An extension tube is a hollow spacer that mounts between the camera body and lens, increasing the distance from the lens’s rear element to the sensor and thereby decreasing the lens’s minimum focus distance and increasing its maximum magnification. The tube contains no optical elements (no glass), so it adds no aberrations or distortion of its own. It allows almost any lens to focus closer than its native specification permits, providing a low-cost path to macro and close-up photography without buying a dedicated macro lens.
The optical principle is direct. A lens’s minimum focus distance is set by how far back the rear element can travel within the lens body. Adding a tube between lens and body pushes the rear element further from the sensor, which mathematically requires the lens to focus closer to maintain a focused image plane. The magnification ratio gained equals the tube length divided by the lens focal length. A 25mm extension tube on a 50mm lens adds 0.5x magnification; the same tube on a 100mm lens adds 0.25x. Stacking tubes (12mm + 20mm + 36mm is a common set) allows finer control over magnification.
Modern extension tubes come in two varieties: dumb tubes with no electronic connections (cheap, often under $30, but disable autofocus and aperture control on most modern lenses) and tubes with electrical contacts that pass through autofocus, aperture, and EXIF data (around $100 to $200 for name-brand sets from Kenko, Vello, Meike). For autofocus or modern electronic lenses, the contact-equipped tubes are essential, since lenses without manual aperture rings cannot stop down without camera communication.
The technique has costs as well. Extending the lens further from the sensor means light has to travel further, which reduces the effective aperture and dims the viewfinder. A 25mm tube on a 50mm lens at f/2.8 effectively becomes about f/4.2 in light transmission, costing roughly one stop. Infinity focus is lost: the lens can no longer focus on distant subjects, only on objects within a narrow range close to the new minimum distance. Depth of field at the new close-focus position is extremely shallow, often a few millimeters at f/8, requiring very precise focusing or focus stacking.
Compared to a dedicated macro lens, extension tubes are cheaper, lighter, and more flexible (any lens works), but provide less convenience. A 100mm or 105mm macro lens like the Canon RF 100mm f/2.8L or Nikon Z 105mm f/2.8 VR offers 1:1 magnification, infinity focus, weather sealing, and image stabilization in a single optic. Extension tubes give the macro behavior on existing lenses but force the user to swap tubes for different magnifications and lose infinity in the process. The economic argument for tubes is strongest when macro is an occasional rather than primary need.
Common working setups: a 12mm tube on a 50mm prime gives strong close-up capability for product shots and small objects; a 36mm tube on an 85mm or 100mm portrait lens enables half-life-size detail work without buying a dedicated macro; tubes on a telephoto zoom let wildlife photographers shoot insects and flowers from a comfortable working distance. Tubes also pair with teleconverters, though combining both pushes light loss into territory where autofocus may struggle and shutter speeds need to compensate. The cheapest entry into macro work remains a basic extension tube set plus whatever lens is already in the bag.