Macro Photography Magnification Calculator
Enter focal length, minimum focus distance, and extension tube length to get reproduction ratio, working distance, effective aperture, exposure compensation, and depth of field. Nothing uploaded.
Lens Settings
Macro Results
Full details
Learn more: macro magnification and extension tubes
What the tool calculates
Wikipedia's macro photography article gives the magnification from an extension tube as m = d / F, the tube length over the focal length. It also gives the effective aperture as the set f-number times (1 + m), and says a strict macro photograph is life-size or larger, 1:1 or more.
The tool uses those formulas. It starts from the magnification your lens reaches at its minimum focus distance, adds tube length / focal length, and applies (1 + m) to the aperture. Minimum focus distance is the figure lens makers quote, measured from the sensor to the subject.
Worked examples
A 100 mm macro lens with a 300 mm minimum focus distance is 1:1. At f/8 the effective aperture is f/16, which is 2 stops of light lost. Depth of field is about 0.93 mm and the frame covers 36 mm of subject on a full-frame sensor.
Add a 25 mm tube and the magnification is 1 + 25/100 = 1.25:1. The effective aperture is f/18 at f/8, the light loss is 2.3 stops and the frame covers 28.8 mm.
A non-macro 50 mm lens with a 450 mm minimum focus distance reaches only about 0.15x, near 1:7. The same 25 mm tube adds 0.5, taking it to roughly 0.65x. The tube helps most on a lens that focuses poorly up close.
Limits of the estimate
The tool treats the lens as a single thin element. A thin lens cannot reach 1:1 closer than four times its focal length, so for a shorter minimum focus distance the tool assumes an internal-focusing macro lens at 1:1. Real macro lenses change focal length as they focus, so tube gains can differ a little from e / F.
The distance shown is from the lens's optical centre to the subject. The gap from the front of the lens is smaller, by an amount that depends on the lens.
FAQ
What counts as true macro?
Wikipedia says the strict definition is life-size or larger, a reproduction ratio of 1:1 or more. Lenses at 1:2 are often called macro but are closer to close-up photography.
Do extension tubes cost image quality?
A tube has no glass in it, so it adds no optical elements. PetaPixel's guide to teleconverters for macro says teleconverters lower image quality more than tubes do because of their extra glass. The cost is light, which you see in the exposure compensation the tool reports.
Why does working distance matter for insects?
Wikipedia notes that working distance is the space between the front of the lens and the subject, and that it depends on focal length and magnification. At higher magnification there is less room, and a longer focal length gives more.