Looking to upgrade 50mm?

I've been researching some different options as an upgrade to my nikon 50mm 1.8G on a nikon D800, but would love to get a more direct opinion. I'm a natural light photographer and would like invest in an upgrade for my natural studio sessions with newborns/young children. I've read that the 1.4 doesn't make a significant difference and I prefer autofocus, so the 1.2 is out. But am considering the sigma 50mm ART. If not, this lens, do you have any other recommendations?

Really, there may not be a good reason to "upgrade" the Nikkor 50 mm f/1.8. It is a very sharp lens.

The new Sigma ART series lenses are worth a look. I shot an event last week using the Sigma 35 mm ART lens on a Nikon D810 and the images were just spectacular.

With cameras performing so well at high ISO settings, the little difference between a f/1.8, f/1.4 and f/1.2 lens is marginal when it comes to exposure. All you will really see is a slightly shallower depth of field and the faster the lens the more light fall off there's at the edges when the lenses aperture is wide open.

The cost of the Nikkor 50 mm f/1.2 ($725) does not justify the slight edge in light gathering.

I suggest that you try using the Nikkor 50 mm f/1.4 ($485) and Sigma 50 mm f/1.4 lens ($959) under the same conditions and then view the results on your much larger computer monitor.

Just a note: Any of the Nikkor lenses you plan on buying should be AF-S lenses because the are designed to work with the newer auto-focus systems found on the latest Nikon digital SLR cameras. The are much faster than the older AF lenses and they "talk" to the cameras auto-focus system

EDIT: Why is it you are looking at a 50 mm lens when your stated purpose is to shoot portraits of newborns and young children? The 50 mm lens is the portrait lens of choice for those using a cropped sensor camera, but you are using a full frame camera. The lenses you should be considering are the Nikkor 85 mm and 105 mm portrait lenses made for that purpose to be used on full frame and 35 mm cameras

To work with faster than f/1.8, accept manual focus. There's no other way.

I agree with fhotoace 100% - consider the 84 f/1.4G instead of the 50mm.