When shooting tightly-framed portraits, it is best to use longer focal lengths to keep the camera far enough from the subject to get a nice perspective. A 135mm focal length works very well for head shot portraits on any DSLR camera.
Using a very wide f/2 aperture on this lens at this close focus distance means that depth of field will be very shallow. The good news is that the background becomes extremely blurred and that the viewer's eye is drawn to the in-focus areas of the image. The bad news is that keeping both eyes and the mouth in focus can be challenging. Here, I placed the plane of sharp focus so that it runs between the subject's eyes and across her mouth.
Lighting for this image was provided by large windows and a heavily clouded sky .