Lens Extension Tubes

If you compare a standard lens and a macro lens of the same focal length, you will notice that the macro lens is usually considerably longer. For example, the Zeiss Planar ZF T* 50mm f/1.4 measures 45 mm, while the Zeiss ZF Makro-Planar T* 50mm f/2 is already 75 mm long and further extends about 30 mm towards close-up.

Why do macro lenses have to be so long? The answer can be derived from equation (15), which describes the image distance h (between lens and image plane) as a function of the selected focusing distance d (between object and image plane) and focal length f. The following diagram illustrates this function for two lenses with fixed focal lengths of 50 mm and 100 mm, respectively:

image distance vs focusing distance

In contrast to what one might intuitively expect, h is actually getting longer as the focusing distance d is decreased, i.e. the lens is moving away from the image plane. Also interesting to note, h is always in the range between 2 f (for 1:1 macro photography) and f (at infinity). The length of the Zeiss ZF Makro-Planar 50mm mentioned above is thus about what we would expect. If you want a pancake lens, cut some of the macro capabilities.

To give a standard lens similar characteristics for macro photography, a lens extension tube or a bellows can be used. An extension tube is basically an empty metal ring which is attached between lens and camera body, thus extending the image distance h between lens and sensor. In contrast to a macro converter, an extension tube does not contain any optical elements and thus does not degrade the sharpness of the lens. A bellows can be considered as an extension tube of variable length. Since the lens cannot be fully retracted any more with a lens extension tube in place, it is usually not possible to focus at infinity.

To see how a lens extension tube modifies minimum and maximum focusing distances of your lens, use the lens magnification calculator and enter either the minimum focusing distance (MFD) of your lens, or a fairly large focusing distance such as 1000 m.

March 03, 2012