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:

• A prismatic rail similar to that of the PB-6 bellows, but without the bottom geared rack. This rail has two millimeter scales, both reading from 208 to 437 mm (including 2mm at each end that are not ticked) but running in opposite directions. You use either one, depending on how you attach the PB-6E to the PB-6. It does make sense to attach it to the rear of a PB-6 and to move the front (lens) standard of the PB-6 all the way to the front of its rail. In this way, you can use the scale on the left side of the rail (looking from behind the camera), because this scale on the PB-6 indicates the extension of the bellows (i.e., the distance between front and rear mount flanges) when the lens standard is located at the front of the rail, and the scale of the PB-6E simply continues and extends the scale of the PB-6.
• A base for the rail, twice as long as the base of the PB-6. You use this long base to connect the two rails together (after removing the plastic thumbscrews at the ends of the rails) and to attach the combined bellows to a tripod head or copy stand. The base has three standard 1/4-20 sockets at its bottom. This base has two tightening knobs, one for each rail. You can insert the rails into this base up to roughly its middle (it is not critical to do this with precision). However, you should not leave any gap between the two rails. This is critical when you move a standard across the joint between the rails. This base does not have geared knobs, so you cannot move the combined bellows in the same way as you use the base of the PB-6 as a focusing rack. In practice, I found it better to attach the combined bellows to an aluminium plate by both bases, for increased rigidity (see above picture). A focusing rack must be provided separately. I even wish there was a third rail base located at the camera end of the rail, for added rigidity.
• A middle standard and bellows. This standard mounts between the front and rear standards on the PB-6 and its bellows connect to the front standard. The original PB-6 bellows connect the rear standard to the middle one. The middle standard has a locking knob but no geared knob, and moves by sliding along the rails. When not in use, the bellows of the PB-6E can be held closed by a metal clasp on each side. The clasps engage automatically also when the bellows are closed to their minimum extension, and sometimes are clumsy to release afterwards, optimally requiring three hands for this operation.

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.