>>3535043I'll explain a bit more.
Every film has something called a characteristic curve (or HD curve). It describes the way the films builds density after development (i.e. how dark it gets to the naked eye) relative to the exposure it has received. Look at picrelated.
Toe, middle part and shoulder are just areas in that curve, in practice they correspond to shadow details, midtones and highlights in your photo.
You see how the density doesn't start from 0? That's because the physical base of the film where the emulsion is coated on, is not completely transparent, but has a density itself. And secondarily, there's "fog", unwanted density that builds up "accidentally" even without exposure. That could be due to the film aging, cosmic rays/radiation, heat, etc. .
That's what base+fog is, the combined densities of the base material and the fog. And when doing calculations, any density up to that point is "discarded", you start counting from base+fog and up.
Notably, the ISO calculation takes the base+fog level into account. The lower the base+fog, the higher the ISO - all other things being equal.
>I see, so the 'only' actual difference between a film that has the ISO you're actually shooting at is the more/less shadow detail and contrast?Yes.
>Right, I know a lot of people recommend pulling 1 time on film because of this exact thing. Would you agree on this?Sure. It's really useful for high contrast scenes. And when in doubt it's always better to have a negative that is a bit too low in contrast, than a bit too high. Cause all the information is there on a low contrast neg so you can increase contrast at will during printing. But a high contrast negative has lost some information so you can't go back.
