Can 3rd party lens designers build an exotic prime that bests top lenses from Nikon and Canon? Sigma appears to have taken a very aggressive approach with their fast 50mm optic and created a lens that is clearly designed to offer more performance than has previously been seen at 50mm. Let's have a look. I paid 469.00 for this lens at Buydig.com. Photo above shot with D700 and Sigma 50mm at F/1.4.
 
BUILD
This is a BIG lens. It feels twice as large as the Nikon version and is substantially bigger than other 50mm lenses on today's market. Even on the D3 or D700 it feels chunky and is not going to be winning awards for being small and unobtrusive. It feels beefier than Nikon's 85mm 1.4 for example. The overall feel is that of a professional lens. Perhaps its single failing is a less than buttery manual focus action, though it's pretty good.
 
FOCUS
The Sigma 50mm 1.4 is a HSM motor equipped lens. It's quite fast and feels no slower than the new Nikon AF-S 1.4 that was recently released. But to be honest the Nikon 1.8 and 1.4 AF-D 50mm lenses were fine and quite snappy to focus on most DSLR bodies.
 
COMPATABILITY
Since this lens has it's own focus motor it will function perfectly on all Nikon DSLRs including the D40, D60 and new D5000.
 
OPTICS
This is what you want to know. Why should you buy a 3rd party 50mm 1.4 lens that costs so much? Quite frankly because it's the best. My copy (obviously a nice example) is as sharp as any of the Nikon or Canon 50mm primes wide open and it has less falloff in the corners. Once again most 50mm optics are pretty sharp, but the Sigma pulls away from the competition by offering truly superb bokeh, on par with Nikon or Canon's finest lenses. It's truly a remarkable lens even if it has to be quite large to get the job done.

 
SUMMARY
Sigma has the finest 50mm auto focus prime on the market. It's not cheap at nearly 500.00, but this is a better lens than even Canon's 1.2 or Nikon's very good 50mm AF-S 1.4. Those who own top shelf zooms will enjoy the Sigma for it's ability to shoot in extreme low light and fine control over dept of field. The only thing to watch out for is Sigma's occasional lemon. Otherwise Sigma deserves high marks. They ignored any regard for portability and simply beat Nikon and Canon with this one. Like the Tamron 90 & 180 and a few other 3rd part lenses, the Sigma 50mm sits at the top of it's class and is worth owning.