How does extension affect magnification?

Magnification from added extension equals the extension divided by the focal length. Adding 25 mm of tubes to a 50 mm lens gives a magnification of 25 ÷ 50 = 0.5×, or a 1:2 reproduction ratio. More extension on a shorter lens yields higher magnification.

Why do I lose light when adding extension?

Moving the lens farther from the sensor spreads the same light over a larger image circle, so less reaches each point on the sensor. This is the bellows extension factor, and it grows with magnification, which is why close-up macro shots can need two or more extra stops of exposure.

How is the exposure compensation calculated?

The light-loss factor is (magnification + 1) squared, and the compensation in stops is the base-2 logarithm of that factor, or 2 × log₂(magnification + 1). At 1× magnification the factor is 4, which is exactly two stops of extra exposure needed.

What is effective aperture in macro work?

Effective aperture is the marked f-number multiplied by (magnification + 1). At 1× magnification a lens set to f/8 behaves like f/16 for both exposure and diffraction. This is why deep macro shots need careful aperture choice to balance depth of field against diffraction softening.

Does my camera's TTL meter already correct for this?

Yes. Through-the-lens metering reads the light actually reaching the sensor, so it automatically compensates for extension loss. The calculation matters most with handheld light meters, flash guide numbers, or fully manual exposure, where nothing measures the light at the sensor for you.

Bellows & Extension Tube Exposure Calculator

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.
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