Sigma 24-105mm f/4 Art Lens + 16x9 Format + Mountain Bikers

First off, this lens is a big hunk of glass and metal, the filter ring is a whopping 82mm.  It feels solid and precise, but does it perform like a pro lens?  Does it auto focus like a high end Nikkor zoom?  Is it sharp at f/4?  I took a few minutes to photograph mountain bikers, and I decided the set looked best cropped in 16x9 format.  I am shooting with my Nikon D4, at f/4, 1/500 second shutter speed, and various ISO.  Continuous AF.  Spot.

My first concern was it's sharpness at F/4.  It's always dark in the woods in wester Washington, and so sharpness at f/4 is important.  It's very sharp, sharp enough for professional work that you would do with a full frame dSLR.  There are tech reviews out there, a lot of them.  Go find them and check all the numbers, but this lens is sharp enough.  I should say I only tested it here from focal lengths 24-35mm.  More testing on the telephoto end later, I have heard it is not as sharp at telephoto.  As a wide angle lens, it's great.  Auto focus was lightning fast and accurate.  I rarely shoot as accurately as I did with this lens today.  My go to for comparisons is the Nikkor 70-200mm f/2.8 VR II.  The Sigma is in that range of goodness when it comes to AF speed and accuracy in a real world situation, like mountain bikers coming by you at speed.

My first impressions are really good with this lens.  When I have time for more elaborate testing I will update.  For now, good night.