Bellows Extension Exposure Compensation Calculator | Stops from Extension
Enter a focal length and bellows extension to see exposure compensation in stops, the exposure factor, magnification, and the multiplier to apply to your metered shutter time. Built for macro and large-format shooters who meter the light loss when the lens racks far from the film.
💡 About this tool
When you rack a lens far out on a bellows or extension tube, the light reaching the film spreads over a larger cone and dims. Shoot at the meter's reading and the frame comes out underexposed, so close-up work needs extra exposure — the classic "bellows factor."
If you meter with a handheld or incident meter on a view camera, you have to add that correction yourself. This tool takes focal length and extension and returns the factor ((focal + extension) / focal)² and the stops log₂(factor), plus the number you multiply your metered time by. A 100mm lens racked out 50mm gives a 2.25× factor, about +1.17 stops, and 0.5× magnification. At 1:1 the extension equals the focal length, so the factor is exactly 4× — two full stops.
Compensation barely matters once the subject sits beyond roughly ten times the focal length; it only bites in the close-up and macro range.
🧐 Frequently Asked Questions
Q. Do I need this on a TTL digital camera? A. No. TTL metering reads the actual light through the lens, so it already folds in the dimming from extension. This tool's correction is for a handheld or incident meter that does not see through the lens. Apply it to both and you will overexpose.
Q. What exactly is "bellows extension"? A. The extra draw added beyond the infinity-focus position, in mm or inches — not the full lens-to-film distance, just the part you racked out. At 1:1 magnification the extension equals the focal length.
Q. Should I correct with shutter speed or aperture? A. Either gives the same exposure. Opening the aperture changes depth of field, though, so in macro work where you want to hold a given depth of field it is cleaner to add the correction to the shutter time.
Q. Does this hold for telephoto lenses? A. Telephoto designs physically extend less than their nominal focal length implies, so they do not follow this simple ratio. Treat the result as an approximation for normal large-format and macro lenses, and check the maker's data for true telephoto designs.
Q. How do I read the magnification value? A. It is the extension divided by the focal length: 0.5 means the subject records at half life-size, 1.0 means 1:1 (actual size on the film).
📚 Why the factor is a square
The bellows factor traces back to the inverse-square law. As the distance from the lens to the film grows, the same light cone spreads across a wider area, and brightness per unit area falls with the square of that distance — which is why the correction is the distance ratio squared, ((focal + extension) / focal)². Equivalently it equals (1 + magnification)².
Before TTL metering, large-format photographers measured the draw with a small metric tape and matched it against a hand-built chart for each lens they carried, then set the shutter from the chart. That is why film-era close-up work was called a craft of experience and arithmetic. Digital cameras with TTL meters automated most of it, but the same equation still governs handheld-meter workflows and exacting film shooting.