Hello all, I am using a Tamron SP AF 2X Teleconverter with my EF 75-300mm f/4-5.6 III USM lens. I understand this means I will lose the equivilent of 2 stops of light and have read that if I had a Canon converter my lcd display would show my maximum aperture as f11. Is this correct? Why do I read about the 2 stops affecting aperture and not just exposure in general. When my lcd display reads f5.6 with the Tamron converter I expected I would be losing the 2 stops through shutter speed. I'm not sure if I'm misunderstanding some of the posts I read and would appreciate some clarity on this subject.

Kind Regards, Jason

-- Jason Eadie (jason_eadie@hotmail.com), April 18, 2001

Answers

You understand correctly.

A 2x TC results in a loss of two stops of aperture (f5.6 becomes f11). Your Tamron TC won't show this, so the LCD will still read f5.6, but to get the correct exposure you'll have to shoot with a shutter speed which is 2 stops slower. The camera takes care of all this for you, it's just that the aperture displayed on the LCD will be wrong.

-- Bob Atkins (bobatkins@hotmail.com), April 18, 2001.