Originally Posted by ashamsuddin 
Okay, I've been reading the forums and so many of you are very knowledgeable about photography, that I know it will KILL you to actually answer this question, but straight answers will be very much appreciated.
My wife and I are wedding videographers, and she is going to start taking some entry level gigs as a photographer. I want to get her a good quality lens that she can use to get some great shots, but I can only afford one right now.
Right now she has the kit lens (18-55) and a Sigma 70-200 (f 4-???) - on a *istDL
We know that she needs something with a lower F stop, but what focal range does she need?
I've done three weddings as an amateur. The flash makes a huge difference, plus you need a great sense of timing, which means either a fast lens with manual focus, or autofocus.
Note that many of the lenses mentioned (DA*) are SDM and will not autofocus on *ist DL.
The 50/1.4 FA can serve as a portrait lens will give you lower depth of field at a given aperture than a normal focal length such as the 31mm/1.8. The in-focus areas are wicked sharp when stopped down, too, and it has very nice bokeh.
I think it would complement the two zooms nicely, especially considering that the zooms are at their worst at this focal range.
Whereas the kit lens' best range is near it's wide end (20-28mm) which will do for group shots, and the kit lens works great with a flash.
So if you have the bucks, go for a 50mm/1.4 plus a flash.
Just watch out for overexposing backgrounds, because like any super-fast lens it can get some serious colour fringing in the bokeh in high contrast areas. (no problem for black and white of course).
(If you had an SDM-enabled camera I'd say wait for the DA* 55/1.4 which comes out in January.)
P.S. shooting portraits with 50mm gives less perspective distortion than 31mm also.
P.P.S. The 40mm/2.8 is said to be the fastest at autofocus with the screw-based focusing, and also makes the *ist DL very pocketable. And it it still has at least a 1 stop advantage vs. the kit lens.