This setup reaches about 0.75× (1:1.33). The 36 mm sensor covers roughly 48 mm of the subject horizontally.
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. | |||||
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.
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:
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 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.
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 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.
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×.
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.
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.
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.
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.
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.
Normally no. Extra lens-to-sensor spacing shifts the focus range closer. Remove the tube to restore the lens's normal distance range.
No. The calculator runs locally in your browser and does not send its input values to a server.
