>>3534777Cause pushing doesn't *actually* increase the iso beyond 0.5 to 1 stop, and that in special pushing developers.
The formula to calculate the ISO takes into account base+fog density, and the HD curve of the film.
Intuitively you can think of it like this: the ISO is determined by base+fog density (think noise floor), and then the length of the curve's toe (i.e. the non-steep part at the beginning) and the steepness of the curve's middle part.
The lower the base+fog, the higher the ISO. The shorter the toe, the higher the ISO. The steeper the middle part of the curve, the higher the ISO.
What pushing does is make the middle part of the curve steeper, but since it also makes the toe longer and the base+fog higher, they mostly cancel out and the ISO doesn't change much.
What it means practically, is that if you push film, you'll get less shadow detail and more contrast compared to a native higher-ISO film. Pushing also increases grain, but a pushed slower film still has smaller grain than the (same family) higher ISO film.
So you decide what you prefer, a less grainy but more contrasty look, or more shadow detail but bigger grain. Other practical points are availability, price, what you have with you™, and that past certain speeds there just aren't any films available natively at higher ISOs.
Pulling has the opposite effect: It reduces grain slightly, and it decreases contrast allowing you to keep lots of shadow detail without blocking the highlights. This is great for contrasty scenes, but can make flat scenes appear even flatter. Still though, you can up the contrast during printing with contrast filters, and pulled negatives are very easy to work with. In general, pulling is a far more useful technique to use regularly - apart from when higher speed is necessary - everybody seems to be hyped about pushing even in broad daylight.
