I don't know that I'd call it a challenger to the new 70-300mm L.
Yes, I realize that. In this post I'm not focusing (as usual) on Canon's market positioning, I'm focusing on what will be best for me. Apologies for not making that clear.
From my perspective, the 70-300mm has a nice focal length range and features, but the maximum aperture is narrow.
The 100-400mm I'm less interested in on the basis of its being an older, heavier, and more expensive design all around, without the benefit of a zoom ring (well, it's a benefit in most situations, plus the dust advantage). I didn't realize until writing this post that the apparent 1/2 or 1/3 stop benefit of the 70-300mm is negated by 155mm, and the 100-400mm surpasses it by 228mm, where it holds on at f/5 until 239mm, where the 300mm has has given way to f/5.6. Not a shattering victory and in actuality a rather small range where the 100-400mm is faster, but the 70-300mm isn't an unambiguous improvement.
Size and weight for the 70-300mm doesn't really sell it to me over the 100-400 - minimum focus is considerably better, however.
The MFD charts (comparing the 70-300mm
and the 100-400mm
product pages) are somewhat ambiguous to me because I'm left attempting to compare different focal lengths:
The 70-300 does seem to start better at the center for many focal lengths (especially wide) and keeps that improvement to 20mm diagonally (where one of the f/8 30LP/mm lines drops off, though the other stays high). The 100-400 MTF lines, sharpening up by 400mm compared to 100, suggest to me that it gets sharper (relative to its wide performance) as you increase the focal length, whereas the 70-300 seems to be the reverse, being sharper at 70mm than 300. Unfortunately this doesn't help a direct comparison - the MTF lines for the 100-400mm sag at about 5-10mm across the frame, at 400mm, but that's apples and oranges to the 70-300 @ 300mm (though it bodes well for the 70-300 that its lines don't sag, but stay high farther across).
I'm starting to think that overall the 100-400mm may have no great handicap at comparable focal lengths, if not an edge...really the only thing that stands out is the rather ugly 80% contrast for 30 LP/mm lines (the thicker / coarser lines) on the 100-400mm at 100mm, but it stays at 80% further across the frame whereas the 70-300mm drops rather quickly past the corners of an APS-C frame (unfortunately, comparing 70 and 100mm, so apples and oranges again). How the newer coatings etc. of the new lens enhance the performance indicated by the MTF charts, not to mention performance on the targeted APS-C frame, may make up the difference remains to be seen.
In any case, the new 120-300mm looks like a potential deal buster to me - if it is more appropriate for full frame is no problem for me, since that leaves full-frame camera options wide in the future. Overall contrast and sharpness, features, and especially AF reliability all remain points I'm worried about, however.
Thankfully I can put off a decision on these zooms for some time.