Extenders and SHOWN aperture value

Okay, I understand how the 1.4 and 2.0 tele extenders affect aperture value. When I put the 2.0 extender on my Tamron 28-75 f2.8, the aperture value in the viewfinder still reads 2.8. Why? If the ap is really 5.6, why doesn't the 10D know the extender is there and adjust the display accordingly? Also, does this affect the metering? For example, does the camera think the ap is 2.8 (when it is really 5.6) and choose a shutter speed based on 2.8 (in P mode)? Sorry, I'm very confused now.

This is because the physical aperture on the lens is still F2.8. What is happening is the aperture is not changing, but the amount of light passing through the optics is being reduced.
Fortunatly because the camera usus through the lens metering then it will continue to meter correctly with the TC in place.

Okay - now it's all clear. I was under the impression that the TE somehow limited how far the aperture opened.

Thanks

Some of the T-cons will electronically tell the Camera the correct number.

But as you are mounting a t-con to a 28-75,. you probably don't have a t-con that has that abuility.. (the dedicated Canon and Sigma T-cons for instance couldn't be mounted on a 28-75)

With the Sigma 2X mounted to an f/2.8 the camera correctly shows me the max aperture as f/5.6...


So it depends on the unit.

"The intersection of the set of things that are obvious to me and the set of things that are not true is embarrassingly large" Webster

You have no idea of the hearty chuckle I got out of this one. Growing self-awareness I guess.

Thanks