Improve Your Photography Exposure Mechanics

Aperture, Shutter Speed, ISO Settings

Learn how these settings control not only photography exposure, but depth-of-field, motion blur, and noise

The topic of exposure concerns the amount of light used to make your digital image. Three factors impact exposure:

• Aperture controls the amount of light coming through the lens to hit the image sensor. Inside the lens is a diaphragm with a variable opening diameter. You, or the camera, set aperture by changing the size of that opening. Aperture is measured in f-stops, which represent the ratio of lens focal length to the diaphragm opening diameter, and which is written in the format f/8.
  
  Smaller apertures (higher f numbers) produce greater depth of field, but pass less light to the image sensor.
  
  
• Shutter speed controls how long the light hits the image sensor. Shutter speed is measured in seconds and fractions of a second.
  
  Faster shutter speeds do a better job of stopping motion, but also reduce the light hitting the sensor.
  
  
• ISO controls the sensitivity of the sensor to the light received. The higher the ISO number, the more sensitive the sensor, and the less light required to create a usable image.
  
  Higher ISO settings allow good exposures with less light, but can create more noise in the images near the top range of your camera.

When you see the need to make an adjustment to your exposure because the image is too dark or too light, you will initially find it best to make adjustments in full 1-stop increments, doubling or halving exposure with each stop. When you are ready to fine tune your exposure, then take advantage of 1/2-stop and 1/3 stop increments if you camera makes them available.

Photography Exposure Equivalents

Combinations of aperture, shutter speed and ISO that are equivalent are said to have the same exposure value. While the same exposure value produces the same image brightness, it produces images that may differ in depth of field, motion blur, noise, or some combination of the three.

The table below shows an array of equivalent photography exposures, where shutter speed is shown at the intersection of a selected ISO and f-stop. Select any two values to arrive at the third. The highlighting in the table shows the equivalent exposure selections for the two examples that follow.

Photography Exposure Examples

Consider the following two examples of a stationary ball swing behind a stationary ball. The goal of these two examples is to show for an identical setup the effect of choosing equivalent exposures with the intent of varying the depth of field while achieving the same motion blur.

The setup for these exposures was a stationary ball mounted on a light stand in the foreground, a moving ball suspended from the ceiling, about 6 inches (15 cm) behind the first ball, and a white board about 12 inches (30 cm) behind the second ball.

While the lighting is incidental to the proving the point about equivalent exposures, you may still be interested in the details: A flash with magenta gel provided the backlight at low power from behind and below the balls, while an overhead incandescent light provided a bit of highlight. White balance was set to "incandescent", which is why the highlight on the near ball is white.

Example 1

In this shot, I was trying to show motion blur and shallow depth of field with the back, moving ball in focus. For shallow depth of field, f/2.8 was a must. With a wide-open aperture of f/2.8, a slow ISO was important so that shutter speed could be slow. Otherwise, the image would not show motion blur. Selecting the right shutter speed and ISO combination required a bit of trial and error to assure the right amount of blur and the right amount of light.

The final solution of f/2.8, ISO 200, at 1/15 second did the trick.

Example 2

In this example, I wanted to show the same amount of motion blur but putting both balls in sharp focus. The deeper depth of field required an aperture of f/16. With the exposure equation understood from the first shot, setting the exposure for this shot only required looking up the correct exposure in the equivalent exposure chart. Looking across the f-stops to f/16, then down to 1/15 second, you then look across the chart back to the ISO setting of 6400.

You'll notice that for some reason (that I can't remember), I shot this second example at 1/20 second. While that screws up the purity of the example, the 1/3-stop difference is negligible compared to the 5-stop changes in aperture and ISO. However, you can see that the 1/3-stop decrease in exposure both darkened the image and shortened the distance of the image blur.

Read Focus Concepts to learn more about depth of field.