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Notes on using a teleconverter with an EOS camera
David Burren
June 2002
Some EF lenses have 7 electrical contacts ("7-pin" lenses) and some
are "10-pin" lenses (typically the later Canon telephotos).
Teleconverters such as Kenko's Pro 300 (and presumably Canon's own
TCs although I haven't checked one lately to be sure) have 7 pins
on the camera side and 10 on the lens side (actually 11: one of the
pins on the lens covers two of the "raised" ones on the camera/TC).
If you attach a 10-pin lens to a TC the camera recognises the
existence of the TC and reports the effective focal length and
aperture of the combination. If however you attach a 7-pin lens to
the TC, the camera does not recognise the presence of the TC and
reports the native features of the lens.
By covering the extra 3 pins on the lens side of the TC (either by
using a small extension tube which only has 7 pins or by using tape
as shown in the
D30/D60 Tips
section on Fred Miranda's website) you
can fool the camera about the TC's presence. The reason you might
want to do this is to trick the camera into autofocussing when the
effective aperture is above f5.6 (or f8 on some cameras). I have
done this successfully with my 100-400mm IS (a 10-pin lens) and a
1.4x Kenko on a D30, although the AF performance at f8 is nothing
to write home about.
This also explains why when you stack TCs the camera does not
recognise the presence of the TC closest to the camera.
Keep in mind that in situations like this the camera is not able to
record the complete details about the settings used in the photo.
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