Extension tubes and bellows let an ordinary lens focus far closer for macro work, but moving the lens away from the sensor magnifies the subject and dims the image. This calculator tells you how much extra exposure to add and how your aperture really behaves.
How it works
Adding extension between the lens and sensor increases magnification:
magnification = total extension ÷ focal length
Because the same light now covers a larger projected image, less reaches the sensor. The light-loss (bellows extension) factor and the compensation in stops are:
exposure factor = (magnification + 1)²
compensation (stops) = 2 × log₂(magnification + 1)
The marked aperture also loses effective brightness:
effective aperture = marked f-number × (magnification + 1)
Worked example
A 50 mm lens with 25 mm of extension tubes set to f/8:
- magnification = 25 ÷ 50 = 0.5× (1:2 reproduction)
- factor = (0.5 + 1)² = 2.25 → +1.17 stops
- effective aperture = 8 × 1.5 = f/12
So you multiply the metered time by 2.25, or open up about 1.2 stops, and treat the lens as f/12 for depth of field and diffraction.
Tips and notes
- Stacking tubes adds their lengths together; bellows let you set any draw on a scale.
- At 1× magnification you always lose exactly two stops, the most-quoted macro figure.
- Watch effective aperture for diffraction: a marked f/16 at 1× becomes f/32, which can soften fine detail.
- TTL metering already corrects for this; the numbers matter most with handheld meters and flash.
- All maths runs locally in your browser; nothing about your gear is uploaded.
Extension tubes versus bellows: which to choose
Both devices add extension between the lens and sensor to achieve magnification beyond the lens’s normal minimum focus distance. The practical difference is flexibility versus convenience.
Extension tubes are fixed-length metal rings. A typical set comes in three lengths — for example 10 mm, 20 mm, and 36 mm — and you stack them to get the extension you need. They are rigid and well-sealed, pass most electronic contacts through to the camera (so autofocus and EXIF data work on compatible systems), and are relatively inexpensive. The limitation is that extension is only adjustable in fixed steps.
Bellows are accordion-like devices with a continuous rack-and-pinion draw, letting you set any extension from near-zero to maximum in fine increments. This is ideal for precise magnification control, focus stacking setups, and copy-stand work. The trade-off is that bellows are larger, heavier, tend not to pass electronic contacts (so manual focus and manual aperture control are required), and are more expensive.
For field macro work most photographers prefer tubes; for studio, copy, and extreme close-up work on a stand, bellows give finer control.
The diffraction problem at high magnification
As effective aperture increases with extension, diffraction becomes a serious concern. A marked aperture of f/11 at 1× magnification is effectively f/22 at the sensor. Diffraction softening begins to be visible to most photographers at effective apertures around f/16 and becomes clearly resolution-limiting around f/22 to f/32.
This creates a balancing act: you need sufficient depth of field to keep your subject sharp (which favours smaller apertures), but you cannot stop down so far that diffraction softens the entire image. The general advice for macro work on a modern high-resolution sensor is to avoid effective apertures beyond f/16 to f/22. If you need more depth of field, use focus stacking — take multiple shots at different focus distances and blend them — rather than stopping down further.