How to push film with an automatic camera?

I have the Nikon N75 and for a project I'm starting to work on I'd like to push my film. However, I can't set the ISO on my camera because it automatically picks up the speed of the film. One of my teachers briefly described a few months ago a way to do it through my exposure. It was something about under exposing or over exposing by a stop or two, but I can't remember what he said. Does anyone know how to do this or have any other suggestions? Also, how long should I develop my film for?

I'm using Tri-X 400 film.

It is easy enough.

The light meter in your camera allows you to under expose your film.

Look on page 61 of your user manual

Or you can set the EV of your camera to underexpose your film for the whole roll

That is on page 64 of the user manual.

If you do not have the manual, you can find it here

http://www.butkus.org/chinon/nikon/nikon_n75/nikon_n75.htm

Just think it through, you don't have to try to memorize things someone has told you. That is the advantage of learning something instead of memorizing.

So you are using 400 film. You have your exposure compensation set to 0, for normal exposure. If you want to push the film one stop, that is ISO 800.So you are actually underexposing the film by one stop, thus you set your exposure compensation to -1. If you wanted to push the film two stops, which would be ISO 1600, you would set the exposure compensation to -2.

You could also increase your shutter speed one stop over a normal exposure meter reading. So if for example, with exposure compensation set to normal (0), and your camera meter reads 1/125 @ f8, if you change your speed to 1/500, you are underexposing by one stop, the same as if you had set the ISO to 800. Same thing with aperture. You could stop it down one stop to equal a one stop push of the film. Keep in mind, the ISO setting on the camera (if you had one) does absolutely nothing except to vary how the light meter determines exposure values of a scene. You can do the exact same thing yourself with the shutter speed, aperture, or exposure compensation.

You will keep it set as such for the entire roll. You will have to research to find how much change is made in the processing time. The Kodak site should tell you specs on your film and push / pull development times.