Calculate the effective aperture.
Determine the actual light-gathering ability for macro shots with bellows factor, exposure compensation and pupil ratio – including reversed mounting.
So your exposure is right even when the lens is extended far and the nominal f-number no longer describes reality.
- ◆Bellows factor
- ◆Light loss analysis
- ◆Reversed mounting
- ◆Pupil ratio
Effective Aperture Calculator
Calculate the effective aperture for macro and close-up photography. The effective aperture takes the bellows factor into account and shows the actual light intensity.
1:1 = 1.0, 1:2 = 0.5, 2:1 = 2.0, etc.
For extreme macro shots with a reverse-mounted lens
The effective aperture is the actual light intensity in close-up shots. It differs from the nominal aperture (set on the lens) due to the so-called bellows factor or extension factor.
Nominal aperture: The aperture set on the lens, valid for infinity focus.
Effective aperture: The actual light intensity taking the changed optical conditions in close-up shots into account.
Standard calculation:
Effective aperture = nominal aperture × (reproduction ratio + 1)
With pupil ratio:
Effective aperture = nominal aperture × (reproduction ratio / pupil ratio + 1)
Retro position:
Effective aperture = nominal aperture × (reproduction ratio × pupil ratio + 1)
Exposure metering: Use the effective aperture for manual exposure calculations or with external light meters.
Flash photography: The effective aperture is crucial for correct flash power in macro shots.
Depth of field: The actual depth of field depends on the effective aperture, not on the nominal setting.
Nominal vs. effective – where the difference comes from.
In macro and close-up shots the optical situation changes significantly. The lens has to move further from the sensor to focus on nearby objects – with two important consequences.
Less light reaches the sensor.
What the factor tells you.
Three formulas, one calculation.
This is how magnification at the lens turns into the real aperture, exposure compensation and bellows factor.
Three magnification tiers compared.
This is what exposure compensation and effective aperture look like for the common macro ratios in concrete terms.
Close-up 1:4
Nominal: f/8
Effective: f/10 (f/8 × 1.25)
Compensation: +0.6 EV
Factor: 1.25×
Macro 1:1
Nominal: f/8
Effective: f/16 (f/8 × 2)
Compensation: +2.0 EV
Factor: 2.0×
Extreme macro 3:1
Nominal: f/8
Effective: f/32 (f/8 × 4)
Compensation: +4.0 EV
Factor: 4.0×
What happens during real exposure.
TTL vs. external meter.
TTL metering: modern cameras meter "Through The Lens" and automatically take light loss into account. Manual corrections are usually unnecessary.
External metering: with a handheld light meter you have to apply the exposure compensation manually.
Flash photography: the effective aperture is decisive for correct flash power in macro shots.
Exit vs. entrance pupil.
The pupil ratio describes the size relationship between the exit and entrance pupil of a lens.
Symmetric lenses: pupil ratio = 1.0 (typical for macro lenses).
Asymmetric lenses: pupil ratio ≠ 1.0 (telephoto, retrofocus wide-angle).
When the lens sits backwards.
For extreme magnifications the lens is mounted reversed. The calculation of the effective aperture changes slightly as a result.
Eight hints for macro photographers.
Answers to common questions.
Related tools.
These calculators match the current tool and extend your workflow.
Solid photography knowledge.
Calculators give you the number. On the blog and in 1:1 coaching I turn theory into results you can see in your images.