The Sigma 50mm F1.4 Art has become probably more popular than the less expensive, smaller, first party versions of the 50mm F1.4. Why? It's a T1.8, meaning it zaps one stop of light before it reaches the camera sensor. It's almost 100% longer than the first party options, 3 times heavier and costs 2-3 times more!
To answer the question, it's because it generally isn't compared to the Nikon or Canon 50mm F1.4 lenses, more often to the ~$4000 Zeiss Otus 55mm, one of the sharpest DSLR lenses available.
When we look at the Canon, Nikon and Sigma 50mm F/1.4's, the Sigma's primary advantage is sharpness when shot wide open. Any lens, especially a prime can be sharp at 2-3 stops down from wide open. However, excellent wide open optical performance is a hallmark of a quality piece of glass. After all, whats the point of an F1.4 lens if your going to stop it down to F2.8?
Additionally, the Sigma has significantly less barrel distortion, vignetting and slightly less chromatic aberration.
For many, this simply won't matter. Shot wide open at F1.4, so little will be in sharp focus that the Sigma's wide open sharpness advantage will barely be noticeable. Additionally, distortion, vignetting and chromatic aberration are also easily fixed in Lightroom or Capture One Pro.
I came to this lens having used the Sony Zeiss 55mm F1.8 ZA lens, which had very similar optical performance. Most importantly, it was also T1.8, but significantly smaller and around the same price.
Topic of Debate: Aesthetic of F1.4 vs. T1.8
Honestly the reason I prefer the Sigma isn't really because it's a optically perfect lens, but because it focuses far quicker on my Nikon D750 compared to the 55mm F1.8 on my Sony a7ii. Additionally, the amount of light the lens actually lets in may be important, but the aesthetic of F1.4 is still mesmerising. But this brings me back to the original question, why aren't more people seen with the smaller, cheaper versions of the 50mm F1.4 from Canon and Nikon. After the above discussion, I don't really know. The lenses are even similar stopped down to F2.8. Can a 25% sharpness increase at F8 really justify 2-3 times the price amongst other sacrifices? Don't get me wrong, I like the lens, but I personally believe for the aforementioned reasons, that it is not really the right 50mm for most people. It's not like the Sigma 35mm F1.4 Art, which has a T stop rating of T1.6, half a stop more than the Sigma 50.
The build quality far exceeds any 50mm F1.4 from Nikon or Canon. It's on par with the Sony 55mm F1.8 ZA, which is also a very well built lens. Build quality amongst lenses doesn't really get much better than this. The lens may be heavy, but for many the large and heavy construction screams premium quality and professionalism. The lens is an amazing value, especially considering the lens delivers F1.4, autofocus (unlike the Zeiss Otus) and is cheaper than the first party and optically inferior first party options.
The lens also has great focusing when attached to my D750. Congratulations to the engineers at Sigma, as large aperture lenses generally require a larger distance for elements to move to acquire focus.
The lens does exhibit quite a bit of focus breathing. It's definitely not the worse I've seen, but it does limit its possible uses in video or close range portraiture.
The lens isn't exactly what you'd call feature packed. It has no optical image stabilisation or weather sealing. I have to cut Sigma some slack here. As of date of publication, there is no stabilised and weather sealed 50mm F1.4 on the market. However, it is a possible future improvement. The closest thing to what I've been describing on the market is the Tamron SP 45mm f/1.8 Di VC USD, which offers image stabilisation and weather sealing, but is F1.8.
This isn't really a gripe, but the lens hood is absolutely huge. In itself, this can also be considered a pro as it is very well built, far higher in quality compared to my Tamron F2.8 zooms.
Sharpness at Varying Aperture Settings (Center of Frame
The above graph illustrates the sharpest aperture when examining the centre of the frame is F4. Additionally, there is a huge difference in sharpness when stopping down from F1.4 to F1.6. The usual reduction in sharpness (due to diffraction) when stopping down towards F16 is also evident, however not as much compared to other lenses. This can become handy for Macro photography. Keep in mind, you'll be needing extension tubes to allow the lens to focus close enough for Macro purposes.
At the end of the day, I'll be keeping the lens. I am a sharpness geek, although I know in reality this doesn't really make that big a difference in real world use. I've wanted to give the lens all the praise it deserves, whilst being practical in my comparisons, and laying out why one might pick this lens over the Canon and Nikon versions. If you crave sharpness, want fantastic build quality and don't mind the T stop rating of T1.8, this lens is definitely for you. Studio, portrait and wedding photographers will absolutely love this lens. If on the other hand you are happy with still good but not world class wide open sharpness, the same F1.4 aesthetic, lighter weight and smaller size and appreciate the money savings, don't be afraid to try out the Canon or Nikon 50mm F1.4 lenses. Their smaller size and weight are great when traveling or when used in street photography settings. The money you save can then be reinvested elsewhere. I personally don't think most people will be disappointed with either of the Sigma, Canon or Nikon 50mm F1.4 lenses, just get out there and start shooting!
Any thoughts? That's what the comments section is for.