Canon PowerShot G12, Using Shutter Priority (Tv) Mode to Stop Motion

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As we covered in the last chapter, Tv mode gives you control over shutter speed, while handing over aperture selection to the camera. The ability to concentrate on just one exposure factor helps you quickly make changes on the fly while staying glued to your camera’s LCD and your subject.

There are a couple of things to consider when using Tv mode, both of which have to do with the amount of light that is available when shooting. Although you have control over which shutter speed you select in Tv mode, the range of available shutter speeds depends largely on how well your subject is lit.

Typically, when shooting fast-paced action or trying to capture something that happens quickly, you will be working with very fast shutter speeds (Figure 5.2). This means your lens will probably be set to its largest aperture. Although the G12 offers a maximum aperture of f/2.8, that applies only when shooting at the lens’s widest setting. As you zoom, the largest aperture available to you reduces; the largest aperture you can expect when fully zoomed is f/4.5. If the available light is not sufficient for the shutter speed selected, you’ll need to raise the ISO of the camera to balance the exposure.

Let’s say you’re shooting a baseball game at night, and you want to get some great action shots. You set your camera to Tv mode and, after testing out some shutter speeds, determine that you need to shoot at 1/500 of a second to freeze the action on the field. When you press the shutter button halfway, you notice that the f-stop readout is blinking at f/4.5. This is your camera’s way of telling you that the lens has now reached its maximum aperture, and any pictures you shoot are going to be underexposed at the currently selected shutter speed. You could slow your shutter speed down until the aperture reading stops blinking, but then you would get images with too much motion blur.

The alternative is to raise your ISO to a level that is fast enough for a proper exposure. The key here is to always use the lowest ISO you can get away with. That might mean ISO 80 in bright sunny conditions or ISO 1600 for an indoor or night situation. Just remember that the higher the ISO, the greater the amount of noise in your image. (This is the reason you see professional sports photographers using those mammoth lenses perched atop a monopod: They could use a smaller lens, but to get those very large apertures they need a huge piece of glass on the front of the lens. The larger the glass on the front of the lens, the more light it gathers, and the larger the aperture for shooting. For the working pro, the large aperture translates into low ISO—and thus low noise—fast shutter speeds, and razor-sharp action.)

The fast shutter speed captures the skateboard mid-air with very little blurring. (Increasing the ISO may have frozen it completely.) Also note that the f/4.5 aperture— the middle of the camera’s range—keeps the background elements in focus, too. [Photo: Anneliese Voigt]
Figure 5.2 The fast shutter speed captures the skateboard mid-air with very little blurring. (Increasing the ISO may have frozen it completely.) Also note that the f/4.5 aperture— the middle of the camera’s range—keeps the background elements in focus, too. [Photo: Anneliese Voigt]
Adjusting your ISO as You Shoot

  1. Press the shutter button halfway and check for the blinking aperture readout in the bottom portion of the LCD.
  2. If it is blinking, adjust the ISO dial to the next highest value.
  3. Lightly press the shutter button and check to see if the aperture is still blinking
  4. If it’s not blinking, shoot away. If it is, repeat steps 2 and 3 until it is set correctly.