When discussing photography it is common to hear people talk about ISO, shutter, and aperture as the "BIG 3". These are the on camera controls that effect exposure, but it greatly limits your photography to think about it this way. The most fundamental part of photography is light. ISO, shutter, and aperture effect how your camera records that light, so the logic generally goes something like this - my camera tells me I need a shutter of 1/60th at an aperture of f4 with an ISO of 200 for a proper exposure. Hence the idea of the "BIG 3", because all three are used in determining exposure.
But let's break this down a little. A shutter of 1/60th is about the slowest you can use and still handhold the camera, you will need a quicker shutter if you are zoomed out, or else camera shake will be evident. With f4 you will get a nice shallow depth of field allowing you to keep the interest on your subject, and an ISO of 200 insures that you will not have noise in the shadows. This could give you a nicely exposed image, or it may not work at all. For instance if you are trying to get a nice tack sharp image of your daughter doing gymnastics. For this you would need a much faster shutter, say 1/250th a change of two stops. You can't makeup that change in the cameras aperture, you might have an f2.8 setting perhaps you even have an f1.4 which will get you close but then your depth of field will be so shallow it will be very difficult to keep your subject in focus especially when she is moving. The only alternative you are left with then is to increase ISO from 200 to 800 which will likely start to introduce noise, which again will effect the over all appearance of sharpness in the image.
The other option is to think of it as a BIG 4 not 3, the fourth element being light. With this approach you gain a lot of flexibility. If we look at the example above and we say well I really need a shutter speed of 1/250th but I don't want to sacrifice ISO and I can't make it up with fstop (remember to change one setting you have to balance it with a change to a second setting) that second setting can be the introduction of more light. In the BIG 3 model you are looking at a triangle where two points have to be changed - but if you ad light to the equation (or subtract it as needed) you then can balance your exposure with this fourth element without having to sacrifice your other settings. In this way you can use f4 to maintain focus on your subject and you don't have to sacrifice shadows to noise with higher ISO settings.
With this approach you can then dedicate the shutter speed to the function of freezing or blurring action, the fstop can be used solely to determine depth of field and the ISO setting can be kept low to reduce noise. Once these settings are decided upon based on your artistic vision, exposure can be determined by adding or subtracting light. The easiest way to start this process is by turning on or off extra lights or to use the cameras flash to add light and neutral density filters or gels to subtract light. This is a good way to learn to start blending artificial light control with the ambient light in the environment, but in the long run you will want to develop this skill far beyond the use of simple lights to include grids, snoots, barn doors, scrims and the like so you can not only determine over all exposure but you can also control shadows, specularity, light fall off and light direction.
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