how does the apperture value mutiples when one use a 1.4x or 2x T/C. the focal length can be changed by addition of an extra lens or creating a gap between the lens and the film plane. but i don't understand how does the apperture changes. for instance if i use 300mmf/4 with a 2x T/C it becomes 600mmf/8. how does the lens knows that a T/C its attached to the camera via a T/C and its 2x T/C so it has to stop down the apperture to double what it is suppose to do.

secondly, assuming that the camera is capable of changing the apperture values in half or full stop, then with a 2x T/C does the apperture values changes in full and double stops or similar to what it is suppose to do, half and full stops.

thnx

-- sajeev (chack74@yahoo.co.in), January 18, 2002

Answers

The camera does not stop down the aperture when a TC is attached. The aperture number (f8, f5.6, etc.) does not represent a fixed diameter, but rather the size of that diameter relative to the focal length. Ansel Adams explains it in "The Camera":

"The lens aperture is simply the diameter of the lens opening, expressed as a fraction of its focal length. Thus a lens of 4-inch focal length with a diameter of one inch has a relative aperture of 4/1, or 4 ... expressed as f4 ... another 4-inch lens that has a diameter of 1/2 inch would be an f/8 lens."

I would think that a 2x TC would double the aperture increments, but I have not used one, so I don't know.

-- Derrick Morin (dmorin@oasisol.com), January 18, 2002.

When using my Canon 1.4XTC on my 300 f4L IS and my 70-200 F4L, the aperture increment is not changed. One stop is lost (ie both lenses go from F4 to F5.6 when the TC is used), but from there the aperture can still be set in 1/2 or 1/3rd stop steps, as dictated by the camera body. The Canon 2X TC works in the same way to the best of my knowledge, and they report the true aperture of the lens to the camera. Third party TCs do not report the true aperture.

-- Isaac Sibson (isibson@hotmail.com), January 18, 2002.

thanks both of you for the piece of information regarding apperture. i never knew whats exactly apperture means and since i have never used a T/C, i didn't know how does it affects the apperture value. thanks again

-- sajeev (chack74@yahoo.co.in)), January 19, 2002.

If according to Derrick, "The lens aperture is simply the diameter of the lens opening, expressed as a fraction of its focal length", then I have one more question. How does the camera knows that there is a T/C between its mount and the lens so that it should reduce the apperture value depending upon which T/C is attached i.e. 1.4x or 2x.

ofcourse the next question arises is how does the camera knows which T/C is attached.

-- sajeev (chack74@yahoo.co.in)), January 20, 2002.

This is where the Canon teleconverters differ from third party ones.

With third party TCs, the camera DOESN'T know that there is a TC there. The electrical contacts on the front of a third-party TC just connect directly with those on the back.

A Canon TC is very different. If you look on the back of a normal Canon EF lens (eg 28-105), you'll see that there are 5 electrical contacts, and two or three power contacts. On an EF lens which can be used with a TC (eg 70-200 F4L), there are 8 electrical contacts, along with two or three power contacts. The front of the Canon TC has all eight contacts, but the back has only five. Within the TC, the correct focal length and aperture are calculated, and reported to the camera. EG if you use 1.4X TCs from Canon and a third party, with a 300mm F4LIS, and an EOS 1V, and then download the data, there will be differences. When using the canon TC, the data will show a focal length of 420mm and a maximum aperture of F5.6. When using the third party TC, the data will show a focal length of 300mm and an aperture of F4, DESPITE the lens now being a 420mm F5.6 due to the TC.

This is why there is a difference in AF between Canon and non-Canon TCs. If you use a 2X TC with the 300 F4, you get a 600 F8. Most cameras won't AF with that (EOS 3, EOS 1V, EOS 1D excepted). However, with the third party TC, the camera still believes it is F4, and so it will TRY to autofocus. And it will probably fail.

Basically, the camera never knows there is a TC there. With a third party TC, it believes that just the original lens is attached. With a Canon TC, it believes that a lens 1.4X longer and 1 stop slower (or 2X and 2 stops) that the original lens is attached.

-- Isaac Sibson (isibson@hotmail.com), January 21, 2002.