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?
There is something going on inside the extenders that is not going on inside the
extension tube. Whether that something is a microchip in the extenders, or
dead-end pin connections in the tube, or something else altogether, is something
I can't answer without cracking these babies open--something I am not prepared
to do at this time. ;-)
Both my EOS 3 and EOS 5 have 8 contacts; my lenses that do not accept the
extenders have 7/8 contacts, whereas the two lenses that do accept the extenders
have 10/11 contacts. Physically there appears no way for the extra contacts on
these two lenses to be utilized with existing bodies. This isn't terribly
surprising--recall that FD lenses had an AE pin long before Canon introduced an
AE SLR to the market; and they also had a "reserved" pin that, to my knowledge,
was never used.
The mystery continues. . . .
fcc
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