Bellows Extension Calculator
How much extra exposure a close-focused view camera needs, worked out from the bellows draw or the magnification.
Rack a lens out past its infinity position and the image gets dimmer. The light from the lens is spread over a bigger circle by the time it reaches the film, so the f-number engraved on the lens stops telling the truth. Your handheld meter has no idea any of this happened. On a 4x5 shooting a tabletop still life, that gap is easily a stop and a half.
The fix is simple arithmetic. Divide the bellows draw (lens to film) by the focal length and square it. That is the bellows factor, the number to multiply your exposure time by. If you know the magnification instead, the factor is one plus the magnification, squared.
How to use it
Focus first, then measure. Run a tape from the lens board (the middle of the lens is close enough for most large format glass) to the ground glass and type that in as the draw. Enter the aperture and the time your meter gave you. The cards show the factor, the stops to add and the aperture the film actually sees.
Pick a film if the new time runs past a second. Long close-ups are where bellows loss and reciprocity failure pile up on each other, and the second one is applied after the first, the same order the reciprocity calculator uses. That is how a half second metered at f/22 turns into something you time with a watch.
A quick sense of scale
| Magnification | Bellows factor | Stops to add |
|---|---|---|
| 1:10 (0.1x) | 1.21 | 0.3 |
| 1:4 (0.25x) | 1.56 | 0.6 |
| 1:2 (0.5x) | 2.25 | 1.2 |
| 1:1 (life size) | 4 | 2 |
| 2:1 (2x) | 9 | 3.2 |
Head-and-shoulders portraits on 4x5 sit around the top of that table. Product shots and flowers land in the middle. Past life size the numbers climb fast, and so does the shutter time.
Frequently asked
What is the bellows factor?
It is how many times longer you need to expose once the lens sits further from the film than its focal length. The formula is the bellows draw divided by the focal length, squared. A 150mm lens racked out to 225mm has a factor of 2.25, which is a little over one stop.
Do I need this with an SLR and macro lens?
Usually not. A through-the-lens meter reads the light that actually reaches the film, so extension tubes, macro bellows and close-focusing lenses are already accounted for. The calculator is for view cameras and any setup where you meter with a handheld meter or a phone.
When can I ignore bellows compensation?
Below roughly a tenth of life size the loss is under a third of a stop, which negative film shrugs off. By half life size you are at about 1.2 stops, and at 1:1 it is a full two stops. Slide film starts to show it earlier.
Why does my telephoto or wide angle lens disagree with the result?
The simple formula assumes a symmetrical lens, which most large format lenses are. Telephoto and retrofocus designs have a pupil magnification other than one, so the real loss differs a little. Measure the actual draw and bracket when it matters.
Zone Light Meter for Android has the same bellows correction built in (see how it works) and folds it into the live reading along with filters, reciprocity and expired film. More on the term itself in the glossary.