Don Weiss, that's a very interesting analysis of the logic of how converters/extenders might work. Although lacking your expertise on the electronics, I had reached rather similar conclusions myself, and also regard your option 1(a) as plausible. Here is a bit more data to fit into the picture. There are AFAIK seven Canon devices with forward and rear facing EF lens mounts: the four extenders (1.4x original and II, 2x original and II), the Life Size Converter, and the 12mm and 25mm extension tubes. All have the basic set of contacts at the rear, and the 12mm extension tube has only the basic set of contacts at the front. All the Extenders, the Life Size Converter, and the 25mm extension tube have the extra contacts at the front.
The conclusion that a lens with the extra contacts 'thinks it is mounted on the camera' when there is open circuit across the extra contacts is irresistible. The simplest way for a device to signal its presence to the lens would be by bridging some or all of these contacts, and there are only four options available, or eleven if bridging to some uninteresting (e.g. 'earth potential') line in the basic set is also possible. This does not sound like future-proofing, since even if the original and II versions of the extenders signal themselves as identical, there are already four distinct devices to be recognised. A more sophisticated device signature could be provided by means of a ROM chip in the device, and this is of course open-ended in terms of the number of devices which could be recognised. Some degree of mixing of the two approaches may even be possible, so finding that one dismantled device contains no electronic components does not imply that is true for all of them. Not only is Canon's lens-camera communication protocol unpublished, but we do not even know for sure what it communicates. I have heard a rumour that at least some USM lenses can communicate focused distance information back to the camera, but the rumour does not say whether any use is made of this. It is well-known that the protocol was extended when IS lenses were introduced, resulting in a minor limitation in the use of IS lens plus extender combinations on legacy EOS bodies. Data readout from an EOS-1V confirms for certain that focal length and maximum aperture are adjusted for the presence of an extender when a lens with the extended contact set is attached. With a zoom lens, the reported focal length is adjusted correctly throughout the zoom range. If working aperture is expressed internally in terms of stops down from the maximum, then of course this would not need adjustment, but absolute working apertures appear on an adjusted basis in the camera display and on the EOS-1V readout. There are, or may be, other subtle adjustments for AF speed and metering corrections. AFAIK the only Canon lenses with the basic contact set that can physically be mounted on the extenders are the three TS lenses. TS24x1.4 is actually a useful and optically quite acceptable combination. The lens operates correctly with the extender in place (it is a MF lens, so you just have to remember to turn the focusing ring more slowly :-)) but the reported focal length, maximum aperture, and working aperture are reported as if the extender was not present. The metering inherently takes account of the 1 or 2 stops loss of light because of the presence of the extender; if any further correction is required, it appears to me to be too small to be noticeable. The 50/2.5 macro lens has an extended contact set to connect with the set on the Life Size Converter. Physically, these contacts appear to be identical to those used between L-series lenses and extenders. The LSC should in theory be optically equivalent to a 1.5x extender with an extra 8.3mm of extension at the front (this is an elementary optical calculation from known data, not a piece of inside information!). In fact the combination is reported as 70/3.2. The reported maximum aperture is not adjusted for the focus setting, either with or without the LSC, but remember this is an AFD lens. The LSC will physically fit any Canon lens, but although it gives excellent results with the 50/2.5 for which it is designed, the optical results with other lenses often show serious chromatic aberration. It does work, operating both aperture and autofocus mechanisms, although autofocus usually does not actually lock on except with the 50/2.5. When the LSC is used with a lens with the basic contact set, it of course cannot be detected, and the effect is like using an extender with a TS lens. Interestingly, when used with an L-series lens with the extra contacts, its presence is again ignored, although at least it does not appear to damage the lens (this test is bad for the nerves!). The conclusion is that there is some basic difference between the extenders and the LSC over how the extra contacts are used. Now, two questions I cannot answer on the basis of the equipment I have. First, does any other Canon lens have the extra contacts apart from L-series lenses designed for use with the extenders, and the 50/2.5? The only plausible candidate is the non-USM 100/4 macro; if so, what devices do they link to? Secondly, what do the extra contacts on the 25mm extension tube do? ... RS __________________________________________________ Do You Yahoo!? Everything you'll ever need on one web page from News and Sport to Email and Music Charts http://uk.my.yahoo.com * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
