Originally posted by Kozlok DOF gets thinner the closer you are to the subject, the greater the focal length, and the larger the aperture. For a given subject framing, the distance to the subject and the focal length are directly inverse, so they cancel each other out, leaving only aperture. If you want a thin depth of field, and don't care about other factors, then you only need to worry about aperture.
All that said, you really don't want the "thinnest DOF possible". The problem you run in to is the picture looks all blurry, but you notice that one little tiny part of the subject is in focus. If you want to experiment, just get an old M50/1.4 It's really a great lens, and has a super fast aperture. Then you can decide where you want to go from there.
nicoprod - As Kozlok observed, you get the thinnest DoF at the closest ranges. However at the closest ranges, you need the wide angle lenses in order to get a person's entire face into the frame - but then it will be distorted. Along with the distortion, the nose will be the only thing in focus. What it boils down to is - everything in moderation. A reasonable distance from the subject to keep a reasonable framing, along with an appropriate aperture selection.
So, with all of that, what is the DoF of a face. Let's say around 12 inches to get everything in - some distance in front of the face to back to the ears. I think what you want to do is to blur the background in order to create some subject separation. In film, it was something like 70 to 135 mm as popular focal lengths. With the smaller sensors, the 70mm essentially turns into 50mm (crop factor). This is great for you, since there are a lot of 50 mm lenses available with fast apertures from f2 to f1.2 available with out costing you your first born.
- A 50 mm at f2.0 at 10' has a DoF of 0.96 feet
- A 50 mm at f1.8 at 10' has a DoF of 0.86 feet
- A 50 mm at f1.7 at 10' has a DoF of 0.81 feet
- A 50 mm at f1.4 at 10' has a DoF of 0.68 feet
- A 50 mm at f1.2 at 10' has a DoF of 0.57 feet
If you want a thinner DoF just move closer, however your frame will get accordingly smaller.
50/f2 are easy to find for $20 (with a film camera attached). 50/f1.2 are difficult to find and can be very expensive. Also, don't forget about the older screw mount M42 Tak lenses (with an adapter). They are plentiful, wonderful glass, and are widely available (ebay, craigslist along with the marketplace here on the Forum). Probably the optimal lens for a reasonable price will be a 50/f1.7 to a 50/f1.4.