TC question

i read on one of these posts that Canon DSLRs do not "read" Tamron converters. I have a Kenko 1.4x converter that i bought for use with my Sigma EX 80-400 lens, and it's provided consistently unsatisfactory results. the lens is optically comparable to a 100-400 "L", and takes marvelous photos by itself. Since Kenko and Tamron TCs are made by the same manufacturer, could the problem be that my 30D cannot communicate with the AF mechanism through the Kenko converter? i thought these were all pretty much just "pass-through" devices electrically... if the problem is that my camera won't communicate through the Kenko converter, would it work with a Sigma TC, which should be made for my Sigma lens?

Rocky

What do you mean by not reading?

I have a Tamron 1.4x teleconverter. My lenses generally work fine with it. On my 300D it would allow autofocus (albeit really poor AF) on all my lenses. My 20D seems to detect it and disable auto-focus the same way it would for Canon teleconverters - if the resulting maximum aperture for the lens is too small it won't let enough light through and the AF refuses to try. In both cases, the aperture read by the camera is the TC-adjusted aperture.

I don't use the teleconverter much at this point - I got it to see if it would be worth upgrading from a 70-300 lens to the 100-400. It served it's purpose and now I don't use it much, since it has too many downsides when used with such a slow lens.

i just read somewhere that Canons won't communicate with Tamron TC's... no idea if it's true or not, which is why i'm asking. my Kenko will autofocus okay with my Sigma - at least it gives the indication of focus lock - but the pictures i get with it are often blurry and out of focus. even the best shots i get with it aren't nearly as sharp as the results i've seen from other people with other converter brands. i just wondered if there was something about the Tamron/Kenko TC's that Canons don't like, or if maybe i just got a bad one...

What may be discussed there is that these focal length converters may not participate in the Canon system for coordinating the effect of focal length converters.

In that system, the Canon converter announces to a compatible Canon base lens its presence and its magnfication (1.4x or 2.0x only). The compatible base lens then does the math to report to the body (straight through the converter) the focal length and maximum aperture of the combination. When the body commands the lens to set an aperture, the command goes straight through the converter to the base lens, which does the math to determine what "native" aperture will, in combination with the converter, produce that aperture, and sets that aperture.

Note that the Canon body does not know that a focal length converter is in place, so there is no notion of the body "reading" the converter or not.

thanks. i thought the body did all the calculations. didn't realize there was that much computational hardware in the lens!

Thank you Doug. I didn't realise that the communications worked quite that way.Do you happen to know if any of the TCs on the market do not have the signal connectors to allow the camera to control lens focus ? That could also be interpreted as a "read error".For those who are not aware of the effect of this communication system... As none of the TCs (canon calls them tele-extenders) have on-board computers they can't tell the lens that there is also another TC between it and the camera (by reporting a combined converter effect), so the lens only knows about the TC connected directly to it and consequently the camera will report the wrong focal length in the image data file. Exposure works OK because it is based on measured light received rather than external incident light readings.