There's probably something to do with the age of the 100-400 and its dislike of TCs, but I've tried the Bower TC on my old 28-135, and the TC definitely degraded the sharpness there too. Maybe I got a bad copy. I imagine the Kenko brand is a tad better than Bower. I hear the Bower/Rokinon/Samyang conglomerate makes fairly good lenses, so maybe they just don't make good teleconverters. It's probably very difficult to optimize a teleconverter for a bunch of very different lenses.jrista said:jiphoto said:It's a Bower. I think it's the same generic brand as Rokinon/Samyang.
It may not be the Bower TC that causes the IQ loss. I mean, it will cause some, but the 100-400 sucks even with the EF 1.4x III (which it doesn't even function properly with), and while it functions properly with the much-loved Kenko Teleplus Pro 300 GDX 1.4x TC, the IQ still sucks (a little bit more than the Canon 1.4x, but the differences aren't huge.) I think it's just that the old 100-400mm lens design was built in the film era, at the early dawn of the digital era, and the bar for quality wasn't as high back then. It is most definitely a softish camera at f/5.6 and f/6.3, and only really starts to sharpen up by f/7.1 and f/8. With a TC, you would be at f/11, which imposes a significant hit on either shutter speed (which increases softness from camera shake) or ISO (which packs on the noise, especially on a 7D).
It is possible that your $80 Bower TC is just fine, and that it just doesn't pair well with 100-400 (because, well, NO TC pairs well with that lens.)
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