Thin Lens Equation Calculator
Enter exactly two of focal length, object distance and image distance to solve the thin lens equation 1/f = 1/do + 1/di, with the magnification and image type.
How to use the thin lens calculator
- Choose which two quantities you know: focal length, object distance or image distance.
- Enter those two values using the real-is-positive sign convention.
- Read the solved third value, the magnification and whether the image is real or virtual.
Examples
Converging lens, distant object
f = 10 cm, object distance do = 30 cm
image distance di = 15 cm, magnification = -0.5 (real, inverted)
Object inside the focal length
f = 10 cm, object distance do = 5 cm
image distance di = -10 cm, magnification = 2 (virtual, upright)
Frequently asked questions
What is the thin lens equation?
It relates focal length f, object distance do and image distance di by 1/f = 1/do + 1/di. Provide any two and the third is fixed.
What sign convention does this use?
Real is positive. A converging lens has a positive focal length, an object in front of the lens has a positive object distance, and a real image on the far side has a positive image distance.
How is magnification calculated?
Magnification m = -di / do. A negative value means the image is inverted, a positive value means it is upright, and the size ratio is the absolute value.
What does a negative image distance mean?
A negative image distance means the image is virtual and forms on the same side as the object, so it is upright and cannot be projected on a screen.
Why must I provide exactly two values?
The equation has three quantities linked by one relation. With two known, the third is determined. Giving one is not enough and giving three is over specified.
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