"F. Craig Callahan" schrieb:
>
> Thomas Bantel wrote:
>
> > Yes. When you think about it, this is to be expected. The tube doesn't
> > have the 2 extra contacts (like the TCs) that are needed to transfer
> > the information about aperture changes. Because only lenses meant
> > to be used with the Canon TCs have these contacts at the rear end,
> > while TCs and tubes don't have them, it is not possible to get correct
> > aperture readout with more than one TC.
>
> Ok; the *lens* has 10 contacts (one contact is double-width). The two extenders
> each have an identical set of contacts on the end that connects to the lens: 11.
> So far, so good-- the 11 contacts on the front of the extenders connect with the
> 10 contacts on the lens.
>
> However, the EF25 extension tube has the same 11 contacts that the extenders do,
> so it also connects to all the pins on the lens. Also, the extenders and the
> tube all have identical contacts on the rear: 7 (including one double-width),
> the same as on the 28-70/2.8L. The EOS 3 has 8 contacts, so it matches up with
> the contacts on the extenders and the extension tube (and 28-70). BUT, the lens
> has contacts that the body does not use, but which both the extenders and the
> extension tube do use (or at least connect to).
>
> It would therefore appear, from visual inspection, that the extenders and the
> extension tube are passing exactly the same information to the body, *if these
> contacts merely pass through the information from the lens.* If the body is
> "seeing" the extenders, then the extenders must be telling the body of their
> presence in some way. No?
My guess is - wild speculation, of course ;-) - that the extenders don't tell
anything to the *body*, they tell it to the lens. And if the tube has the extra
contacts as well, it likely tells also something to the lens.
In case of extenders, this something is likely
a) Change of aperture/focal length
b) Information for fine tuning AF (speed of the motor, ...)
The extenders will not report a change in aperture. If anything, they could
report "there is an extra 25mm of extension" and the lens would have to
adjust the effective aperture accordingly. Now, I don't think that EOS lenses
do adjust the aperture with respect to extension. Does the 100mm f/2.8 macro
allow you to set f/2.8 at 1:1? The maximum possible aperture should read f/5.6
if the adjustment is done. The extenders *could* report some info for AF fine
tuning, though.
According to my theory, the lens will report the "summarized" info of the
combination to the body through the normal set of contacts (7 or 8). Now,
threre is no EOS component which has the additional contacts on both sides,
AFAIK. Therefor, only the one component attached directly to the lens can
report info to the lens through the additional contacts. The rest is cut off
because of the missing contacts (11 at the front of the second TC, but only
7 or 8 at the rear of the tube it is attached to. Probably having the full
set of contacts at the rear wouldn't help either, because the interface
might not be prepared to "add up" and forward info from more than one
component to the lens. The TCs and tubes are likely quite "dumb" and don't
use processors, they just forward info to the lens through the additional
contacts. The coding of this info might be as simple as voltage on the extra
connectors or resistance throught those connectors. Anyway, this *is* WILD
speculation, although it explains most of what actually happens.
Thomas Bantel
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