The best low-light Canon lens for event photography…
If I'm on an outdoor shoot, I'm always carrying at least 2-3 lenses "on standby", and even during business events, a camera rucksack of lenses is often on my back (or slung over one shoulder) throughout, just so not to miss a moment. All this to say that weight was a big reason why I only wanted to adopt only one prime lens as part of my "always with me" gear. The 50mm focal range was the obvious choice.
I've used the Sigma 50mm F1.4 DG HSM Art Lens for several months now. For event coverage I use it mainly for low-light situations and for my private natural light photo sessions I use it to isolate subjects from whatever Paris backdrop they've chosen. Often, however, I'll reach for that lens purely because there's nothing quite like shooting on a 50mm prime.
Having exclusively used Canon L-Series lenses for client work, my initial inclination was to purchase the Canon EF 50 mm f/1.2L USM Lens. The Sigma Art lens was the only competitor in the professional range and costing half the Canon, I first thought it surely wasn't worth consideration. Nevertheless, before parting with 1K+, I decided to take the time to research, reading the many online comparisons of the two lenses and, more importantly, trying them hands on. I'm very glad I did.
Sharpness:
The great thing about the L series zoom lenses I own (16-35mm, 24-70mm, 70-200mm) is their consistent sharpness across all apertures. Unlike non-professional lenses, there's no need to stop them down a little "to be on the safe side". It's therefore surprising that the Canon 1.2, being both an "L" lens and a prime, contrary to it's highly-regarded 85mm and 35mm Canon equivalents, just isn't sharp at 1.2. Or at 1.4. If I shot only lifestyle photography, this wouldn't be as much of an issue - one could argue sacrificing sharpness for the mood created by a swimmy 1.2 bokeh. However, for corporate photography, where anything short of truly sharp (as appose to "that'll do") is usually undesired, such performance is simply not adequate. For assignments requiring more spontaneity (e.g. event photography), your gear should be the one thing you must be able to completely rely on. The Sigma is not only reliably sharp at all apertures, but (as one can see in numerous example images that have been posted) it's sharp right to the edge; for candid photographs, which often require at least some degree of cropping, this is extremely important.
Auto-Focus:
I found both the Sigma and the Canon 50mm primes to focus about the same speed - a tad behind my "L" zoom lenses, but still very fast.
Bokeh:
The Canon and Sigma's bokehs are different, with the Canon being perhaps more "arty". I wouldn't by any means use the word "better", as this really is subjective; it depends on both what look you like and on what you're shooting. Having read/watched 20+ reviews (no, really), I'd say that as many photographers prefer the Sigma's bokeh to the Canon's. Personally, I can't say I prefer one over the other. Having bought this lens predominantly for it's low-light capabilities during events, the style of bokeh wasn't really a factor for me.
Build Quality / Feel:
Where is the Canon better then?! That's the million dollar question (or, rather, the £650+ question, that being the price difference at the time of writing). Both lenses feel reassuringly heavy and sturdy. The Canon wins over the Sigma here only because it has weatherproofing. Other than its slightly more compact size (and ever so slightly lighter weight), that's about the only practical advantage of the Canon over the Sigma, and that said, even if I'd wanted a 50mm lens exclusively for photo journalism and travel, I'd still have gone hands down for the Sigma; weatherproofing really is a piece of mind thing when you consider that the major factor in how a lens holds up "on location" will always be how well you look after it.
Using Sigma with Canon
One of the most important things for me when giving a client a catalogue of images is consistency, hence a concern in purchasing the Sigma, was how it's "look" would compare to that of my other lenses, all of which are Canon. I remember that, years back, when comparing photos taken with a Tamron lens I'd borrowed and its Canon equivalent, I could literally tell image-for-image which photograph had been taken with which lens. Sometimes, but not always, I can tell the Sigma images from those shot with a Canon lens, but such cases really are nitpicking. The Sigma's image quality really is on par with my Canons', and any visual differences their images do have does nothing to change the overall "look" of a photograph. The Sigma compliments my Canon setup extremely well.
Conclusion:
What would make this Sigma Art lens better? Weatherproofing and a shorter, more traditional 50mm prime length. That's it.
Overall I am delighted with this 1.4 prime. It's consistently sharp, focuses very reliably, and produces the same professional level of image as my workhorse "L" lenses. For lifestyle portraiture, the choice between the Canon 1.2 or the Sigma 1.4 might be subjective, but for events, weddings, and any assignment where sharpness is of great importance, the Sigma 50mm F1.4 Art lens is by far the best choice on a Canon camera.
For an in-depth technical analysis of this lens, I fully recommend Ken Rockwell’s review.