Bellows Extension & Macro Exposure Factor Calculator (Magnification, Effective Aperture)
Work out the exposure compensation for close-up and macro work from lens extension or magnification — the bellows factor, effective aperture, stops of light lost and the corrected shutter speed — for cameras without TTL metering, view cameras and extension tubes.
How it works
magnification m = (extension − f) ÷ f = tube ÷ f · exposure factor = (1 + m)² = (extension ÷ f)² · stops = log₂(factor) · effective aperture = marked f-number × (1 + m) · corrected shutter = metered × factor
A lens marks its f-numbers for a subject at infinity, when it sits one focal length from the film. Focus close and the lens moves forward, spreading the same light over a larger cone: at life size it is two focal lengths out and the image receives a quarter of the light — two stops. Cameras that meter through the lens see this automatically; a view camera, a hand-held meter or a flash guide number does not, so you apply the bellows factor yourself. The effective aperture also tells you how much diffraction you are inviting.
Worked example
A 150 mm lens on a 4×5 camera racked out to 300 mm is at 1:1 (magnification 1). The factor is (1 + 1)² = 4, two stops: a metered 1/60 s at f/16 becomes 1/15 s at f/16, or 1/60 s at f/8. The effective aperture is f/32, and depth of field is only about 4 mm.
Frequently asked questions
Does this apply to digital cameras and modern SLRs?
Metering through the lens takes care of it, so you only need the factor when using a hand-held or flash meter, manual flash with guide numbers, or a view camera. The effective-aperture consequence for diffraction applies to every camera.
What about internal-focusing macro lenses?
Many change focal length as they focus close, so the physical extension is not visible; the (1 + m)² rule still holds for the light loss with m the actual magnification, which is why a 100 mm macro reports f/5.6 as f/11 at 1:1.
How do I measure magnification quickly?
Focus on a ruler: the millimetres visible across the frame width divided into the sensor or film width gives m (36 mm frame showing 18 mm of ruler = 2×). On a view camera, magnification = (bellows draw ÷ focal length) − 1.
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