Freeze!Shutter priority mode is ideal when you need to freeze or blur motion, and you're not as concerned with depth of field. Some examples of this are:
- Fast-action sports
- Live concerts
|Motion Blur Using a Slow Shutter Speed (1/2 sec)|
[Jerry, the Muddled Rambler, made an excellent follow-on point in his comment below, so I'm expanding this section a bit:] Shutter speed also provides the ability to independently control the mix of ambient lighting and flash lighting in an image. One day I intend to do a whole series on flash lighting because the rules change and they're not intuitive (although they do make perfect sense once you understand them). But I'll give you the short version here. Flash lighting occurs in a small fraction of a second (around 1/2000 of a second depending on the flash power level). In fact, time-wise the flash is a tiny blip compared to your shutter speed, assuming you're at the X-sync speed or slower (which you have to be in order for the flash to work properly). The full power of that flash asserts itself in the image regardless of the shutter speed. Or put another way, your shutter speed has no effect on flash exposure (so long as it's no faster than the X-sync speed). You're going to get the same flash exposure level when you shoot at 1/200 as when you shoot at 15 seconds. However, any ambient light (that is, non-flash lighting already in the scene) is going to be affected by the shutter speed, just as it is when you're not using flash.
A classic example of this: Let's say you're shooting a portrait of your mother at night, outside, with the lights of a distant city shining in the background. If you shoot at a relatively fast shutter speed, like say 1/200, the flash is going to light your mom nicely, but the city is going to disappear into a black void in your photo. Unless you're using a nuclear option, the flash is not powerful enough to light the city in the background, and the ambient light of the city itself is too weak to create sufficient exposure at that shutter speed. If you go with a longer shutter speed, say 5 seconds, the shutter is going to be open long enough for the city lights to expose the sensor so you'll see them in the photo, and the blip of flash is going to expose your mother exactly the same it is did when the shutter speed was really fast. So you'll get a photo with your mom and pretty city lights in the background. So shutter speed provides a way to control the ambient light exposure independently of the flash exposure. Cool!
We talked about shutter speed being able to freeze moving objects in the photo. It will also freeze movement of the camera itself. The camera shake that occurs when you handhold your camera can really do a number on the sharpness of your image. This is especially true when you using long telephoto lenses because when you magnify an image, you also magnify any movement of the lens. There's a fairly useful rule of thumb from the film days that when you're hand-holding a camera, you want to have a shutter speed equal to 1 over the focal length of the lens or faster. So with a 200mm lens, you want a shutter speed of 1/200 or faster. (Now, there are exceptions and modifications to this rule resulting from modern digital camera design, but that's a topic for another time. For now, I'm just going to stick with the simplest form of the rule to show when you might want to use shutter-priority mode.) Obviously, if you shoot on a tripod then there's little or no need to reduce camera shake, making sharp photos at ultra slow shutter speeds possible (assuming your subject isn't moving).
As for me, I use shutter-priority for about 15% of my photos, mostly when shooting my kids, who bear a strong resemblance to the Tasmanian Devil but with slightly more intelligible speech. I also use shutter priority when I shoot photos with TTL flash, which isn't that often. I mostly go full-manual when shooting flash because not all my speedlights support TTL But if all my flashes were TTL, I'd probably use shutter-priority a lot more.