Not a silly question at all. It gets complicated fast. Bear with me while I lay out some basics. I suspect you know most of these, but it's all pertinent.
When you hear about "exposure", what's really being talked about is the total amount of light hitting a light sensitive medium - film, paper, sensor...
For the moment, assume we're refering to entirely average conditions: an arbitrarily chosen film with, let's say, medium speed, or sensitivity to light, and trying to record an average subject under plenty of light. For people new to working out their own exposures, the basic rule is that with a given film, to render the same subject the same way, the film will need the same amount of light hitting it regardless of whether the scene is lit more or less. (yes, there are exceptions, but under ordinary conditions they can be put off a while.)
And as long as I'm simplifying to get some basic points across, I'l make the assumption that there is just one correct exposure for a given scene. Later you will discover I'm full quite wrong with these simplifications, but they'll help get things clearly stated for the moment.
Exposure is always the result of a combination of the rate with which light hits the film (or sensor) and the length of time the light does so. So, you have two basic controls, one for the rate light flows, and one for duration of illumination of the film.
Controlling the rate light hits the film.
The rate with which light hits the film is determined by the opening of the lens, variously called the f/stop or aperture or just opening. Basically if the diaphragm is more open, the lens lets light through faster, and if the lens diaphragm is closed down to a smaller opening, less light will pass through per unit of time. The openings are usually listed in a series.
f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90 ...
Most lenses offer only a limited portion of this range due to physical, design and cost limitations. My first normal lens for 35mm photography went from f/2 to f/16, quite typical.
The key here is that each step jumps you to half or double the rate of exposure of the last. With apertures, the small numbers correspond, perversely, to a big opening and a lot of light coming through, so f/2 lets in lots more light than f/16, 32 times as much per unit of time.
Controlling the time light light hits the film.
The time light goes through the lens to hit the film is generally controlled by the "shutter" which does pretty much what the name implies, opening and shutting. For the most part, it's an all or nothing operation, either open or shut. Expert leaf shutter users will note I am making one more simplification here, again, for the benefit of a direct explanation.
The time intervals are generally a series like: 1/1000 sec, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1 second, which was the series that was available on my first camera. The key here is, again, that each step jumps you to half or double the rate of exposure of the last. With shutter speeds, the change in exposure is directly proportional to the tiem the shutter is open, so that a one second exposure will be twice a half second exposure all other things being equal.
Barring fairly old equipment, the fstops and shutter speeds avaiable to you will be at least similar to some portion of what I have listed above.
To get a given exposure (total amount of light) you'll need a lot of light briefly, or a slower flow of light for longer, or a middling rate for a middling time. Your meter will generally indicate some combination of fstop and time, based on the brightness of the scene and the responsiveness or speed of the film.
But sometimes you will want a different shutter speed or aperture for creative purposes - how to convert from one set to another?
Think of it as a staircase.
An example - your meter says f16 and 1/1000 of a second
If you are at the right exposure at, say, f16 and 1/1000 of a second - and you want a different f stop or shutter speed - you are going up the staircase with, say, f stops, to f11, but to keep the exposure constant, you must come back down with shutter speed.
Going from f16 to f11 will double your exposure since the lens opening gets bigger letting in more light, so you'll need to cut the exposure time in half to compensate. Half of a 1/1000 of a second would be 1/2000 od a second.
Your new exposure setting in this example is f11 at 1/2000th.
The key is that each step in the series jumps you to half or double the exposure.
So:
If you go one step up one, go down one step on the other.
If you go two steps up one, go down two steps on the other.
If you go one half step up one, go down one half step on the other.
And so on.
Whether you fiddle with f/stops or shutter speed, each full step doubling or halving of exposure is reffered to as one stop, up or down.
The confusing part is the f stops, since the small numbers make for greater exposure, and the big numbers mean lesser exposure.
A more general example:
Imagine two tape measures. One has f/stops and the other exposure times. Each increment on the tape is - say - an inch.
I'm going to write it out horizontally to save space:
One tape is f/stops - 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90 ....
at each step right exposure is reduced to half.
One tape is exposure times - 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1, 2sec, 4sec, 8sec ... at each step right exposure is doubled.
If you know any combination of these is the right exposure, line the two tapes up side by side, but, here's the vital part, match up the correct exposure combination side by side.
Now I'm going to use an arbitrary example where your meter says f5.6 at 1/125th, and line up the tapes vertically for clarity.
f/stop vs exposure time
1.4 1/2000
1 1/1000
2.8 1/500
4 1/250
5.6 1/125*
8 1/60
11 1/30
16 1/15
22 1/8
*note that here we've set f/5.6 and 1/125 side by side - this lines up everything else!
We already know f/5.6, 1/125th is good.
The tapes also tell us f/4 at 1/250th works too.
and f/2.8 at 1/500
and f/8 at 1/60
and f/11 at 1/30th
etc.
All the side by side pairings will provide the same exposure.
Now, here's the payoff that may clarify your query. If you want to change the exposure to more or less for creative or technical ends, you can deliberately step up or down the staircase by whatever number or steps or stops you wish. Lets say you want one stop more exposure, slide the tapes so that 5.6 is matched with 1/60th. Any of the side by side combinations will give you one stop overexposure. To get one stop under, match 5.6 with 1/250th. Again, any of the side by side combinations will give you one stop underexposure. Move the tapes less or more to get a half stop or more than a stop.
It's good you're looking at the zone system. It provides an unequalled way of understanding many aspects of exposure.
Hope that helps.
Best,
C
