Does anyone know why cutting edge innovator Canon (along with every other 35mm camera maker) chose 1.4x for teleconverter factor instead of 1.5x? A 50% increase in focal length would be a lot easier & more convenient to calculate than a 40% increase!

I'm guessing it has something to do with trying to keep to a 1 stop aperature loss rather than a fraction, but that's just a wild guess. Any learned answers? :)

-- Hung James Wasson (HJWasson@aol.com), August 10, 2001

Answers

1.4x is 1 stop. 2x is 2 stops. That's why they use 1.4x and 2x.

Fractional f-stops aren't such a big deal with autoexposure and electronically controlled lenses, but back in the days of manual operation and mechanical f-stops they were, so that's why 1.4x and 2x are standard.

-- Bob Atkins (bobatkins@hotmail.com), August 10, 2001.

Bob,

Thanks for the answer! I assumed that it all had to fall back onto the days of manual metering, and then just sticking with it because that's just they way it's always been done -- but it's nice to get confirmation from someone as learned as yourself! :)

-- Hung James Wasson (HJWasson@aol.com), August 11, 2001.

Also, 1.4 is the square root of two. So, if a 1.4x tc looses one stop, then two 1.4x tcs loose two stops, and give 1.4 x 1.4 magnification, which is 2. Thus, a 2x tc is effectively two 1.4x TCs in one.

-- Isaac Sibson (isibson@hotmail.com), August 31, 2001.

Isaac!

Trust an engineering student to give a clear, concise (& mathematical) explanation of a question that's been baffling me for ages! Thanks! :-)

-- Hung James Wasson (HJWasson@aol.com), August 31, 2001.