How is bellows magnification calculated?

Magnification equals the bellows extension beyond infinity divided by the lens focal length. A 100 mm lens drawn out an extra 100 mm reaches 1.0 times, or 1:1 life-size reproduction.

What is the bellows factor and effective aperture?

As the lens extends, the same f-stop passes less light to the larger image. The effective f-number is the marked f-number times (magnification plus one), so an f/8 lens at 1:1 behaves like f/16.

How many extra stops do I need at a given magnification?

The exposure compensation in stops is two times the base-2 logarithm of (magnification plus one). At 1:1 that is exactly two stops; at 0.5 times it is about 1.17 stops. The table lists this for every row.

What does extension beyond infinity mean?

It is the amount the bellows is drawn out past the lens position that focuses at infinity. That additional draw, not the total bellows length, sets the magnification, so the tool asks for the extra extension.

Does through-the-lens metering already account for this?

Yes. A camera that meters through the lens automatically includes the bellows factor. The table is for hand-held meters, flash exposure, and large-format work where you set exposure manually.

Bellows Draw Magnification Ratio Table

When you rack a lens out on a bellows for macro work, two things change together: the subject grows on film or sensor, and the image dims. This tool prints a single reference table that ties bellows draw to magnification, reproduction ratio, effective aperture, and the exact exposure compensation in stops.

How it works

Using the thin-lens relations, with extension measured beyond the infinity-focus position:

magnification m   = extension / focal length
reproduction      = 1 : (1 / m)
effective f-number= marked f-number × (m + 1)
exposure stops    = 2 × log2(m + 1)

The (m + 1) term is the bellows factor: it is 1 at infinity, 2 at life-size, and keeps growing the deeper you go into macro territory.

Example and tips

A 100 mm lens at f/8 reaches 1:1 (1.0 times) at 100 mm of extra draw, where the effective aperture is f/16 and you must add two full stops. Build your table once for the lens you use most and tape it to the standard. For hand-held flash macro, the added stops translate directly into needing more flash power or a wider base aperture; through-the-lens metering already compensates, so this table is mainly for manual and large-format setups.