What is it?
The Lens Calculator works out where a lens forms an image and how big it is. It uses the thin-lens formula to show the image distance, height and magnification, whether the image is real or virtual, and the lens power.
Who is it for?
Physics students (Class 10–12) and photography learners.
How to use it
Enter the Focal length (negative for a diverging (concave) lens), the Object distance and the Object height. The answer appears as you type; there is no Calculate button. Tap Copy result to copy it, or Share on WhatsApp to send it.
Sign convention
It uses the thin-lens equation 1/f = 1/u + 1/v (real-is-positive). A negative image distance means the image is on the same side as the object (a virtual image, as in a magnifying glass).
Many school textbooks in India use the Cartesian sign convention, 1/v − 1/u = 1/f, where object distances are negative. The numbers are the same size but signs differ; follow your textbook’s convention in exams.
Reading the result
- Magnification above 1: the image is bigger. Negative magnification: the image is upside down (real image).
- Object at the focal point: no image forms (rays go out parallel).
- Lens power in dioptres = 1 ÷ focal length in metres. Spectacles of −2.0 D have a focal length of −50 cm.
Errors and how to fix them
- “Enter the focal length”
- “The object is at the focal point: the image forms at infinity”: move the object.
Open the tool: Lens Calculator
Still stuck?
Contact support. Tell us which tool or page you were using, what you tapped, what you saw, and your phone or computer and browser (for example: Android phone, Chrome). We reply by email.
Questions, or an error not listed here?
Tell us in the comments: what you were doing, the exact message you saw, and your device and browser. We read every comment and add new fixes to this article.