Canon focal length converters - communication

[Doug Kerr]

Traffic in another forum reminds me that there is still some lack of clarity floating about as to how Canon focal length converters (often, but incorrectly, called "teleconverters") interact with the associated lens (and camera?). I thought it might be worthwhile bringing everybody up to date.

The electrical interface between a Canon focal length converter (here, "FLC") and the associated basic lens (which would have been called the "prime lens" until that term was inexplicably hijacked to mean "a lens of nominally fixed focal length") has three contacts ("pins") beyond those of the regular EF lens interface. These, though a simple contact-closure scheme, advise the lens that it is sitting on an FLC and which one of the two recognized types it is, 1.4x or 2.0x. (There is no provision for any other factors.)

There are no "special" interface leads between the FLC and the body.

The "regular" interface leads pass directly through the FLC from the body to the lens. The FLC does not interrupt any of them, look at any of them, nor apply any signals to any of them.

The body in fact is never aware that an FLC is in place.

When the basic lens finds it is sitting on a 1.4x [2.0x] FLC, it modifies its behavior in these ways (at least):

• It reports to the camera, as its current focal length, 1.4 [2.0] times its own actual current focal length. (Any other focal length reporting, such as the focal length range, is similarly adjusted.)

Note that this is not an "effective focal length". It is the actual focal length of the optical system that is mounted on the body (a "new" lens made of two subassemblies).

• It reports, as the f/ number of its maximum available aperture (for the current focal length, if the maximum aperture is not constant) 1.4 [2.0] times the actual f/number.

Note that this is not an "effective f/number". It is the actual f/number of the aperture of the optical system that is mounted on the body.

•When it receives an aperture setting command from the body, it sets its aperture to that whose f/number is 1/1.4 [1/2.0] times the f/number in the command.

•There may be some changes enacted in its AF algorithm. (I knew something about this once, but it has gotten away from me. I think motor speed is one change, and I forget exactly why that makes any sense.)

Again, keep in mind that the body is wholly unaware of the presence of an FLC in the optical chain. Note also all the information described above passes directly from the lens to the body or vice-versa, the FLC merely serving as an "extension cord".

Many encounter this matter in connection with the refusal of an EOS body to attempt auto focus when the reported available maximum aperture is smaller than a certain threshold (dependent on the AF system of the particular body model). For many lenses of interest, with an FLC in place, the maximum aperture of the "new lens" (which is reported to the camera as discussed above) will be two small to qualify.

Some users disable the three-pin auxiliary interface between the FLC and the basic lens (typically by insulating the pins on the FLC with electrical tape). The lens is now unaware of the presence of the FLC, and it reports its own properties to the body, unadjusted. Thus the body is (incorrectly) advised of a maximum aperture larger than the optical assembly actually exhibits, and thus may be willing to attempt AF. And in many cases, the AF will work.

Note that when this is done, aperture commands to the lens are not properly executed. However, this does not usually result in automatic exposure errors, since the body is misadvised of the aperture of the "new lens" in the metering state, and the two errors cancel out.

Finally, let me return to my comment at the outset about the term "teleconverter". That term was originally applied to a focal length converter whose purpose is to work with a "normal" focal length lens to give the user a new "telephoto" lens.

None of the Canon lenses with which the Canon FLCs can be used reasonably qualify as "normal focal length". Thus the term is not apt for the Canon FLCs (and in fact Canon does not use the term for them, at least in careful writing).

Breakfast today will be Menu A.

[Gary Ayala]

Thank You Doug.

This morning ... fresh ground Sumatra this morning, brewed in a French press w/ a splash of raspberry flavor just for a difference.

Gary

[Doug Kerr]

Hi, Gary,

Originally Posted by Gary Ayala

This morning ... fresh ground Sumatra this morning, brewed in a French press w/ a splash of raspberry flavor just for a difference.

Sounds lovely!

Best regards,

Doug