Macro & Extension Tube Calculator

Extension tubes and bellows move a lens farther from the sensor, so it focuses closer and magnifies more. They are an inexpensive way into macro photography, but they also cost light and change the effective aperture. This calculator shows the magnification you will get, or the extension you need, plus the exposure and sharpness consequences.

Magnification from extension

For a lens focused at infinity, magnification equals extension divided by focal length. A 25 mm tube on a 50 mm lens gives 0.5×, half life size; 50 mm of extension gives 1:1, life size. The same tube adds much less magnification on a long lens: 25 mm on a 100 mm lens gives only 0.25×. If the lens is already focused closer, enter its own magnification at that setting and the two are combined.

Extension for a target magnification

Choose “I want a magnification” and enter, for example, 1 for life size. The calculator gives the extension needed. A common tube set of 12, 20 and 36 mm totals 68 mm, which gives about 1.36× on a 50 mm lens.

Effective aperture

As the lens moves away from the sensor, the light spreads over a larger area, so the effective aperture becomes smaller than the number set on the lens. It equals the set f-number times one plus magnification divided by pupil magnification. At 1:1 with a symmetric lens, f/8 behaves like f/16.

Pupil magnification

Lenses are not all symmetric. Telephoto designs have a pupil magnification below 1 and lose more light at the same magnification; retrofocus wide-angles have values above 1 and lose less. If you know your lens’s value, enter it; otherwise 1 is a reasonable starting point.

Light loss and shutter speed

The bellows factor is the square of the ratio of effective to set aperture, shown in stops. If you meter with a handheld meter or calculate flash exposure, add that correction. The calculator gives the shutter speed to use, from the metered time without extension. With through-the-lens metering the camera compensates automatically.

Diffraction

At high magnification, effective apertures quickly reach f/22 and beyond, where diffraction softens fine detail. The calculator warns when that happens. Open the lens up, accept a thinner zone of sharpness, or use focus stacking, combining several shots focused at different depths.

Practical tips

Use a tripod and a focusing rail, since small movements change focus dramatically. Focus by moving the camera rather than the focus ring. Use flash or continuous light, because light loss and small apertures demand a lot of it. Manual lenses with an aperture ring are easiest on tubes without electronic contacts.

Tubes versus a macro lens

A dedicated macro lens focuses to 1:1 on its own, keeps autofocus and is corrected for close distances. Tubes are cheaper, lighter and work with lenses you already own, but optical quality depends on the lens: normal primes such as a 50 mm usually do well, while zooms can soften at the edges.

Related tools

For flash power at close distances, use the Flash Exposure Calculator. For depth of field at normal distances, the Depth of Field Calculator helps. To sharpen macro shots after stacking, try Sharpen Image.