What does negative image distance mean?

With this sign convention, negative image distance means a virtual image on the same side of the lens as the object.

How is magnification calculated?

Lateral magnification is -di/do. Its magnitude tells the size ratio, while the sign describes upright or inverted orientation.

Thin Lens Calculator

Calculate image distance, object distance, focal length, magnification, image height, image type and lens power from the thin-lens equation.

Lens parameters

Lens type

Solve for

Focal length f, cm

Object distance d_o, cm

Image distance d_i, cm

Object height h_o, cm (optional)

Thin lens equation

English search intent for this page is classroom optics: solve the thin-lens equation, then interpret image distance, magnification and whether the image is real or virtual.

\frac{1}{f} = \frac{1}{d _{o}} + \frac{1}{d _{i}}

f is focal length, d_o object distance and d_i image distance.

M = - \frac{d _{i}}{d _{o}} = \frac{h _{i}}{h _{o}}

M is lateral magnification. A negative value means the image is inverted.

P = \frac{1}{f _{m}}

P is lens power in diopters when focal length is entered in meters.

Sign convention and limits

Quantity	Positive result	Negative result
Focal length	Converging lens	Diverging lens
Image distance	Real image on the opposite side of the lens	Virtual image on the object side
Magnification	Upright image	Inverted image

Physics model, not an eyeglass prescription

This calculator uses an ideal thin-lens model. It does not prescribe glasses, contact lenses or medical vision correction.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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