How do extension tubes work?

Extension tubes fit between the camera body and lens, moving the lens further from the sensor. This shifts the focus range closer, increasing magnification. A 25mm extension tube on a 50mm lens adds 0.5x magnification. Tubes have no optical elements — they just add distance, preserving optical quality.

How much magnification does an extension tube add?

Added magnification = extension tube length / lens focal length. A 25mm tube on a 50mm lens adds 0.5x. On a 100mm lens, the same tube adds only 0.25x. Extension tubes are most effective with shorter focal length lenses.

Do extension tubes affect aperture?

Extension tubes reduce the amount of light reaching the sensor because the lens is further away. The effective aperture increases by: f_eff = f x (1 + M). At 1:1 magnification, you lose 2 stops of light. TTL metering compensates automatically, but you may need to increase ISO.

Can I stack extension tubes?

Yes. Stacking extension tubes adds their lengths together. A 12mm + 25mm tube gives 37mm of total extension. However, more extension means more light loss, potentially softer images (especially at the edges), and a very narrow focus range. Most macro photographers use one tube or a dedicated macro lens.

PhotoCalcs

Extension Tube Calculator — Added Magnification

Calculate the magnification, light loss and effective aperture when using extension tubes.

Focal Length

Lens focal length

Extension Tube Length

Total extension (can stack)

Lens Aperture

Set aperture f-number

Sensor Width

e.g. 36mm for full frame

How We Calculate This

Extension tubes increase magnification by moving the lens further from the sensor:

Added Magnification

M_added = extension length / focal length

Effective Aperture

f_eff = f-number x (1 + total magnification)

Light Loss

Loss (stops) = 2 x log2(1 + M)

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.