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.
The bellows factor and why it matters
Photographers moving from close-focus accessories to a dedicated bellows unit quickly discover that their light meter gives incorrect exposures. The reason is geometric: as you extend the lens further from the film or sensor, the cone of light spreads over a larger image circle. The same aperture opening now illuminates a bigger area, so the light per unit area on the film drops. A meter reading taken through the lens automatically accounts for this, but an incident meter, a flash meter, or a studio strobe set manually does not.
The correction factor (sometimes called the bellows factor or exposure factor) is:
(magnification + 1) squared
At 1:1 that is (1 + 1)^2 = 4, which is exactly two stops. At 2:1 it is (2 + 1)^2 = 9, roughly three and a half stops. The table this tool generates gives you the exact stop value at every extension step so you can adjust flash power or shutter speed without guessing.
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.
Worked reference table (100 mm lens at f/8)
| Extra extension | Magnification | Reproduction | Effective f-stop | Exposure add |
|---|---|---|---|---|
| 50 mm | 0.5x | 1:2 | f/12 | +1.17 stops |
| 100 mm | 1.0x | 1:1 | f/16 | +2.00 stops |
| 150 mm | 1.5x | 3:2 | f/20 | +2.64 stops |
| 200 mm | 2.0x | 2:1 | f/24 | +3.17 stops |
These are clearly illustrative figures for a 100 mm lens. Generate your own table in the tool above for whichever focal length you use most, then tape a printed copy inside your macro kit.
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.