Lens Magnification

This formula tells you how large a subject’s image forms on the sensor relative to the real subject size when using a thin lens. It comes from the standard lens geometry: as the subject moves closer to the focal length, the image distance grows quickly, so magnification rises sharply; if the subject is far away compared with the focal length, magnification becomes small. The quantity is dimensionless, and the result is often used as a size ratio rather than a percentage.

Use this when you need to estimate how big a subject will appear on a sensor or film at a given shooting distance, especially in close-up, macro, or product photography. It is also useful for comparing lenses or planning framing before taking the shot.

• Using subject distance in metres without converting it to millimetres first
• Using the formula when the subject distance is measured from the sensor plane but then mixing it with lens-to-subject distance instead
• Forgetting that the result is a ratio, so 0.5 means 0.5× magnification, not 50×