so everyone knows by now that Canon has refreshed its telephoto extender lineup with the 1.4x III and the 2.0x III. they're really great (even without having the chance to use them on the lenses they're really designed for, the series-II super teles) ... but as I mull over a future purchase of either 300 f/2.8 or the 500 f/4 (many years in the future), I started wondering:
the gap between the 1.4x and 2.0x is pretty significant. if you were to say purchase a 300 f/2.8 with the intent of using extenders to increase its reach, you end up with the following focal lengths: 300, 420, and 600. the gap between 300 and 420 is pretty minor, but the gap between 420 and 600 is huge. a 1.7x would bridge that perfectly, giving you the option of also creating a 510mm f/5 lens.
is that just me, or would that be awesome? I would totally shell out another $500 for a 1.7x extender of the quality of the new series-III extenders, and I'm curious to see if others would as well.
even on the new 70-200, the gap between the 1.4x and 2x is significant. you're stuck with either a not-so-long 280mm f/4, or a long-ish but slow-ish 400mm f/5.6. I've found myself shooting a bunch in the mid-300mm range but stuck at a f/5.6 max aperture, which kind of sucked, and probably slowed down AF speed quite a bit as well. a 1.7x would make the 70-200 into a 120-340mm f/5 lens, which would be great for a number of mid-telephoto applications.