Macro Magnification Calculator — Extension Tubes, Focal Length & Working Distance
Lens and extension inputs
Estimated macro setup
This setup reaches about 0.75× (1:1.33). The 36 mm sensor covers roughly 48 mm of the subject horizontally.
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Compare extension-tube lengths
Rows use the lens, sensor, and native specifications above. The current custom length is included automatically.
| Total extension | Added magnification | Total magnification | Reproduction ratio | Subject width | Est. working distance |
|---|---|---|---|---|---|
| Enter valid values to compare tube lengths. | |||||
How to use this macro magnification calculator
- Enter actual focal length. A 50 mm lens stays 50 mm in this calculation on any sensor format; do not enter its crop-equivalent field of view.
- Add the tube lengths. A 12 mm and 20 mm tube form 32 mm of total extension. Empty extension tubes contain no optical power.
- Enter native maximum magnification. Find “maximum reproduction ratio” or “maximum magnification” in the lens specification. Enter 0 only when modelling a lens focused at infinity.
- Add native working distance when known. This anchors the physical clearance estimate to the real lens instead of pretending its optical principal plane is at the front glass.
- Set sensor width and aperture. Sensor width gives subject coverage; marked aperture gives a simplified effective-aperture and light-loss estimate.
- Use the table to compare stacks. Check whether extra magnification is worth the reduced clearance, darker effective aperture, and loss of infinity focus.
Formulas and assumptions
The primary magnification estimate follows the standard extension-tube planning rule documented in B&H's extension-tube guide. The object-distance relationship is derived from the thin-lens and magnification equations presented by OpenStax Physics.
Total magnification ≈ native magnification + added magnification
Reproduction width = sensor width ÷ total magnification
Thin-lens subject distance = focal length × (1 + 1 ÷ total magnification)
Required extension = focal length × (target magnification − native magnification)
Effective f-number ≈ marked f-number × (1 + total magnification)
Light loss ≈ 2 × log₂(1 + total magnification) stops
The calibrated working-distance estimate first compares the thin-lens subject distance at native maximum magnification with the published native working distance. It then applies the calculated change in optical subject distance to that real baseline:
+ new thin-lens subject distance − native thin-lens subject distance
All distances use millimetres and magnification is unitless. The model assumes a fixed focal length, fixed principal-plane relationship, a thin lens, and pupil magnification of 1. It is best used to compare possible setups, not to certify mechanical clearance.
Working distance: what the estimate can and cannot tell you
Working distance is the physical gap from the front of the lens barrel or front element to the subject. Subject distance in the thin-lens equation is measured from an optical principal plane. Those reference points are not interchangeable.
Real photographic lenses contain many elements. Internal-focus and floating-element designs can shorten effective focal length, move principal planes, and change pupil magnification at close focus. Lens hoods, recessed front elements, filters, and tube barrels also affect usable clearance. The optional native working-distance input improves the estimate by calibrating it to one known point, but it cannot model every internal change.
How to measure actual magnification
Photograph a millimetre ruler square to the optical axis at the closest usable focus. If a 36 mm-wide sensor frames 48 mm of ruler, magnification is 36 ÷ 48 = 0.75×. This direct measurement automatically includes the behaviour of the real lens and extension stack.
Sensor size and crop factor
Sensor format does not multiply optical magnification. At 1×, a 10 mm object still forms a 10 mm image on any sensor. A smaller sensor simply records a narrower part of that image, which is why this calculator uses physical sensor width for subject coverage and never applies crop factor to the reproduction ratio.
Macro extension-tube FAQ
How much magnification does an extension tube add?
A common planning estimate is extension divided by focal length. A 25 mm tube on a 50 mm lens adds about 0.5×, while the same tube on a 100 mm lens adds about 0.25×.
Do extension tubes change focal length?
The tube itself has no glass and no optical power; it increases lens-to-sensor spacing. However, a real internal-focus lens may change effective focal length while focusing, so its measured result may differ from the simple fixed-focal-length model.
What does 1:1 or 1× macro mean?
At 1:1, a subject projects onto the sensor at life size: a 10 mm subject creates a 10 mm sensor image. At 0.5× the image is half life size; at 2× it is twice life size.
Is subject distance the same as working distance?
No. Optical subject distance starts at a principal plane, while working distance starts at the physical front of the lens. Use the calibrated working-distance result only as a planning estimate.
Why does a shorter focal length gain more magnification?
The same extension is a larger fraction of a short focal length. That raises magnification efficiently, but it can also leave very little room for lighting or the subject.
Do extension tubes reduce light?
Yes. At close focus, the image is projected farther behind the lens and illumination falls. The displayed effective aperture assumes a symmetric thin lens; through-the-lens metering often compensates automatically, but flash and manual exposure may need testing.
Can I still focus at infinity with a tube attached?
Normally no. Extra lens-to-sensor spacing shifts the focus range closer. Remove the tube to restore the lens's normal distance range.
Are my lens settings uploaded?
No. The calculator runs locally in your browser and does not send its input values to a server.