Extension Factor Calculator.
Calculate precise aperture adjustments for extension tubes, filters and converters – so your exposure stays right even with extra gear.
The factor becomes a number of stops, which becomes a concrete recommendation for aperture, ISO or shutter speed.
- ◆Logarithmic
- ◆With examples
- ◆All accessory types
- ◆Stackable
Aperture Stops Extension Factor Calculator
Calculate aperture adjustments for extension factors with extension tubes, filters and converters
Input parameters
>1 = light loss (extension tubes), <1 = light gain (filters/converters)
Used to calculate the new aperture after adjustment
Calculation basis:
- • Formula: aperture stops = ln(extension factor) ÷ ln(2)
- • Factor > 1: open aperture (smaller f-number)
- • Factor < 1: close aperture (larger f-number)
- • 1 stop = doubling/halving of the amount of light
- • Each stop = √2 times difference in aperture value
Results
What are extension factors?
- • Measure of light change caused by additional equipment
- • Extension tubes: factor > 1 (light loss)
- • ND filters: factor < 1 (light loss)
- • Teleconverters: factor < 1 (light loss)
- • Calculation enables precise exposure correction
Extension factors and stops.
Extension factors describe how the amount of light changes due to extra gear – the ratio of available light with and without the accessory.
What the factor tells you.
- Factor = 1: no change in light
- Factor > 1: less light at the sensor
- Factor < 1: light reduction by filter
- Factor = 2: light is halved (1 stop)
- Factor = 4: light quartered (2 stops)
Where factors come from.
- Extension tubes: for macro photography
- ND filters: for long exposures
- Polarisers: for reducing reflections
- Teleconverters: for extending focal length
- Close-up filters: for close-up shots
Exposure compensation via: aperture (depth-of-field effect), ISO (noise effect), shutter speed (motion blur) or a combination of all three.
From factor to stops.
A logarithmic conversion turns the linear extension factor into photographic stops – the language you can dial in directly on the camera.
Extension tube with factor 4.
Stops = ln(4) ÷ ln(2) = 1.386 ÷ 0.693 = 2 stops
From f/5.6 → f/2.8 (open up 2 stops to compensate for light loss)
Alternative: ISO 100 → 400, or shutter speed from 1/125 s → 1/30 s.
Extension factors of typical gear.
A quick overview for common filters, converters and tubes – so you don't need the calculator for every standard case.
| Gear | Extension factor | Stops | Adjustment |
|---|---|---|---|
| 12 mm extension tube | ~2× | +1 stop | Open the aperture by 1 stop |
| Macro extension tube set | ~4× | +2 stops | Open the aperture by 2 stops |
| ND filter 1 stop | 0.5× | −1 stop | Close the aperture by 1 stop |
| ND filter 3 stops | 0.125× | −3 stops | Close the aperture by 3 stops |
| Polariser | ~0.25× | −2 stops | Close the aperture by 2 stops |
| 1.4× teleconverter | ~0.5× | −1 stop | Close the aperture by 1 stop |
| 2× teleconverter | ~0.25× | −2 stops | Close the aperture by 2 stops |
| Close-up filter +2 | ~1.5× | +0.6 stops | Open slightly |
Five hints from practice.
Three typical scenarios.
From macro setups to long exposure to wildlife – this is what the maths looks like in the real world.
Macro with extension tubes
Situation: 25 mm extension tube on a 100 mm macro lens, extension factor = 3.5
Calculation: ln(3.5) ÷ ln(2) = 1.8 stops
Correction: aperture from f/8 → f/4 (open about 2 stops)
Alternative: ISO 200 → 800 or 1/60 s → 1/15 s.
Landscape with ND filter
Situation: ND 3.0 filter (10 stops), extension factor = 0.001
Calculation: ln(0.001) ÷ ln(2) = −10 stops
Correction: shutter speed from 1/125 s → 8 s.
Result: long exposure with motion blur for water or clouds.
Wildlife with teleconverter
Situation: 2× teleconverter, extension factor = 0.25
Calculation: ln(0.25) ÷ ln(2) = −2 stops
Correction: ISO from 400 → 1600 (increase 2 stops)
Benefit: double the focal length at acceptable noise increase.
Answers to common questions.
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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.