Zeno Pelgrims posts an update to Zoic, an extended camera shader for the Arnold Renderer. Zoic is an open source camera shader that is capable of simulating a few optically imperfect lens effects.
Zoic 2 offers twi different lends models. The latest version of the lens shader introduces a raytraced lens model and better sampling for thin lenses when using optical vignetting. The new raytraced model comes with an increase to the rendering times, due to the complexity of the calculations, but the trade off is that it will offer a much more photorealistic effect in the end.
Zoic can be downloaded for free. If you want a ready to use compiled version however, Zeno is asking for a small contribution. Otherwise, you can compile it yourself with the instructions that are posted.
Download Zoic, the open source camera shader for Arnold Render