The basic elements of exposure are shutter speed, f-stop and ISO settings. These three elements work together and balance each other to produce what can be a very well exposed photo. Before we go on, let’s talk about what good or proper exposure means.
This is a very subjective judgment. In the end, I go by an old Duke Ellington maxim. He said, “If it sounds good, it is good." You can worry all day long about what proper exposure is, but if you got the image you wanted and it looks great, then it’s properly exposed for what you want. To me, an ideal exposure will contain all colors or shades of gray between white and black. That is, the whites are a true white and the blacks are a true black. Often the exposure can be either too dark or too light and that will change the appearance of the colors.
The important thing to remember about all this is that the exposure settings on your camera are tools for getting the picture you want. There is no right or wrong way to get the picture you want, only successful and unsuccessful ways.
ISO, f-stop and shutter speed are like the three legs of a tripod, to use an appropriately photographic simile. As one of these factors changes, the others have to change to accommodate and preserve the proper exposure.
If you make your shutter speed very fast, you have to accommodate by allowing more light through the aperture. This is done by changing the f-stop. Alternatively, you can change the ISO setting so the sensor does not need as much light. If you choose to change the f-stop to decrease depth of field for a portrait, you then have to decrease shutter speed so the right amount of light gets to the sensor.
Something nice about digital cameras is that you can change the ISO settings. Back in the film days you had to stick with the same speed setting for the whole roll of film. If you do change the ISO, just be sure to adjust the other settings to compensate for the increased or decreased light sensitivity.
The best way to really get a grasp on how all these things work together is by experimenting with the settings to see how they change the shooting experience and the final image. Be scientific about it; keep two of these settings the same and change the third. Then switch off and try changing another setting. Doing this with one setting at a time will allow you to isolate which settings do what.