slclick said:
And on the other hand I and others who have used, rented and owned most or all of the Art lenses (minus the wider new varieties) have experienced the opposite.
It's not really "on the other hand" when I explicitly stated
"Some other people may luck out and get a great-focusing copy right away. I don't doubt that there are some copies of Sigma lenses out there which manage to focus well enough for their owners."
rlarsen said:
This lens looks very heavy. I wonder at f2 how it compares with the Canon 100 f2.
One would have to assume better. Most f/1.4 lenses are better than f/2 equivalents when stopped down vs wide open. Sigma are reporting, by their own admission, that this 105mm design (currently) has extremely strong vignetting, but other Sigma lenses also have strong vignetting and clean up very nicely with just one stop. The 85 and 135 Sigmas don't have any other particular optical flaws, so it's hard to imagine this 105 will have any, too.
Conversely, the Canon 100mm f/2 doesn't have the resolving power to hold up on a 5DS R sensor—though less-dense sensors are fine for it—and it has some slight distortion, altered by focus breathing; noticeable barrel distortion at closest focus and strong pincushioning at infinity. Granted, in regular use you never notice these things, but we should all know by now that these Sigmas are sold primarily on their superiority in controlled lab testing, and that's where the Canon 100mm's faults show up.
In short, I expect this Sigma
stopped down to f/2 to have a more uniformly-bright image, less distortion, more saturation, more broad contrast, more micro-contrast, less astigmatism toward the edges and corners, and higher resolution.
All that said, given my experience with multiple copies of the 'best'-focusing Sigmas, I don't trust the AF will be anywhere close to the Canon 100/2 (which is still one of the best AF motors in any Canon lens), and manual focus probably will be a tie. Of course then there's the weight, size, and price; it's very hard to ignore the fact the Canon 100mm f/2 can be found for as little as £200 in good condition and isn't much bigger than a cheap 50mm, with a very cheap 58mm filter thread to boot.
Then there's the rendering. Sigma lenses, while
technically very good, have pretty boring rendering. Generally with lenses you can have technical perfection or pleasing rendering, but rarely both. (Zeiss have both, but then, they give up on autofocus, they're not very well sealed,
and you're paying a huge premium.) Sigma, so far, have always gone with technical quality above all else. Conversely, the Canon 100mm f/2 is one of the leading examples of the value of rendering quality over technical quality. That lens may not win the resolution battle anymore, but for its common use as a portrait prime, it's
exactly as sharp as it needs to be and not being sharp enough to show up every single pore—which will then have to be blurred out anyway—is a positive.
There may also be a slight framing difference. The Canon 100mm is actually about 108mm, while Sigma tends to go a little wider than their stated lengths—the 85mm is actually about 82mm and the 135mm is actually about 128mm—so I expect this 105 to actually be a straight 100mm. Those few millimeters don't mean as much to a telephoto lens as they do to a wide-angle, but it can still make a difference if working distance is tight.
All-in-all, I don't think these two lenses will really be in competition. The Sigma will most likely for be for those people who want absolute maximum background blur without the overly-compressed look of something like 200-300mm, and at f/2 and smaller it will probably be the winner for anybody who has to demand absolute technical perfection. The Canon 100mm f/2 will continue being the go-to 100mm for anybody who values size, values autofocus, or values... value. It will probably also continue being the #1 100mm choice for portrait shooters with expressive styles rather than gigantic-print, technical styles.
To put it another way: I have the Canon 100mm f/2. I also have the 100mm f/2.8L IS Macro, a Mamiya 110mm f/2 I use with an adapter, my 70-200s spend most of their time at 100mm, and I'm strongly considering buying the Laowa 105mm STF. My #1 complaint with Fuji is they still don't have a 100/105mm equivalent, and if I buy into Sony any more I'll definitely be picking up their 100mm STF. To say I like the 100mm focal length is an understatement, and each lens I have or have had in that length has had its own use.
It should be no surprise that this Sigma 105mm will have to really drop the ball for me to not pick one up, and I fully expect that, if I do buy one, it will find a specific use just as all my other ~100s have.