I will be using FS100 cameras for small club band shoots.
Invariably these small clubs have terrible lighting.
Yes, I know the best solution is to light the stage properly before the shoot. I won't give you all the reasons why this doesn't happen (no room on the dance floor, club owner liable when inebriated dancer trips, etc.), but the fact is it rarely can be done in these particular clubs. I am going to have to adjust the FS100's as best as I can during the shoot, and then use the best tools available to deal with it in post.

In the club I just shot in (with an XF100, the FS100's haven't arrived yet), there is a very large picture window right beside the stage. As a result, for the first two hours (until it became darker outside) the light was very harsh on the right side and the two musicians on that side have shiny pasty faces on one side. The stage is wide, so the musicians on the far left side are in low light.

In this case, I need the post processing equivalent of a graduated ND filter. It would start strong on the right and gradually disappear. It would need to be high quality, because I don't want an "effect"...........I want it to be as natural looking as possible.

In most other clubs, the issue is very bright spots, poorly placed, maybe shining on a small area behind the band. In this case, I would need to be able to shape the area for graduated filtering.

What is the best tool/plugin for doing this in FCP X?
I am assuming there is a high quality tool for this, since it must be a fairly common problem.
I'm too new at this to know what is available........if there is a good resourse on this, please let me know.

I have found Tiffen Dfx, which has a plug-in for FCP X.
The many filters include one called ND Grad, which appears to do what I am looking for.
Has anyone used Dfx?

Well, I downloaded Dfx for a trial, and used the graduated ND filter.
Then I used the Mask built into FCP X, and I would say the Mask did at least as well as the Dfx filter.
And I didn't have the problem of dropping frames that I had with Dfx (I've got to sort this problem out, it happens whenever I use a 3rd party plug-in).

I am not adept with either tool, so this isn't a fair test.
But my initial reaction is that I underestimated the quality of the FCP X Mask.

Tiffen's Dfx filters are very good. I used them for still before video. And yes, you have used them to solve a problem that they are not usually used for. Bravo for inginuity!

Now I am accomplishing most of the same effects with Magic Bullet's "Looks"

Thanks for the input, Chuck.

If I could please ask you a little different question:
I see that Looks has some of the features of Colorista II. Do you use these features for color grading?

I ask because I enjoy color grading more than anything else (a bit perverse, yes) and the poor lighting in the clubs make this a major focus of every editing session. So, I have been looking for tools to augment FCP X's capabilities.

I looked in detail at what it would take to use Resolve and it makes absolutely no sense for me.............I would spend tons of times learning complex capabilities designed for much more complex movies, and also spend a fortune on a separate computer system. Not for a hobby.

I am now looking at more powerful color grading tools/plug-ins in between FCP X and a full blown Resolve system. I am just wondering if Colorista II is a more powerful tool than FCP X, or just a little different way of doing the same thing.

Thanks for your help!