Digital Photography: Creating the Perfect Exposure - Part II

In a previous article, Part I of Creating the Perfect Exposure, we talked about shutter speed. In Part II, we discuss the other two sides of the Golden Triangle, aperture and ISO.


Also called f-stop, aperture is the size of the lens opening when the shutter button is pressed and controls how much of a scene will be in focus both in front of and behind the subject. The smaller the setting (larger the number), the more of the photo will be in focus.

The higher the f/stop number, the smaller the opening; the smaller the opening, the less light strikes the sensor. Typical f/stop settings are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 and 32. Here again, just as with the shutter speeds, there is a direct relationship between the settings. Starting at the lower end, each setting is half as big as the previous one as you go up the scale (even though the numbers themselves might not reflect that).

One technique that uses f/stops at the lower end of the scale is called selective focusing. Using a small f/stop setting, such as f/4 or less, will yield a shallow zone of sharpness so only your subject will be in sharp focus. Because the human eye will go to what is in focus, it will automatically be drawn to your subject. Selective focusing is also used to throw a distracting background or foreground out of focus, thus eliminating the potential competition with the subject.

If each successive stop going up the f/stop scale makes the opening in the lens half as large and if each successive stop in shutter speed is twice as fast as the previous one, can you see as you move one in one direction, you have to move the other one in the opposite direction to maintain a same amount of light hitting the sensor thus maintaining the perfect exposure.

For example, if you have your mode selection wheel set to Program and your camera selects 1/125th shutter speed at f/8 aperture, but you want more depth of field, you can move the f/stop two stops up to f/16. In turn, your shutter speed will move two stops slower, down to 1/30. Now the same amount of light hits your sensor at the settings of 1/30th of a second and f/16 as it does at 1/125th of a second at f/8. You reduced the amount of light hitting the sensor by two stops, so you had to slow down the shutter speed by two stops to compensate.


This is the last part of the Golden Triangle. ISO (International Standards Organization) settings still follow the old traditional film speeds of 100, 200, 400, etc. Use a low ISO setting, such as ISO 100, when shooting requires the use of fast shutter speeds and large f-stops, such as shooting in the bright outdoors. When it is overcast outdoors or you are going to use the camera's flash, use a higher ISO setting, such as ISO 400. For indoor shooting or in high shutter speed outdoor situations, use an even higher setting, such as ISO 800 or even ISO 1600. OF course, you will use these high ISO settings also when photographing outdoor sports at night or stage shows so you can get a fast enough shutter speed to prevent blurring.

So what is the perfect exposure? For the technical answer, let's look at the lighting, focusing and subject placement. If the image is not over or underexposed, the lighting was correct. If the subject is in sharp focus and the foreground and background slightly blurred, focusing was correct. And finally, if the subject was placed in the image using the Rule of Thirds, placement was correct. From the esthetic standpoint, if the image shows what you wanted it to artistically show, then you have a perfect image.

Now, you should see how the three variables of shutter speed, aperture and ISO work together as a team to produce the perfect exposure and how changing one of them changes at least one of the other variables.