Let's start from the beginning:

Opening the aperture results in a more "selective" focus, LESS depth of field.

Closing the aperture results in a less selective focus, MORE depth of field.

In many movies, they "overlight" scenes with lamps and lights etc. and then turn down the amount of light in the aperture.

But then, how do they achieve the "selective" focus we constantly see in movies? Even when using 50mm lenses.

Is this because they adjust the shutter and leave it so that more light hits the film?

Confused!
Help me!

I guess it's the matter of compensation, mostly in the aperture and lighting control. If you know how Gregg Toland make deep focus technique in Citizen Kane, then selective focus would be the opposite of deep focus. (ie. less lighting and bigger aperture)
I don't know if shutter speed have a great deal in selective focus.

You know, I wish we had a forum where we could discuss movies...

There's always density filters to cut down on the amount of light entering the lens.

i use neutral density filters to cut down... oh wait, yeah... what filmerp said.

I don't know cinematograpy, but in photography the limited depth of field is due to a wide open aperture. Not sure if this adds anymore to your understanding.

I'm not sure what you mean by "overlight" the scene. Usually I take that to mean a scene is overwrought in a "Hollywood" sense where every actor is perfectly lit with a key light, fill light, eye light, etc. It's more a stylistic comment than a technical one.

If you mean that most sets tend to be very brightly lit, then that's usually due to the fact that they're shooting relatively slow stock. Or it could be the lens or the shutter angle or a filter. It all sort of depends on the DP and the particulars of any given setup.

3 factors control depth of field:

* apeture setting (wider = less DoF)
* focal length of the lens (longer = less DoF)
* focus distance (closer = less DoF)

As noted above, in brightly lit scenes, selective focus can be achieved by using a slower film stock or ND filters (or both) to allow the use of a wider aperture setting.

Thank you, Audrey. I actually knew all the stuff you wrote, but you kinda just clarified it. Easier to "manage"