Hello Steven,
to your first question:
The mask is mainly useful (and was introduced) for optical printing on CN paper. And for us as photographers it is important to differentiate between more technical and theoretical factors and our "real world": If you are using colour negative film and look at the whole imaging chain up to the final result you see several factors that influence the colour rendition:
- with optical printing it is your personal filtering process (your visual impression / your taste) and the choice of your colour paper, and both have huge influence on the resulting colour
- with scanning you have the characteristics of the used scanner, of the scanning software, and the "filtering" = editing you use as a scanner operator.
Or to say it in a different way: Give one negative to 10 different photographers or labs and you will most probably get 10 different results in colour rendition.
So in "the real world" colour accuracy of negative film is often a more theoretical concept.
That is by the way one of the reasons why in pre-digital times in professional photography for book publications and in advertizing almost exclusively positive film was used: You always have the transparency as an original and as reference. And all who are involved in the process can refer exactly to that.
And then you have the very important point that often film manufacturers simply don't go for "most accurate / most precise" colors, but for "most pleasing colours" for the target audience.
Best example ist Kodak: None of the current Kodak CN films is really "accurate" in a technical sense. All current Kodak CN films have a warm (yellowish) rendering. Most a quite strong one (especially all the amateur films, Ektar 100, but also Portra 400 and Portra 800), others a bit less (ProImage, Portra 160). Portra 160 is currently the only Kodak CN film which is oriented more to a neutral, less warm colour rendition (but not completely neutral).
Kodak is doing that because due to their experience and market research their customers go for that warm look, have a preference for it.
Fujifilm has a different approach.
And then of course you have the problem that different people indeed see colours differently.
E.g. if you ask 100 people looking at the same colour chart you will not get 100 same descriptions of these colours.
If I look at my personal colour assessment, the most natural / neutral / accurate colour films I've used and tested have been Reala, Superia Reala, Astia 100F and Provia 100F.
Therefore my recommendation as a photographer for a very pragmatic approach:
Just use the film and a certain colour rendition which fits your subject and your creative idea. Use what you like.
And don't care too much for theoretical or technical concepts.
To your second question:
Color reversal film generally has a bit finer grain (and higher resolution and better sharpness) than color negative film of the same speed. The main reason is the reversal process: When the film is exposed, mainly the larger silver-halide crystal are exposed. And in the reversal process these are removed, and the finer/smaller crystals remain forming the final positive picture.
And with Velvia 50 and Velvia 100 we have in addition the unique characteristic and advantage that they are delivering a superior, unsurpassed resolution already at extremely low object contrast (1.6:1) with 80-85 lp/mm. No other colour film is offering that, especially no colour negative film.
Best regards,
Henning