Extension Tube
A hollow spacer placed between lens and camera that shortens the minimum focus distance and increases magnification.
Formula / rule
m_added = extension / f
Measured in
mm
In depth
Because it contains no glass, an extension tube adds no optical degradation of its own — but it does cost light, since the lens is now further from the sensor. It works best on shorter focal lengths, where a given extension buys proportionally more magnification, and it is the cheapest honest route into close-up photography.
Examples in practice
A 25 mm tube on a 50 mm lens adds 0.5× magnification, turning a standard prime into a capable close-up lens.