Exposure, not exposure time. Exposure time or shutter speed, would be one factor on exposure.
The exposure in this sense would be how much light does your sensor collects.
As in the previous post, it works in a triangle (thanks Laurie Excell for the analogy and the great book in Composition). We have now control over the light sensitivity of the sensor (ISO), the amount of light allowed to reach it (the aperture) and the amount of time that light is reaching it (the shutter speed).
Each has effects and consequences I’ll try to talk about here. As before and in the whole blog, I’m not a professional, so there might be some mistakes or incomplete information. Feedback is ALWAYS welcome!
Starting with shutter speed.
First of all the shutter speed has to be adequate to your camera “holding”. Whatever part of your photo that you want sharp, has to be really sharp, doesn’t it? You really don’t want the “sharp part” to be blurry. Also, the longer the focal length (the longer the glass, in the slang I learned not long ago), the harder to keep things sharp. So, if you’re hand holding your camera, hand/body shaking will play an important role in the sharpness of your photo. The rule of thumb is basically a shutter speed, in fraction of second, smaller than the focal length. If you’re using a 50mm you could go close to 1/50th of a second, if your using a 100mm you shouldn’t get slower than 1/100th of a second, and so on.
If you’re using a flash, and your subject is in the flash range (just remarking this, because a lot of people turn the flash on to shoot something that is tens of meters away … “ain’t gonna help much!”) you can freeze the action with your flash. The flash lightens the scene for a very short time and though the exposure can be longer, what will “remain” of your subject is the flash lighten image.
If you’re using a tripod, and even better if using a remote shutter release command, you’re free to go as large in shutter speed as you want.
What can you do with shutter speed? A lot! There are of course many more things than I will and than I can mention here. There are whole (and great) books about what to do with each parameter (like Exposure from Jeff Revell). I’ll limit myself to the part I understand and am already able to use.
Give the idea of motion, or stop the motion, is what you may want to control. You’re shooting a moving subject, like your kid running. A very short shutter speed, will show everything sharp and no motion idea will be given. Like shooting a statue. That might or not be what you want. A larger shutter speed can be used and you can follow the moving subject, panning, and get a motion blurred background, while the main subject is pretty much sharp.
Happening things, like a waterfall, you might want to give a silky look to the water, using a large shutter speed, so a lot of water goes though each part of the fall and get “blurred” (using a tripod is high recommended). Or you might want to take a snapshot of the moment, with drops all around, giving a “crunchy” or “crispy” look to the water. It’s your decision, just a dial of shutter speed away.
There’s a lot that can be done with shutter speed, but this gives an idea of the possibilities.
Aperture comes in the next post.