Extender problem

Started Sep 9, 2012 | Discussions thread
40d_dane Senior Member • Posts: 1,034
Re: Extender problem

I think that it's about time to close this one...

It's doesn't take a rocket scientist to figure out what's going on.

Nothing is wrong with my 300mm f4 when it operates as I described. The system operates exactly as one would expect given the circumstances (taped TC pins).

Adding a 1.4 TC in the optical path is like introducing a magnifying glass in the optical path. The image projected on the image sensor is magnified... as is the image projected on the PD sensor. Thus the phase difference seen by the camera is magnified by the TC and the camera will ask the lens to change focus too much if the lens AF parameters are those of the bare lens. Thus the hunting.

A non-reporting TC that doesn't hunt modifies the lens AF parameters to fix this.

40d_dane wrote:

From what you write I gather that your experiments were carried out using a non reporting TC and not a Canon 1.4 TC with taped pins ?

Correct ?

Karl Gnter Wnsch wrote:

40d_dane wrote:

Well Karl, sorry to say so, but you've got it all wrong even though you don's seem to think so. And I didn't say that there wasn't a limit in the beam splitter. I just said that it wasn't that limit that causes AF to hunt with taped pins.

You are misinterpreting your results. If your implications were true using a non reporting TC on a f/2.8 lens would result in the lens hunting no matter which AF point was being used - well after testing this scenario I can only say that for the three f/2.8 lenses I have at my disposal all hunt when being used with the center AF point (which happens to be the one used in f/2.8 mode which is out of it's working range) and none of the three hunt on the outer AF points when used with a non reporting TC. I can't say what went wrong with your 300mm lens test but I wouldn't jump to invalid conclusions like you do! Neither the 400mm f/5.6L+TC nor the 100-400L focus properly with my two non reporting TC's at all - which is only natural given that the beam splitters are mostly blocked.

You should read up on how EOS lenses report to the camera with and without TC.

There is no difference in the communication with or without the TC (if you disregard the new version III extenders, which actively alter the sent messages). The way a TC is detected is simply a matter of a pull up circuit on the 3 extra contacts of a TC-ready lens, which then prompts the lens to report different focal length and speed parameters for the AF.
Karl Günter Wünsch

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