I can't tell you which one to use but just multiply the focal length and
f-stop by the magnification to figure out the effective focal length and
focal ratios.
Someone already mentioned that the 1.4X gives a one stop loss (square
root of 2). The 2X gives a 2 stop loss (square root of 4) and the 3X
gives a bit more than a 3 stop loss (3 stops would be the square root of
8 = 2.83)
A 200mm f/4 with a 1.4X converter becomes a 280mm f/5.6 (200x1.4 =
280mm, 4x1.4 = 5.6)
A 200mm f/4 with a 2X converter becomes a 400mm f/8 and
A 200mm f/4 with a 3X converter becomes... hard to focus (600mm f/12).
I've always wanted a 1.4X converter figuring that I might actually use
it since it's not so extreme. I have 2 2X converters and never use
either one. I'm sure that I personally would never use a 3X because of
the light loss even if there were no other optical penalty.
Chuck Norcutt
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