Where are aperture breaks for 100-400/4.5-5.6?

I'm interested to know where the max aperture changes for the EF 100-400/4.5-5.6L IS. Specifically I want to know whether it's a 300/4.5 or a 300/5.6 or something in between. I'm trying to weigh the zoom convenience against the (slightly) lower cost and (slightly?) higher quality of a 300/4 IS with(out) 1.4x tc.

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The

<A HREF="http://www.usa.canon.com/camcambin/cameras/eflenses/ef100n400.html">

Canon website</A> helpfully mentions that the max aperture is printed on the lens barrel (presumably the part that's exposed by the push/pull zoom). Unfortunately no one ever seems to have any in stock to play with.

Pop Photography (Feb 99) report it as a 100/4.5, 200/4.5, 300/5.6 and

400/5.6, so it looks like it goes from 4.5 to 5.6 somewhere between

200mm and 300mm. Of course they may be working in 1/2 stop steps. If

you have an EOS-1(n) or EOS-3 which work in 1/3 stop steps you might

get back 1/3 stop at 300mm.

When Photodo runs MTF tests on a zoom lens, they report its maximum

aperture at each tested focal length. Surf to http://photodo.com and

click on "Lens" near the top of the home page, then persevere until

you bring up the right lens review.