When Should I Override the Auto Exposure on My Camera? (Part 2)



Priority Modes

Most high-end digital cameras offer shutter priority and aperture priority modes.

Aperture Priority - Aperture priority lets you set the aperture, and automatically sets the shutter speed to the right setting to produce a correct exposure. The primary reason you might want to use aperture priority is to control the Depth of Field. DoF (Depth of Field) refers to the depth of the photo's focus plane. That is, the amount of the photo that's in focus.




Figure 1                                                                


DoF is determined by the focal length, the distance between the camera and the subject, and the camera's aperture setting. Once you have composed your photograph, however, the only variable that's easy to change is the aperture, so photographers often think of the aperture as a DoF control. Some effects you can achieve in aperture priority:

Create a shallow Depth of Field - Shallow DoF is often desirable to separate your subject from the background or foreground. With shallow DoF, the area that is in sharp focus is small compared to the area that is blurry. Shallow DoF is a great tool for placing emphasis on a particular point in a photograph. To create a shallow DoF, open the aperture up wide (smaller numbers). In this example image, only the reflected face is in focus. (Figure 1)

Create a Deep Depth of Field - Sometimes you want the opposite effect. For instance, you may consider the background an important feature of the photograph. In that case, you need to stop down the f number to create a smaller shutter opening. Keep in mind you'll be selecting larger numbers on the aperture dial. In (Figure 2), exposed at f/11, I wanted the trees and buildings to be fairly sharp, because I considered the view from the top of the city library an important feature of the image.

Shutter Priority - You set the desired shutter speed, and the camera sets the aperture automatically for a correct exposure. There are several reasons you might want to override the camera's shutter speed controls:

Freeze Motion - You can freeze water drops in mid-air, football tackles, dancers leaping and so on with fast shutter speeds. (Figure 3)

Capture Motion Blur - Intentional motion blur can add a sense of motion to a photograph. (Figure 4)

Figure 2

Figure 3

Figure 4


To create a panning photo, focus on a subject moving from left to right or visa verse, and twist at the waist as it moves past you. Keep twisting even after you click the shutter release button. Experiment with the shutter speed until the subject is in focus and the background is blurred out. (Figure 5)

To create a zoom photo, focus on something moving toward you or away from you and zoom the lens while you're pressing the shutter release. (Figure 6)

Figure 5

Figure 6

 Manual - The manual setting gives you full control of all of the exposure settings for the most precise control possible over exposures. This is the mode that I use most of the time, because I find that I'm usually better at exposure than the camera is. If you shoot a lot indoors, you would do well to master full manual control. However, if you shoot a lot outside in quickly changing light conditions (a partially cloudy day, for example), Aperture or Shutter priority may suit your needs better.

Auto ISO - Many cameras have auto-ISO features, where you set the aperture and shutter speed, and the camera automatically selects the appropriate ISO to get a proper exposure. This feature can come in handy when the light conditions are changing rapidly. For example, on days when clouds are passing the sun, it might make sense to dial in the settings you want, and let auto-ISO keep up with the changes for you. However, high ISO settings come with an undesirable side-effect: Digital noise. Noise is literally background static, like you hear on the radio, or see on TV. When you turn up the ISO, you also turn up the noise floor - the point at which the usable image data gets lost in noise. Put another way, when you turn up the ISO, you turn down the signal-to-noise ratio. As technology marches forward, this problem gets less and less severe, but for the time being, it's helpful to keep in mind that you should try to keep your ISO set as low as possible to maintain the best image quality possible.

In (Figure 7 and Figure 8) the same Buddha statue photographed at ISO 3200 and ISO 100, respectively. Notice that the ISO 3200 is not only spotted with digital noise, but that the noise reduces the image contrast and color saturation.

Figure 7

Figure 8


A good part of the art of photography is understanding the cause and effects between Aperture, Shutter Speeds, and ISO.  If you master the basics, you’re taking a big step on the pathway to photographic nirvana.