So lets see what the lens is and what it does.
It is a 120mm f6.3 lens constructed with 5 elements in 4 groups. The lens is fairly old so has the older coatings. This should not be much of an issue for macro work, but doesn't seem to flare in my work and has nice contrast. The lens is small, with a 38mm front thread and a M39 rear thread. There is no focal helicoid and will require bellows or other extension means to focus at various distances. The iris is 6 blades and is operated similarly to the 105 mm f4 Bellows Nikkor. That is, there are two rings. The first ring has 1 stop detents on it that will allow you to set a minimum closure and then a second ring that will allow continuous change of the diaphragm from fully open to the setting of the first ring. In a departure from photographic lenses, the stops are marked from 1-7 where 1 is 6.3 and each number denotes a 1-stop reduction in iris side.
What does this lens give you that a standard micro lens does not? First is image circle. This lens is designed to cover 4x5 inch film so will allow as much swing and tilt as your bellows will normally allow. Don't expect any vignetting from movements like you can see with standard or even PC-E micro lenses. Second is a flat field. There is no appreciable field curvature which is critical in macro photography (greater than life size) where depth of focus is in fractions of a millimeter.
This lens requires substantial extension and I am often testing the limits of my PB-6 Bellows. Bellows extension will allow for more magnification but the lens is optimized for about 1:2 to 4:1 so you will see some degradation in performance outside of these ranges on 35mm/FX images. Working distance is much better than what I am used to with other lenses (e.g. a reversed 50mm) which makes lighting substantially easier.
To fit this lens to the bellows, a method to convert the Leica M39 screw thread to Nikon F bayonet is required. This adapter (nikon's is BR-15) is pretty rare, but off brand adapters can be had on the internet if you are looking.
During my experimentation, the exposure difference between settings 1 and 2 on the lens barrel were actually 1.3 stops at the same extension. This means that setting 2 is f10. The remaining settings are close to 1 stop each, so 2 is f14 and 3 is f2. I would not suggest going beyond setting 3 (f20) as diffraction starts to get the better of you, especially if you are using high-MP cameras. Better to keep a wider aperture and use focus stacking techniques than trying to stop down the lens that much. Also remember that the effective aperture as it relates to exposure also has to take into account the large extension you are using (usually over 100mm) so exposure times, and the noise inherent in those settings increase dramatically.
In the end this is a fine lens for bellows offering a small package to get into the macro range of photography with a lens specifically designed for this application.
For the best write-up on this lens (and the source for this entry) visit Enrico Savazzi's website here
