« on: January 16, 2014, 02:29:12 AM »
Hi,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.
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. )
The Kenko is decent, but it is definitely not as good as the Canon TCs. It allows just barely visible improvements when attached vs. when not attached. Subjects are definitely larger in the frame, but you don't get the same kind of increase in overall detail as with a Canon TC.
Something else I've noticed with the Kenko TC...boke circles look TERRIBLE. They have this funky warped star effect which just looks rather bad, so I don't really use it much anymore. It's great though, for people who want f/8 AF on camera bodies, like the 7D, that don't normally support it (so the boke issue just doesn't matter in those cases). It does allow f/8 AF, and in good light, even the 100-400 will focus automatically, albeit slowly.
If you have a good lens, and a body that supports AF at the smaller apertures, get a Canon TC. No question they offer better quality. If you have a body that does not support f/8 AF and you need it (probably best with the 400/5.6 L prime), the Kenko is the best bet. (Actually, the Kenko MC4 seems to produce better IQ overall than the Kenko Pro 300 DGX, so I actually recommend getting that one...I simply couldn't find one for sale when I bought mine.)
I use the Kenko Pro 300 DGX 1.4x (blue dot version) on my 400mm F5.6L and 6D all the time and the bokeh look normal to me... focusing speed is only slightly slower. Anyway, bokeh is cause by the aperture blades, right? So the quality of the bokeh should be determine by the lens design.
Have a nice day.