With the exception of the very last one, all the images below are the result of
stripping down and combining the optics from several lens systems to make a hybrid macro lens.
Main lenses used were a Zeiss Vario Sonnar 10-49mm (primary) and a Pentacon 50mm F1.4 (secondary, reverse mounted on the front of the primary).

Take f1 and f2 to be the focal lengths of the primary and secondary lenses respectively (i.e. f1 = 49mm and f2 = 50mm in this case). Combining two suitably matched lenses by mounting the secondary lens back to front on the primary will cause (if both lenses are focused to infinity) the real focal plane to be at the position the virtual focal plane would normally occupy if the secondary were being used according to its original design (i.e. in a camera). In other words, the region that is brought into focus lies only a couple of centimetres from the front element (originally the rear element of the secondary lens).  This results in images being formed on the sensor (or film) that are f1 / f2 times lifesize. Approximately actual size when both primary and secondary lenses are the same focal length. On a 12mm x 9mm sensor, the entire image represents an area only 12mm in diameter, which clearly allows some very interesting photographs to be taken!

As with most things, however, a compromise must be reached. At these magnifications, depth of field is razor-thin. Using a small-sensor camera means a smaller field of view and thus greater magnification, but due to diffraction effects at small apertures, typically the smallest aperture size of just a few mm equates to stopping down only to about f/8.  This further limits the depth of field, as normally you would hope to use something around f/22.  Coupled with the obviously low light levels at small apertures, and the short shutter times required with highly magnified live subjects, is the fact that autofocus is largely impossible and focus has to be achieved by moving the camera back and forth by hand.

For more about optics and lens design, see my lens design software