If you compare a standard lens to a macro lens of the same focal length, you will notice that the barrel of the macro lens is usually considerably longer. For example, the Zeiss Planar ZF T* 50 mm f/1.4 measures 45 mm, while the Zeiss ZF Makro-Planar T* 50 mm 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 (M11), 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 diagram shown in fig. 4 illustrates this function for two lenses with fixed focal lengths of 50 mm (blue) and 100 mm (red), respectively.
In contrast to what one might intuitively expect, as the focusing distance d is decreased for close-up photography, the image distance h is actually getting longer, 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 50 mm mentioned above is thus about what we would expect (give and take a little for lens construction and the distance between flange and image sensor). If you want a flat pancake lens, cut some of the macro capabilities.
To give a standard lens some macro capabilities, 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. Extension tubes are available from some of the original lens manufacturers, such as Canon, Fujifilm, Hasselblad, Leica and Nikon, or third parties such as Kenko. In contrast to a close-up filter (i.e. a lens which decreases the focal length), an extension tube does not contain any optical elements and thus does not degrade the sharpness of the lens [Peterson 2003]. Since the lens usually cannot be fully retracted any more with a lens extension tube in place, it is no longer possible to focus at infinity.
To see the effective minimum and maximum focusing distances of your lens with an extension tube, use the extension cube calculator and enter its minimum focusing distance (MFD) and a fairly large focusing distance such as 1000 m, respectively.
For example, a 50 mm lens with a minimum focusing distance of 0.3 m offers a magnification of 0.27. With a moderate extension tube of 12 mm, the effective minimum focusing distance falls to about 0.22 m and the magnification almost doubles to 0.51. The effective maximum focusing distance is at 0.32 m, with a magnification of 0.24.