Written for a buddy of mine awhile back for the D70, but it'll carry over to other cameras as well.
When thinking about photography in general, think of it as in stops of light. I mean, what is photography without light? This becomes very useful when you consider that ISO, aperture, and shutter speeds affect how the light hits the sensor.
ISO. ISO varies from ISO 200 - 1600 on the D70. When someone is referring to the base settings, it is referred to the lowest setting that the camera is capable of and in this case it's ISO 200. ISO in digital terms refer to the same thing as ASA in reference to film. As you go higher up in ISO, there's more noise and in film terms, it's the same as grains. By going to a higher ASA film, or bumping ISO to a higher number, you're effectively increasing the amount of light that the sensor is gathering. So how does this relate to stops of light? By doubling the ISO # at each step, it's the equivalent to one stop of light.
200 -> 400 -> 800 -> 1600
Going from 200 to 400 is one stop of light, from 400 to 800 is another stop of light, and so on. With that in mind, going from ISO 200 to 800 is 2 stops of light.
Aperture. This is the f/stop setting that you see. It is really dictated by the lens since it is referring to how large of an opening the lens will let through. Apertures varies from f/1.4 all the way to f/32, but it's the lowest number (which conversely is the maximimum aperture) which is counterintuitive. At f/1.4, the lens is letting through a lot of light, but at the same time, the focus area is extremely thin. It's very rare that you have to shoot at such a large aperture unless you're really lacking light or shooting indoors without flash (hence termed natural light). The change in apertures can also be viewed in stops of light.
1.4 -> 2.8 -> 4 -> 8 -> 11 -> 16 -> 22 -> 32
Similar to ISO above, going from f/1.4 to f/2.8 is one stop and going from f/1.4 to f/4 is 2 stops of light. You might also wonder where the other apertures are, such as 1.8 on the 50mm f/1.8. 1.8 is actually right in between f/1.4 and f/2.8. It's actually 1/2 a stop of light in between the two.
Lastly, shutter speed. Shutter speed is how fast the shutter remains open/closes. Shutter speed will vary between bulb to 1/8000 on the D70. Bulb is where you basically set the camera to bulb mode and trigger it by using the remote (you can do so without the remote, but then you have to sit there and hold the button down). Hitting the remote button while the camera is in bulb mode opens the shutter and keeps it open. To close the shutter, hit the button again and it closes the shutter. Likewise, you should view it in stops of light.
... 1/15 -> 1/30 -> 1/60 -> ...
Shutter speed is shorter and longer than those on both ends, but I just chose those to use since the list is much longer, haha. Going from 1/15 to 1/30 is one stop and going from 1/15 to 1/60 is two stops of light. You can click through the D70 to see all the available shutter speeds (set the camera to shutter priority; for aperture above, set the camera to aperture priority).
So, with the above in mind, you probably wonder, what does the above have to do with anything and each other? The most important aspect of the above is lighting (as stated earlier, it's all about stops of light). If you can see all three above as stops of light, you can plug in any combination to get the correct exposure!
So for instance, if you have a perfectly exposed picture at ISO 200, f/4, with a shutter speed of 1/30. Taking a look at this picture you noticed that the exposure isn't blown (check your highlights menu to see if any of it is), but at the same time, you noticed that the focus is rather shallow and you want a bit more. In this case, to get more depth you want to stop down, which is the same as going to a smaller aperture. Say that you want to take the picture at f/8 instead. How is f/8 different from f/4? Well, from the info above, you know that it's 2 stops of light's difference (4 to 5.6 is one and 5.6 is another). f/8 is s stops of light less than f/4 so to compensate for it, you can chose to go with either bumping ISO or go with a shorter shutter speed. In this respect, if you want to keep ISO constant then shutter speed will have to be altered and vice versa. If you were to keep ISO at 200 and shoot at f/8 you have to vary shutter speed by 2 stops of light (f/8 is two stops slower than f/4). Starting at 1/30, if you half that, it's 1/15 and that's one stop of light. To get another stop, you have to half the 1/15 and you arrive at 1/8, which is two stops of light. So, the new picture is snapped at ISO 200, f/8 with a shutter speed of 1/8 secs. Say that instead of altering shutter speed, you want to vary ISO instead. With that in mind, you would have to snap the new picture at ISO 800, f/8, with a shutter speed of 1/30. Then there's also the middle ground, altering both ISO and shutter speed. Let's say you want 1 stop from each of them. The picture would then be taken at ISO 400, f/8, with a shutter speed of 1/15. As you can see, when it comes to exposure, you have a choice with what you want to do.
Well, that's enough info for you to digest at the moment. Welcome any question that you may have after having read the above, haha.
