Part 1 of this five-part series of articles started to give digital photographers the knowledge to tame the three-headed exposure monster: ISO, shutter speed and aperture. Part 2 explains shutter speed. Part 3 focuses on the third head, aperture. Many photo experts will tell you that learning how to control aperture is what separates the digital photographers who don’t seem to know (or care) what they are doing with a camera from those who want to advance to a higher level of accomplishment.

Aperture is a number that represents the size of your camera lens opening. That number is called an f-stop; “f” is for focal. Those numbers are found on the lens of your digital camera, and may appear as f/2.8, f/4, f/5.6, f/8, f/16, f/22, etc. What can be confusing about f-stops is that the smaller the number, the bigger the aperture opening. If f/2.8 is the smallest number on your lens, then that setting allows the most light to enter the camera. An f/22 opening is very small, so the least amount of light strikes the camera’s sensor.

Another bit of confusion is that, like shutter speed, each f-stops represents a doubling or halving of the amount, although the numbers are not twice as much or half as much as the previous number. For example, f/8 is an aperture setting that is only one-half the opening of f/5.6, or f/5.6 opens the lens twice as much as f/8. Remember, the shutter speed settings are also double or half of the next number in sequence: 1/500^th is double the speed of 1/250^th, or 1/250^th is half the speed of 1/500^th.

This is where the three-headed monster often bites digital photographers because there is a direct relationship between the aperture and shutter speed settings, since both follow the doubling or halving concept. This means that you can cause the same amount of light to enter your camera at more than one combination of aperture and shutter speed settings.

• For example, your camera’s fastest shutter speed is 1/500^th, with the next choice at 1/250^th, and your smallest lens aperture is f/22, and then f/16.

• You’ve decided that to shoot a properly exposed picture under certain conditions, you should set your shutter speed at 1/250^th and your aperture at f/22. (This could be on a very sunny day.) The speed is at the high end of the scale, so the shutter opens and closes quickly, reducing the amount of light that enters. The aperture setting of f/22 is the smallest openings, so that also severely reduces the amount of light.

• You could also set your shutter speed at 1/500^th and your aperture at f/16, and expose the picture correctly. The setting of 1/500^th doubles the shutter speed from 1/250^th, but moving from f/22 to f/16 doubles the size of the lens opening. You allow more light to enter through the lens, but the faster shutter speed doesn’t give it as much time to enter.

• This is why changing either the shutter speed or aperture settings directly affects the other. If, based on the example above, you changed the shutter speed to 1/500^th, which halves the amount of time light can enter, but kept the opening of the lens at f/22, your digital picture would be overexposed. The opposite would also be true: Keep the shutter speed at 1/250^th, but change the aperture to f/16 (doubling the size of the opening).

Part 4 of this article reveals the direct correlation between aperture and depth of field to help you improve your digital photography.

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