MANUAL SETTINGS ON A CAMERA

aperture is the F stop ( indicated by an F number)

Oddly, the higher the number, the smaller the aperture.

You can think of the aperture as a certain size hole that lets light into the lens, which is adjustable via changing the F stop setting.

F2 is very big hole

F 22 is very small hole.

The F stop is determined in part by some match formula type stuff that has to do with the size of the lens ( focal length). That gets complicated. But generally, if you have a light meter and it tells you that the light requires a certain shutter speed and F stop combination then that works for all lenses, no matter the size of lens.  This is also related to ISO speed.  One F stop is worth 1 traditional shutter speed.

For example, if your light meter says " shoot this at ISO 400 with a shutter speed of 1/60th of a second, and F 8... then you can change the ISO to 800 (twice as fast ), and HALF the F stop ( F 4.5 )

You can also relate the shutter speed to the F stop the same way. At 1/60th of a second shutter speed ( as light meter told you to do) with F 8, you can reduce the shutter speed by half ( 1/30th of a second ) and close the F stop one stop... from F 8 to F 5.6.

It's kinda easy once you get the hang of it.

The depth of field ( amount of stuff in front of and behind the subject you are focused on, INCREASES the smaller the aperture ( once again this means ( odd as it seems ) a HIGHER F stop number.

More stuff is in focus in front of, and behind, your subject focused on, the higher the F stop NUMBER.

The speed of light is a constant, and has nothing to do with this. It is a matter of the AMOUNT of light (F stop), and time exposed (shutter speed), and SPEED OF FILM ( ISO).