October 2008 HelpLine

This Article Features Photo Zoom

The image on the left is the result of choosing a small aperture in aperture-priority mode. There’s a lot of depth of field, but the camera compensated for the small aperture by selecting a slower shutter speed, which blurred the fountain water. The image on the right was shot in shutter-priority mode, with a shutter speed fast enough to freeze the water droplets. To match this shutter speed, the camera needed to select a large aperture, which reduced the depth of field. 

Q) I typically use my camera in automatic mode, but occasionally I step out of my comfort zone and move into some of the other modes. I really understand the full manual mode but, quite honestly, I don’t get the whole aperture priority and shutter priority. I know what they are supposed to do and I sort of get when you are supposed to use them, but when I take pictures, I often switch between the two and I really don’t see much difference. Is it me or the camera?

J. Perry
Via the Internet

A) I’m glad that you’re trying modes other than automatic mode. There’s nothing wrong with automatic exposure, but I really feel that the more you know your camera and its different operations, the less it becomes an obstacle to taking great pictures.

When you talk about controlling exposure in a camera, you’re really talking about controlling the amount of light that reaches the light-sensitive part of the camera. For a digital camera, this is the image sensor; for a film camera, it’s the emulsion of the film. These exposure concepts aren’t unique to digital photography. Film photographers have used these same exposure controls for a long time.

There are really only four things you can do to change the amount of light coming into the camera. First, you can put something in front of the lens to reduce the light, such as a neutral-density filter or a polarizing filter. Second, you can change the lens mounted on your camera—if your camera has interchangeable lenses—to one that’s more (or less) efficient at passing light.

The third option is to adjust the opening in the lens that allows light to pass. This opening is a diaphragm that works like the iris in your eye-opening wider to let more light in and closing down to limit light. The term “aperture” refers to the measured diameter of the diaphragm opening. The incremental adjustment of aperture is described as ƒ-stops.

The last method for adjusting exposure is controlling how long the light-sensitive device is exposed to light. This is done through the shutter, which is typically a pair of fairly thin pieces of light-blocking material. These shutter “curtains” move in rapid succession to uncover and then recover the sensor. You adjust the timing of this via the appropriately named shutter speed.

The first two options require additional gear and a commitment to stop to reconfigure your camera setup. In other words, they’re not achieved by quickly rotating a dial or pressing a button on your camera. But the last two methods for changing exposure are just that—they’re quick and easy to adjust, and are directly related to your question.

You can set your camera’s exposure system so that when you select the aperture, the camera controls the shutter speed. This is called “aperture priority,” and on many cameras it’s labeled either Av or A. You also can let the camera control aperture while you control the shutter. “Shutter priority” is the name of this mode, and it’s usually labeled Tv or S. The small “v” stands for “value,” so the labels technically read “Aperture value” and “Time value.”



You mention you’ve been trying aperture priority and shutter priority but don’t see much difference between the two. It could be that the settings result in the same exposure. For example, let’s say you set your camera for aperture-priority exposure mode and you set your aperture for ƒ/11. You compose your shot, and the light meter built into the camera reads the light in the scene and determines that 1?125 sec. is its choice for shutter speed. When you press the shutter release, you capture an image with exposure of ƒ/11 at 1?125.

Now, you flip your camera to shutter priority, set the camera for 1?125 sec. and compose the same shot. The camera sees the same light coming into the camera and chooses an aperture of, you guessed it, ƒ/11. I guarantee that if you press the shutter release and take the picture, it will look the same as the first image. It’s the same exposure.

But let’s take a look at another exposure example—you can compare the two exposures displayed in the images at the top of this column. I recently was in Rochester, N.Y., and saw this fountain in the Maplewood Rose Garden. I found it a perfect opportunity to shoot examples of different exposure-mode settings.

The image on the left was shot using aperture-priority mode—Av. I set my lens to get maximum depth of field (smallest aperture) so I could capture a scene showing that the fountain was in a rose garden. I used ƒ/32, and the camera picked 1?5 sec. shutter speed. With the increased depth of field, the garden comes into focus, but the fast-flowing water becomes a blur.

Next, I shot using shutter priority—Tv, as shown in the image on the right. I wanted to freeze the water and make the background subtler. I chose a shutter speed of 1?2000 sec., and the camera picked an aperture of ƒ/2.8. The fast shutter speed was more than enough to freeze some of the droplets in midair. Of course, the depth of field was as narrow as I could get it with the lens I was using, so the background is unrecognizable.

This is an extreme example, but it’s something you can prove to yourself with your own camera. In this scene, a tripod is a must because of the slow shutter speed, but you could try other scenes without a tripod. For example, photograph athletic events or street scenes. There’s usually fast movement that you can capture. See how choosing between aperture or shutter priority can change the feel of the image. Just pay attention to the shutter speed and aperture settings. Make sure that you don’t end up with the same exposure settings for both exposure modes.

After seeing the example images and reading about the explanation of aperture versus shutter priority, it’s easy to fall into the trap of thinking that if you want to control shutter sp
eed and its effect on the image, you should always have the camera in shutter priority. That isn’t always the case. Although somewhat counterintuitive, if you’re in a situation like a sporting event and you want to freeze the action as much as possible, aperture priority may be your best setting!

Why? If you use shutter priority, you might limit the maximum shutter speed you can use. You might set your camera for 1?125 sec. and find that the camera is choosing ƒ/4, but the lens you’re using could go down to ƒ/2.8. At the very least, you’ll spend a lot of time fiddling with the shutter-speed setting. Instead, use aperture priority and set your camera for the largest aperture the lens provides—in this case ƒ/2.8. The camera will then select the fastest shutter speed possible.

Try it yourself and you’ll see that even aperture priority can make the shutter a priority. 

If you have questions, please send them to HelpLine, PCPhoto Magazine, 12121 Wilshire Blvd., Ste. 1200, Los Angeles, CA 90025 or [email protected]. Visit our website at  www.pcphotomag.com for the web-exclusive HelpLine Weekly and past HelpLine columns.

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