But since I was shooting at 200mm (within the 70-200mm range) most of the time, on my second outing I thought I would experiment and use the 200mm f/2 fixed lens saving me a stop of light, theoretically cutting my ISO to 1600.
Now, the 200mm f/2 lens is shown below on a D800 body and while that to the left of the gold ring is lens hood, it can nevertheless be seen to be a little monster. It weighs a whopping 2.9kg which compares with the 70-200mm which is a lightweight at 1.5kg. Aside of the extra stop of light, the other key advantages to the 200mm f/2 is the focus speed, it's possibly the fastest focusing lens I have ever used, it's effectively instant, and it is supersharp since there are no optical compromises that you get with a zoom lens. You'll also get shallower depth of field and improved bokeh. But...
Despite VR then, I actually felt that the minimum realistic shutter speed to guarantee results was 1/320th meaning I am giving back half a stop of light by needing a faster shutter speed. So while I did shoot at ISO 1600, realistically I think I was underexposed by around half a stop and some pictures needed that adjustment in post production. Overall though, the pictures came out pretty good. Whether for all practical purposes they came out better than those shot on the 70-200mm is a different matter. pictures from both lenses are shown below.
There's another problem too, stage lighting changes as lights go on and off while the actors themselves move around, sometimes in the light, sometimes out. All in all, it's a bit of a nightmare, so what should you do?
For all the worries we discussed, stage lights will be modestly constant and if the actor momentarily steps into a darker area of the stage, well, that's life. The action is simply too quick and too unpredictable to change your settings between every picture taken. Aim for a sort of broad average. Keep checking however along the way to make sure you are in the ball park and if you are, and if you are, that should be good enough, especially in light of the ability to correct plus or minus a stop of light in post production.
Colour casts can also be a problem when these are caused by colour stage lighting and while these can be corrected a little if felt necessary (consider using the saturation sliders), if it is too much, just popping the whole thing in to black & white can also be a life saver while giving it an extra edge of moodiness.
Whether then it is worth the extra weight and effort of the heavier prime lens versus the very capable f2.8 zoom , my gut reaction says the answer is no. Had it been darker still, and ISOs creeping up to 6400 or 12800, maybe. As a lens, it is utterly bonkers, but it is without doubt a lovely piece of very high quality kit.
ps, the advantage of going twice: knowing how it would end, fire, torches etc, I popped a wide angle in my bag specifically and switched lenses as they were setting it up. It seems a fitting end to the post also.