Instruction: Extract information from the supporting document to answer the given question.

Input:
Question: What is the difference between this lens for $400 and the similar Sigma lens that says Art for about $1000?
Document: Today, the Canon is $400 and the Sigma $450, which includes the lens hood. So the price is a wash.I'm completely happy with the Sigma-- it's fast, sharp and the bokeh is lovely. I've heard of quality control issues with some Sigma lenses, but this one is well built and a good performer. It's heavier than the Canon, but compared to the Canon 24-105mm I normally have attached, this is a feather.
Although it is more expensive than its Nikon and Canon counterparts - the sigma outperforms them both in auto-focus speed and sharpness. The Sigma 50mm 1.4 is no doubt one of the most underrated lenses around. When compared to the 1,400 dollar Canon 50mm 1.2, the Sigma even outperforms that lens in sharpness (even if barely) at 1.4 aperture. What does this say about Sigma's lens?
I had both Canon 50mm models and this Sigma is far superior on both construction and image quality.The HSM motor makes a world of difference and even in very low light conditions, the lens seems to have no issue focusing correctly. I've tested on both high and low ISO setting with my 5D Mark II and 1D Mark II, it is simply incredible.
I Am so Very Happy with this Lens! It Really Is the Perfect lens for my New Nikon D7000! So Very Fast and Accurate! If you love Taking photos in Natural light without using flashes, this lens will exceed your expectations! Thank You Sigma for a True Masterpiece work of art Lens!! !
AF speed is very similar on both Canon and Sigma models. To me, this lens feels like a step between the Canon 50mm f/1.4 and the f/1.2L... I used the 1.2L once and was impressed with the weight and size and overal sharpness wide open or even stopped down to 1.4, but for the price, I'd rather have this Sigma 1.4!I'm becoming more and more a fan of Sigma Prime lenses!
For 95% of photographs you'll be taking, this lens is the best 50mm I've ever used for a Canon camera, that includes the Canon 50mm f/1.2L. For the price, you will be hard pressed to tell the difference between this lens and the Canon, in fact, the Sigma is sharper.For astrophotography shooters and night sky photographers, the coma performance is only OK. You need to stop down to at least f/2.8 to completely eliminate the comatic aberration in the corners.
Can't get the amount of light and bokeh with zoom's, and love the drama in my shots. It isolates the subject, and gives a 3D feel. I hate busy backgrounds, as they detract from the shot.I do own the Canon 50mm f1.4 lens, and will be selling shortly (there is a noticeably visible difference from the Sigma). The Sigma is superior in bokeh and saturation. I've taken some dreamy shots wide open that the equivalent Canon cannot match.
Following the success of Sigma 30mm f/1.4, Sigma launches 50mm f/1.4. It is Sigma premium prime lens ($475 street) The build quality of this lens is very sturdy.Mounted in Canon Rebel XSi / 450D, the lens body texture is very pleasant to grip. It is heavier than other prime lenses but it is not a big issue. The lens cap from Sigma has a similar built quality like the lens body.
I bought this lens becasue I need a low light lens for some particular shooting sessions. I read this lens has a better boken and blurry background than the new and similar priced Nikon 50 f1.4 G. And I saw quite some people sold their new Nikon 50 f1.4 G and replaced with this Sigma instead.The lens I received is very sharp even at f1.4.
$1000.
Answer: It seems the art lens has higher quality optics which give better contrast and sharper images, although to the average photographer the difference would be slight. The Sigma 50mm 1.4 for $400 works great, and I have no complaints at all.