The length of time the plate is exposed to light. The length of time the light has to “sink in”. It’s measured in seconds, or most commonly, fractions of seconds. The longer the exposure, the more light hits the plate, and therefore, the brighter the resulting picture.
If there is a lot of light, such as a sunny, outdoor setting, then the exposure needs to be short. If overexposed, the light portions of the shot will be pure white and the detail will be lost. In extreme cases, the shot might come out solid white. If it’s getting dark, if it’s past dark, if you’re indoors, then you’ll have the opposite dilemma. Not enough light and the shot will come out too dark. The solution is to increase the exposure time. There is a limit, however, around 1/32 of a second. Anything shorter than that and you’re safe, but if you dare go longer, then you will get a motion blur. And not just a blurring of subject matter, but if the camera moves even slightly, such as the amount it moves when you press the shutter button, your shot is ruined. If it’s too dark to keep it above 1/32, use a tripod. Carry a small, light-weight tripod with you on photo-shoots, even if you don’t expect to need it. I also have one of those tiny tripods, about three inches tall, that fits in my camera bag. If you don’t have a tripod, there are a few tricks you can use. Set the camera on a flat or steady surface, prop up anything that wants to wobble, and be very careful pressing the button. Use an external or remote shutter button if you have one. Leaning against a wall can help. Prop your arms either on another object or against yourself, but don’t hold them free. Don’t breathe while taking the shot. Exhale all the way and then snap the shot. Many people instinctively inhale, as though it’s going to take a long time and they might run out of air, but you move a lot more than you realize with a lungful of air. You can also set the exposure to something ridiculously long, several seconds or even minutes. This is how you get those cool “time-lapse” shots, where car headlights and stars in the sky are blurred along a trail. You need a tripod—you can’t take an extended-exposure shot without one. Most cameras go up to thirty seconds, but have an option to manually control the time. You’ll have to experiment to get the other settings to compensate the over-exposure, but the results can be exciting. Set up the camera on a rooftop in the city at night. Take a shot of a moonlit field. Water of any kind will look surreal when motion blurred. One way I’ve used this technique is at events such as weddings. I often try to get my shots before the event, during the setup (since most of my clients have been caterers and venues, not wedding parties). The reception area is usually dark with stragetic and often beautiful lighting. The problem is the people constantly walking in front of the camera. But, if I set up my camera for a very long exposure on a tripod in the back of the room, any people walking in front of the camera disappear. Because they’re in the dark, they won’t expose in the shot. As long as they don’t stay very long in one place or step into bright light, dozens of people can walk through your shot and not show up in the final shot.