Olympus Telescopic Auto Tube 65-116mm
The Olympus OM system (centered around the OM film SLRs) includes several unique items for macrophotography and photomacrography. One of these is the Olympus Telescopic Auto Tube 65-116mm, which is a variable-length extension tube with a continuously variable length range (as the name indicates, 65 mm to 116 mm) that almost rivals typical bellows. This accessory is discontinued since 2003.
In comparison, the Nikon PB-6 bellows (which are somewhat larger than average) provide an extension range of 48 to 208 mm, and the discontinued Olympus OM bellows 36 mm to 198 mm. The total travel of bellows is generally higher, and starts at a lower ertension than the Telescopic Auto Tube. There is of course no way to reduce the minimum extension of the Telescopic Auto Tube, but shorter focusing helicoids are broadly available. At the opposite end of the extension range, a couple of extension tubes mounted at the end of the Telescopic Auto Tube would suffice to reach the maximum extension of bellows. Weight of the Olympus Telescopic Auto Tube is also somewhat less than ordinary bellows (about 0.4 kg including the tripod shoe, versus 1 kg for the Nikon PB-6)
The main unusual feature of this variable extension tube is that it is not equipped with a helicoid for changing its length. Instead, the large rubber-clad ring is a brake clutch. Turn it right to free the brake, then push it forward or pull it back to change the tube extension. Turn the ring to the left to re-engage the brake and lock the tube at the desired extension. The tube is reasonably stiff even at full extension, in spite of its built-in system of double coaxial sleeves. It also seems to provide adequate internal blackened surfaces to avoid flare and loss of contrast, and is unlikely to suffer from pin-point light leaks and the slight transparency to visible light or near-infrared.radiation that sometimes plague old or low-cost bellows.
The tube is equipped with a useful tripod shoe (available in two slightly different variants), which can be removed and, unfortunately, is often absent in second-hand specimens. The front and rear lens mounts follow the Olympus OM standard, and two sets of mechanical cams provide full-aperture metering and automatic aperture with OM cameras and lenses. Of course, with other cameras the cams are useless and the aperture operation is only manual.
The Olympus Telescopic Auto Tube carries two magnification scales for the Olympus OM 135 mm and 80 mm bellows lenses (or, in some versions, four magnification scales for the OM 135 mm, 80 mm, 35 mm and 20 mm), plus an extension length scale with ticks every 5 mm.
Leitax sells replacement mounts for several Olympus OM lenses (and the Telescopic Auto Tube) to use them on Nikon F and other cameras (albeit, of course only in manual aperture mode). Since I still use plenty of Nikon equipment, I converted a Telescopic Auto Tube with one of these Nikon kits, which includes only the rear bayonet and screws.
The front OM bayonet of the tube is more problematic. I built an M39 to Olympus OM adapter to mount at the front of the tube by modifying an old OM extension ring. A few types of adapters for mounting camera lenses onto OM bodies and accessories were available in the past, but have become difficult to find. Many adapters advertised on eBay as OM compatible are instead for 4/3 or Micro 4/3 cameras, so be careful of what you are actually ordering.
Just to make things clear:
As an alternative, Olympus OM reversing rings (with a male OM bayonet and a male 49 mm thread at the opposite end) are not difficult to find on the second-hand market (but also in this case, beware of erroneously advertised 4/3 and Micro 4/3 reversing rings.
If you plan to use the Olympus Telescopic Auto Tube only with 4/3 or Micro 4/3 cameras, it is easy to find a real OM to 4/3 or OM to Micro 4/3 lens adapter, and there is no need to replace the rear bayonet of the Telescopic Auto Tube.
As shown above, the Olympus Telescopic Auto Tube is more compact than bellows. It is also less delicate and easier to use in the field. Its most likely use today is together with third-party bellows lenses of short to medium focal lengths, but the original Olympus bellows lenses can also be used. No current focusing helicoid I know of completely overlaps the extension range of the Olympus Telescopic Auto Tube. On the other hand, if you need a lesser amount of extension than 65 mm, then a focusing helicoid is the logical solution. As a result, the Telescopic Auto Tube is today primarily interesting as an accessory for macrophotography and photomacrography. Originally, it was also designed for focusing from infinity to the close-up range with the Olympus 135 mm bellows lens.
It is best to use the tripod collar of the Telescopic Auto Tube for attaching the camera equipment to a tripod, stand or focusing rack, rather than using the tripod attachment at the bottom of the camera body (and this is the reason why the tripod collar is essential for the operation of the Telescopic Auto Tube).
Some of the Olympus lenses designed for use with bellows and the Telescopic Auto Tube have very short, built-in focusing helicoids. Although using a focusing ring to focus does change the magnification and is generally a no-no technique in scientific photography, doing so may be acceptable as a way to acheve fine focus after using other means to achieve a preliminary, coarse focus. This does result in the magnification and subject size not being documented with precision. In the particular case of the Olympus photomacrographic lenses, the resulting change in magnification is not major. A focusing rack sufficiently precise for use in photomacrography was never provided by Olympus as part of the otherwise extensive range of photomacrographic Olympus OM equipment, and the short focusing helicoid was perhaps meant as a substitute.