Getting the right exposure is fundamental in photography. It's like getting your balance in riding a bike, you're never going to win a competition unless you have the awareness of your balance from the get-go. Three settings will factor into your exposure: Aperture, Shutter Speed and ISO.
So the question then becomes "How do we achieve correct exposure?"
Study this and learn it, then you will KNOW how to control exposure.
Okay, remember how the lens bends the light rays into the camera and onto the film plane? Good. Between the lens and the film plane are TWO devices we use to for exposure control, to control the exact amount of light hitting the film.
THAT'S RIGHT, THERE ARE TWO DEVICES TO CONTROL EXPOSURE....
THE SHUTTER, is usually a curtain-like device just in front of the film. Think of a shade pulled down on a window, and then quickly open it and close it. FOR AN INSTANT THE ROOM WAS FILLED WITH LIGHT and the length of time that burst of light filled the room is shutter speed! That is basically how a shutter controls the amount of light getting to the film.
The time the shade was open determined - to some extent - how much light came into the room - but so did the SIZE of the window! That window opening acted as...
THE APERTURE, which is built inside each lens and controls how much light enters the lens.
Now for some clarification on shutter speeds. Looking at the photo below, you will see the numbers that are changing are the shutter speeds in fractions of a second (i.e. 30 = 1/30, 60 = 1/60). This is the time taken from when the shutter opens to when the shutter closes, after you press the shutter release.
Moving from one speed to the next one halves the amount of light that enters the camera. Moving the other way, to a slower shutter speed, doubles the amount of light that enters the camera. This change from one speed to another is called moving a stop. For instance, moving from a speed of 1/30th to 1/60th of a second is going 1 stop faster, and from 1/60th of a second to 1/250th of a second is moving 2 stops.
HERE IS A RULE OF THUMB FOR PROPER EXPOSURE OUTDOORS:
First, take the film speed number as your shutter speed (100 ISO = 1/125th of a second or, if your film speed is 400 then the shutter speed would be 1/500th of a second for instance), your aperture setting is:
For bright sunny days and the sun is on the subject. f16.
For overcast, cloudy. f8.
Sunsets and sunrises, low light. wide open @ 1/30th
These are basic starting points that usually work. The film package also has some excellent, basic exposure suggestions.
So, you have two methods of controlling exactly how many light rays get on the film and if you understand the above, you then understand how to control exposure for different types of film or different ISO. Re-read it until you understand it, because this is the crux of exposure for daylight photographs.
In order to become more sophisticated with exposure control you need to learn how to use a light meter. This can get very complicated because there are so many light metering systems out there, and so many ways of using those meters. I submit, after 40 years of shooting, that the only true metering system you NEED to master is the light metering system offered in any good SLR camera.
The light meter reads the light coming off the subject matter, through the lens you are using, and is controlled by the film speed you have already set that meter to. It simply is the most sensible, accurate way to meter those light rays. SLR meters are getting more advanced all the time offering "spot" metering (you can zero in on one particular spot on the subject, get the right exposure, and lock in that setting and make your photo) ... overall metering, reflected metering, incident metering, ......and on and on. It is no longer necessary to "bracket" your exposure ( shoot one frame over by one stop, one frame at the indicated exposure, and one stop under the recommended exposure). I quit bracketing twenty years ago and have not exposed a frame improperly.
Therefore, I will not get into other methods of metering. All of the recommended exposures from now on will be based on through-the-lens metering with an SLR camera.
Enough with Exposure Control, Shutter Speed, and aperture.