Macro Magnification Calculator
Compute the image magnification ratio for macro photography from focal length, focus distance and extension tube length. Use it to predict how large your subject will appear on the sensor before you shoot.
Last updated: September 2026
Formula below · 1 source (Wikipedia) · Updated Sep 2026
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About this calculator
Magnification (M) in macro photography describes how large the image of a subject appears on the sensor relative to real life. With an extension tube added to a lens, magnification is: M = focalLength / (subjectDistance − focalLength) + extensionTube / focalLength. The first term is the lens's own thin-lens magnification when it is focused at the subject distance you enter (measured from the lens, without the tube); the second is the extra magnification the tube adds, and it is exact for a thin lens. With the tube fitted the lens then focuses closer than the distance entered. Magnification is optical and does not depend on the sensor; the sensor only sets how much of the subject fills the frame (subject width covered = sensor width ÷ M), so the sensor setting does not change the result. A magnification of 1:1 means the subject is reproduced at life size on the sensor. Values above 1 (e.g. 2:1) indicate the image is larger than the subject, common with stacked extension tubes or dedicated macro lenses. Knowing M before shooting helps you frame insects, flowers, or small products precisely.
How to use
Suppose you use a 100 mm macro lens set to focus at 200 mm (its own 1:1 point) and add a 25 mm extension tube. M = 100 / (200 − 100) + 25 / 100 = 1 + 0.25 = 1.25, so the beetle is reproduced at 1.25:1. On a full-frame sensor (36 mm wide) the frame covers 36 / 1.25 ≈ 28.8 mm of subject; on APS-C (23.5 mm) it covers about 18.8 mm — a tighter crop, but the same magnification.
Frequently asked questions
What does a 1:1 magnification ratio mean in macro photography?
A 1:1 magnification ratio means the subject is reproduced at exactly life size on the camera sensor. For example, a 10 mm insect will occupy 10 mm of the sensor's width. This is the standard minimum for a lens to be classified as a true macro lens. Above 1:1 (e.g. 2:1 or 5:1) the image is larger than the subject, revealing details invisible to the naked eye, while below 1:1 the subject appears smaller on the sensor than in real life.
How do extension tubes increase magnification for macro photography?
Extension tubes are hollow rings mounted between the camera body and lens that physically increase the distance from the lens's optical center to the sensor. In thin-lens optics, moving the lens farther from the sensor forces it to focus on closer subjects and increases the image size. The magnification gain equals the extension tube length divided by the lens focal length; a 25 mm tube on a 100 mm lens adds 0.25× magnification. Longer tubes or stacked tubes provide higher magnification but reduce working distance and require more light.
Does sensor size change magnification in macro photography?
Sensor size matters because magnification is measured relative to the sensor area. A smaller APS-C sensor crops the image compared with a full-frame sensor, so the subject fills a larger proportion of the frame even though the optical magnification of the lens is unchanged. So the calculator's magnification does not change with the sensor setting; the subject width that fills the frame is sensor width ÷ magnification. In practice an APS-C shooter gets a tighter crop of the same image — useful for small subjects — without any change in optical magnification.