Just 2x checking, not trying to sound dense...i ask because i am a beginner.
You are saying if you push Kodak TMax 400 to 800 ASA, you use the same development times, and the negatives look fine.?
Doesn't that, effectively, underexpose by one stop if you do not adjust the time.?
Thank You
You are under exposing by one stop even if you do adjust the development time. In either case, you lose shadow detail, and we tend to rate the "speed" of film based on its ability to render shadow detail.
Increasing development has very little effect on the sensitivity of the film. Increasing development does have an effect on the contrast of the film.
So when you under-expose the film and then increase the development time (a "push") what you are doing is accepting that your dark shadows will be rendered with very little detail, in return for having more pleasing contrast in the mid-tones, lower mid-tones and near shadows. In addition, the "push" will make the highlights more dense and contrasty and therefore may make them harder to print in a pleasing way. It is, however, a compromise.
TMax 400 exhibits a very long straight section in its characteristic curve (with most developers). For that reason, when you under-expose it by one stop (meter at an EI of 800 instead of the ISO speed of 400) it isn't as necessary to increase the contrast in the mid-tones, lower mid-tones and near shadows in order to print those areas in a pleasing way. And if you develop it normally, the highlights are more likely to print in a pleasing way. So Kodak's recommendation about how best to make the compromise when light is low is to meter at an EI of 800, develop normally and use the inherent qualities of the film along with the tools available to the printer to obtain the best print.
With other films, Kodak's recommendation about the low light compromise is different.