So, next we should take each item and discuss how it helps us make images within the camera. First off the Aperture. The aperture opens to various different diameters or stops as they are called. The stop numbering system is counter intuitive because a large stop number means a small opening. Stops are referred to by the letter f. So an f stop of 1/16 has a narrower opening than an f stop of 1/8. And an f stop of 1/8 has a smaller opening than an f stop of 1/4. You may see a pattern here, the f stops appear to be in increments of one half. This is no coincidence. An f stop of 1/16 is twice as small as 1/8 and so let's in half as much light. This will be crucial later on and fundamental to the entire capture process. More later on the half/double rule.
Following on from Aperture, is Shutter speed, and as the name suggests, this is the speed with which the shutter opens and closes determining how much time the light entering from the Aperture gets exposed to the sensor. The degrees or measures of time are called stops (similar the stops of measurement for Aperture) and stops of time are also subject to the half/double standard. In other words, a shutter speed of 1/60 keeps the shutter open for half as long as a speed of 1/30. And similarly, a speed of 1/4 is open for twice as long as a speed of 1/8. What this all means is that twice the amount of light will enter when you slow the shutter speed one stop and will halve when you increase the speed by one stop. And now for the bit that connects the dots.
As both Aperture and Shutter speed exhibit the same half/double characteristic and both control the amount of light beaming it's way toward the sensor, it becomes clear that the two are directly related. In other words, if both have timing in common, then by adjusting one in ratio to the other, we should be able to get the same result. What result? The amount of light hitting the sensor should be the same. Example: an Aperture of f/4 and Shutter speed of 1/125 will leave the same amount of light onto the sensor as an Aperture of f/5.6 and Shutter speed of 1/60.
And last but not least, the ISO (International Organization for Standardization) is a number that denotes how sensitive the sensor is to light. And surprise, ISO also follows the half/double convention, in other words, an ISO stop of 400 is twice as sensitive to light as ISO 200. Using a high ISO requires less light and using a lower ISO requires more light. As in the previous paragraph, by adjusting the ISO levels one stop, you can adjust either the Aperture or the Shutter speed one stop. Adjusting the ISO from 200 to 400, you could adjust your Aperture up from f/8 to f/16 and leaving the Shutter speed alone, you would be allowing the same amount of light to enter. As the sensors is now more sensitive to light, less light through the Aperture is required for the same exposure. Again, by adjusting the ISO from 200 to 400 and leaving the Aperture constant, you could increase the shutter speed from 1/125 to 1/250. This would leave half the light into the sensor but the sensor is now twice as sensitive.
So there you have it, playing around with these 3 elements will take time, patience and practice. Bring it on,