Will a Sigma 24mm f/1.4 ART autofocus on a Nikon D5100?

I have hears that the ART series is only for F mounts and will only autofocus on full frame Nikon bodies. But then I have also seen that it will autofocus on a D5100.

The, "HSM," designation is Sigma's version of Nikon's AF-S, so yes.

It's stupidly expensive for what it does, though.

I hope you know that the Sigma 24 mm ART lens has a field of view similar to a 35 mm lens, when the 24 mm lens is used on your cropped sensor D5100 -- about 60°50'. This is a great lens if you use it on a full frame camera like the Nikon D610, D750, etc and you are shooting landscapes.

As long as you understand this, go ahead and spend $850 for the lens and enjoy it.

Note: What brainchild told you, "… That the ART series is only for F mounts and will only autofocus on full frame Nikon bodies"? If you look on the Sigma website, you will see that this lens is made with either a Sigma, Nikon, Pentax, Canon or Sony mounts and since it is a lens designed for full frame cameras, it can also be used with cameras with APS-C sensors like your D5100

You should watch this video on using FX lenses on DX bodies:

The presenter talks about how using FX lenses are not nearly as sharp on DX bodies as they are on FX bodies. However, the points made in the video are moot for those who plan on upgrading to an FX body. In which case it makes sound economic sense to future-proof your lenses. While some people question this theory, Tony mentioned in another video that he bases his opinion on an actual lens test - albeit just one test. So take what is said with a grain of salt.

A great place to compare lenses is photozone.de. They have reviews of some Sigma Art lenses, but not the 24mm… Yet. Here's the link: http://www.photozone.de/...e/nikon_ff