What Lens would be best for this type of photography?

Http://lookbook.nu/look/2706493-Fresh-Air

Id really like to get this depth of feild, i have a nikon d5100 but i just can't get it!

The 50mm f/1.8 could do that with no problem.

I agree.

Any of the AF-S prime lenses like the 50 mm or 85 mm will do that for you.

If you have the bux, look at the 24-70 mm f/2.8

Any f2.8 or better makes it easy to do this. In this case photoshop has helped a bit.

First off you need to take it on a cloudy day like this picture is. You want to be able to open up the appeture which reduces the depth of field but to be able to do that, you have to be in a lower light environment or force the camera into it. If your camera lets you set the appeture while it sets the speed, open it up. You can also ad some gray filters to reduce the lighting. One critique I have on the picture is that it is slightly out of focus. While it may have been intentional, it looks like it wasn't focused (distance wasn't set right). I might have added a little fill light on the right to put a bit more light on her face. Back to the lens, if you want to play with the lens, zoom lenses typically have a smaller depth of field vision because of all the lenses and mirrors in the zoom lens. Z.

To get depth of field you must stop the lens down as far as possible. Of course this means a longer exposure, and you might want to consider a tripod too. Most lenses will stop down to f2.8. Anything bigger than that is going to limit your depth of field.

The smaller the f/number you use, the easier to get shallow DOF. Set camera to aperture priority and select the largest aperture size (lowest f/number). Position subject as far away from the background as possible. Position camera as near to the subject as possible. Shoot.

50mm f1.8 (140$)
24-70mm f2.8 (1600$)
70-200mm f2.8 (2600$)

Basically, anything that can shoot at f2.8 or lower. Your kit lens with the d5100 has a small maximum aperture (the smaller the f#, the WIDER the opening… It. A ratio of focal lenght vs aperture diameter so a lens at 100mm focal lenght opened at f2 would have diameter of 50mm for example) that get's even smaller as you zoom in.

There are 4 things that control DoF.

-Aperture
-Distance to subject
-Focal Lenght
-Sensor size.

Here is a good calculator:
http://www.dofmaster.com/dofjs.html