Nikon PB-6E bellows extension
The Nikon PB-6E is an extension that more than doubles the maximum length of the Nikon PB-6 bellows. It must be used together with PB-6 bellows (see above picture), and cannot be used alone.
The PB-6E consists of:
Mounting the PB-6E to a PB-6 involves removing the plastic thumbscrews at the ends of the rails of the PB-6E and PB-6 to be joined, detaching the bellows of the latter from its front standard (with its small locking knob), sliding the front or rear standard off the rail, connecting the two rails with the long PB-6E base, sliding the standard back on at the front or rear end of the combiner rail, and reconnecting the two bellows to the proper standards. The combined PB-6 and PB-6E allows a "monster" maximum extension of 437 mm, more than twice the length of any other general-purpoase bellows built for an SLR/DSLR. The dedicated bellows of certain photomacrographic stands, however, are even longer (e.g., the Nikon Multiphot has 300 and 600 mm interchangeable bellows, and the Leitz Aristophot similarly long bellows for 35 mm Leica cameras).
Although a comparable extension can be achieved by adding a long stack of extension tubes to a PB-6, or even by connecting two PB-6 together with an extension ring, these alternatives have drawbacks. The PB-6/PB-6E combination is the only combination among these alternatives that allows a minimum extension of 83 mm without disassembling anything and without forcing the front standard to move back along the rail, which causes the latter to stick out in front of the lens and often in the way of the subject. This minimum extension is sufficiently short to leave the PB-6 and PB-6E permanently connected together (at least, if you have a good variety of photomacrographic lenses of different focal lengths). Actually, I have a second PB-6 that I always keep without extension for whenever a shorter extension range is necessary.
The PB-6E manual contains tables of magnifications that can be achieved with several Nikon lenses, but the PB-6E is really best when used with dedicated photomacrographic lenses, which are discussed on several pages on this site. With a Macro-Nikkor 12 cm, for instance, about 4x is achieved at maximum extension. A Zeiss Luminar 25 mm reaches nearly 20x (however, it is diffraction-limited even with the aperture fully open). The exact magnification depends, among other things, on the length of the adapter used to connect a lens to the bellows, so it would be pointless for me to provide here a table of magnifications for photomacrographic lenses. A rough estimate can be achieved by considering that
where m is the magnification, f the focal length, l0 the registration distance (46.5 mm for the Nikon F bayonet) and l the extension as read from the PB-6/PB-6E scale. However, this equation gives exact results only if l is measured to the rear nodal point of the lens, not its mount, so the actual magnification usually will be a little higher than the one otained with the above equation.
More to the point, vibration and minor sagging of the stand and other parts of the equipment have a substantial effect on image quality above 3x-4x, so the aluminium plate I added at the bottom of the bellows is very important in keeping it reasonably rigid, and by no means exaggerate in size and weight - heavier and thicker metal would not be out of place here.