When using an artistic medium such as film, there are many considerations that are in play when deciding just which film to pick. Color and B&W aside; box speed is a primary determinant factor in what, how, and where you’re going to be able to shoot. If it’s golden hour almost dusk, most shooters most likely will stick to a 400-and-above speed film, just the same as someone may choose a 100 speed film to shoot in that harsh mid-day sun.
For those unaware of pushing and pulling film, this simply refers to shooting your film over or under the box speed. i.e. 400 speed pushed to 800 would be +1. The +() only really comes in handy when you need to make a personal note of, or let your developing lab know what you’d like the film to be developed at. BW and Slide films are often more finicky about being pushed or pulled in the sense that they need to be developed as they were shot. Color film is quite different, you can have a roll of Portra 800 and shoot it at 100 or even 200 and still get beautiful results. This is what’s known as latitude; essentially how much a film can handle before having issues. Therefore, by intentionally over or underexposing your film you can achieve some pretty cool results.
All of the fancy terms aside, shooting your film over or under can mean a few things: color shifts, highlights, shadows, and grain alterations. Take that Portra 800 for example, it’s a rather well saturated stock with a medium amount of grain. When shot at 400 or 200 it can appear almost dreamy, kind of heavy air if that makes sense. This is a trick often used by portrait and wedding photographers. The biggest key is that while you may shoot your film over or under the box, you do nothing different when developing. This can be nice for home developers because that means you won’t have to wait for multiple rolls to get the most life out of your chems.
As most things in photography are easier to show, this happens to be one of them. So, in order of top left to bottom right we have: Portra 800 shot at 640, Portra 800 shot and developed at 3200, Pro400h shot at 200, and Portra 800 shot at 400.
Hopefully this helps give those out there the push they needed to experiment! As always, have fun and keep shooting!