About the Macro Magnification Calculator
This macro magnification calculator shows how close you can get with extension tubes, close-up (diopter) filters, or both. Enter your lens’s focal length, the magnification it already reaches at the focus setting you will use, the total length of extension tubes and the strength of any close-up filter, and the calculator returns the new magnification and reproduction ratio, the working distance, the size of the area you will fill the frame with, and how much light the extension costs.
It is aimed at photographers getting into macro on a budget — turning a 50 mm prime into a 1:2 macro with a cheap tube set — and at macro shooters pushing a dedicated 1:1 lens beyond life size. Comparing the field of view with the size of your subject (an insect, a coin, a flower) tells you which combination you need before you buy.
The maths uses a thin-lens model, which is accurate for simple unit-focusing lenses. Internal-focusing zooms and many modern macro lenses shorten their focal length when focused close, so real results with them can be lower; treat the numbers as a planning estimate and check the effective aperture your camera reports.
With the default inputs, the magnification is 0.5 ×. Change any value above to recalculate instantly.
How to use the macro magnification calculator
- 1Enter the focal length of your lens.
- 2Enter the magnification the lens gives at the focus setting you will use (0 if focused at infinity).
- 3Add the total length of your extension tubes and the strength of any close-up filter.
- 4Choose your sensor size and the aperture you plan to use.
- 5Read the magnification, field of view, working distance and effective aperture.
Formula and method
In a thin lens, magnification equals (image distance − focal length) ÷ focal length. A lens set to magnification m₀ sits f(1 + m₀) in front of the sensor; extension tubes add their length e to that distance, so with no filter the magnification becomes m₀ + e ÷ f. A close-up filter of D dioptres adds optical power, so the combined focal length is 1000 ÷ (1000/f + D) mm, and magnification is the image distance divided by that combined focal length, minus one.
Subject distance is the image distance divided by the magnification. Field of view is the sensor size divided by magnification. Moving the lens away from the sensor spreads the light over a larger area, so the effective aperture is N × v ÷ f — which equals N(1 + m) with tubes alone — and the light loss is 2 log₂(v ÷ f) stops. A close-up filter shortens the combined focal length instead of adding extension, so it raises magnification without dimming the image. This assumes a symmetrical lens (pupil magnification of 1).
- f
- Lens focal length (mm)
- m₀
- Magnification of the lens alone at its focus setting (0 at infinity)
- e
- Extension tube length (mm)
- D
- Close-up filter strength (dioptres)
- P
- Combined optical power (dioptres)
- m
- Resulting magnification
- N, N_eff
- Marked and effective f-number
Worked examples
50 mm lens at infinity with a 25 mm tube
A 25 mm tube on a 50 mm lens adds 25 ÷ 50 = 0.5× magnification, giving half life size (1:2). The subject sits 75 ÷ 0.5 = 150 mm from the lens, a full-frame sensor covers 72 × 48 mm, and f/8 behaves like f/12 — about 1.17 stops of light lost.
100 mm macro at 1:1 plus 36 mm of tubes
At 1:1 the lens is 200 mm from the sensor; 36 mm more makes 236 mm, so magnification rises to 236 ÷ 100 − 1 = 1.36×. The frame now spans just 26.5 mm, and f/8 becomes about f/18.9 — nearly 2.5 stops darker.
+4 close-up filter on an 85 mm lens
With the lens at infinity, a +4 filter focuses at 1000 ÷ 4 = 250 mm, and magnification equals focal length × dioptres ÷ 1000 = 85 × 4 ÷ 1000 = 0.34×. The frame covers about 106 mm across, ideal for flowers, and because there is no extension f/8 stays f/8.
Frequently asked questions
How much magnification does an extension tube add?+
An extension tube adds roughly its length divided by the focal length: a 25 mm tube adds 0.5× on a 50 mm lens but only 0.125× on a 200 mm lens. That is why tubes work best on shorter lenses, and why a long lens needs a lot of extension to reach life size.
What does 1:1 magnification mean?+
1:1, or 1×, means the subject is projected onto the sensor at life size: a 10 mm insect covers 10 mm of the sensor. On a full-frame camera a 1:1 frame covers 36 × 24 mm of the subject; 1:2 (0.5×) covers 72 × 48 mm.
Are close-up filters or extension tubes better?+
Close-up filters add no light loss and work best on longer lenses, but cheap single-element filters can soften the edges. Extension tubes contain no glass, so image quality is unchanged, but they cost light and work best on shorter lenses. Both can be combined.
Why does my camera show a smaller aperture with tubes?+
Moving the lens further from the sensor spreads the same light over a bigger image circle, so less reaches each pixel. With tubes the effective aperture is about N × (1 + m), so at 1:1 an f/8 lens behaves like f/16. Cameras metering through the lens compensate automatically.
What is a dioptre on a close-up filter?+
A dioptre is the optical power of a lens, equal to 1000 divided by its focal length in millimetres. A +2 filter has a 500 mm focal length; with your lens at infinity it focuses at 500 mm, and stacking filters adds their dioptres together.