I quite often shoot D100 at ISO 50. It works fine, you just develop normally.
D100 has a straight-line region that extends from about 3 stops below nominal exposure for ISO 100, to about 7 stops above. So if you shoot it at ISO 50 you gain a stop on the low side and lose a stop on the high side, giving you a straight-line region from about 4 stops below nominal (at ISO 50) to 6 stops above. This gives you more protection against blocked shadows, provided you don't have any extreme highlights to worry about.
My only concern would be if you have bright highlights or over-expose, you might find the negative density exceeds the Dmax of your scanner. I have no practical experience of this as I print in the darkroom.
Really depends on your developer. But the way I do it, Delta 100 has quite a long toe, and I like most
of the exposure on the relatively straight line part of the curve; so I rate it at 50. TMX on the other hand
has quite a bit of contrast not only overall, but way down into the shadows, with relatively little toes;
and it's a film that can block up in the highlights under extreme brightness range, so I expose it at 100,
just as mfg rated. These two films might have similarites in grain, but not in curve shape. They are very
different animals in that respect.
If you're planning on shooting in Bright Light, don't overlook Microdol-X 1:0, if you don't mind a slight softening of grain. It has a nice effect on strong highlights for Tmax 100, like Panatomic-X IMO.
Thanks all, some great info in here. I should have mentioned that I'm shooting with a Leica IIIf using a 5cm Elmar. The options for using a filter are rather slim and expensive, so I've ruled that out for now. I do have a CV 35 as well, so maybe I should just find a 35mm viewfinder so I can use that with filters. I may have to research Microdol-X/Perceptol...etc. I do plan on shooting a few rolls this weekend as we are expecting about 2 feet of snow Friday night, so come Saturday afternoon when the suns out, my scenes will be more than bright!
Forget about Microdol(-X), Perceptol etc with TMax 100 or Delta 100. You'll gain nothing there. Use what you're using, or use a general purpose developer like D-76 or XTOL, Ilford DDX etc.