Understand: exposure time
Exposure is the amount of light detected by the photosensitive element inside a camera. For analog cameras, the photosensitive element is photographic film, whether it is a digital sensor in digital cameras. You can change exposure by changing the value of three parameters which are aperture, exposure time and sensitivity. These three parameters form together what we call the exposure triangle.
How to quantify exposure time?
Exposure time is the time during which you let light inside the camera to the photosensitive element. Behind the lens (and behind the mirror in DSLR cameras) lays the shutter. It works like a door that won’t let light in the camera unless you push the shutter release button (in other words, the ‘take picture’ button). The time during which the shutter remains open to take a picture is called exposure time and it’s way easier to quantify than aperture, because we already quantify time everyday. We quantify exposure time in fraction of seconds most of the time. If exposure time is longer than one second, we quantify it in seconds. So if the shutter remains open during 1/125th of a second, exposure time is noted as 1/125th. Yes, it is that simple. Like aperture, exposure time is calibrated in stops. Common exposure time stops you will find on all cameras today are 4 seconds, 2 seconds, 1 second, 1/2, 1/4th, 1/8th, 1/15th, 1/30th, 1/60th, 1/125th, 1/250th, 1/500th and 1/1000th but a lot of cameras can go up to 30 seconds and down to 1/4000th or even 1/8000th. From one stop to the next or previous one, exposure time is multiplied or divided approximately by two. It means that if exposure time is increased one stop, light that enters the camera to reach the photosensitive element is multiplied by two. If exposure time is decreased one stop, light that reaches the photosensitive element is divided by two.
In a nutshell:
Exposure time is one of three parameters that you can use to change exposure.
It corresponds to the time that the shutter is open to take a picture.
Changing exposure time is changing the quantity of light you let into the camera at the moment the picture is taken.
Exposure time is calibrated in fractions of seconds, in seconds, or sometimes more.
Common exposure time stops are 4 seconds, 2 seconds, 1 second, 1/2, 1/4th, 1/8th, 1/15th, 1/30th, 1/60th, 1/125th, 1/250th, 1/500th and 1/1000th.
From each exposure time stop to the previous or next one, the amount of light that reaches the film or sensor is divided or multiplied by two.
How to change exposure time?
You can change exposure time in two modes: manual mode (M) or shutter speed priority mode (S or Tv). Change exposure time by turning the exposure time dial on the top plate of your camera. If your camera does not feature an exposure time dial, change exposure time by turning the wheel on your camera.
What it does
Changing exposure will change the time during which the shutter remains open, and therefore the amount of light that reaches the film or sensor inside your camera. If you increase exposure time, the shutter will remain open for a longer period of time, letting more light into the camera to the sensitive element. As a result, in manual mode (M), your picture will appear brighter. If you decrease exposure time, the shutter will remain open for a shorter period of time, letting less light into the camera to the sensitive element. As a result, in manual mode (M), your picture will appear darker.
Manual mode (M)
In manual mode, if you increase exposure time one stop but want to keep the same exposure, you need to either decrease aperture one stop or to decrease sensitivity (ISO) one stop. Aperture and sensitivity are calibrated as stops that match exposure time stops, so if you increase one of them by a given amount of stops, you have to decrease the other two by the same total amount of stops to keep the same exposure and vice-versa. For instance, in manual mode, if you increase exposure time four stops but you want to keep the same exposure, you can either :
Decrease aperture four stops.
Decrease sensitivity (ISO) four stops.
Decrease aperture two stops and sensitivity (ISO) two stops.
Decrease aperture one stop and sensitivity (ISO) three stops.
Decrease aperture three stops and sensitivity (ISO) one stop.
In manual mode, if you decrease exposure time four stops but you want to keep the same exposure, you can either:
Increase aperture four stops.
Increase sensitivity (ISO) four stops.
Increase aperture two stops and sensitivity (ISO) two stops.
Increase aperture one stop and sensitivity (ISO) three stops.
Increase aperture three stops and sensitivity (ISO) one stop.
Shutter speed priority mode (S or Tv)
In shutter speed priority mode, all you have to do is to change exposure time. Your camera will calculate proper aperture for you (and also proper sensitivity if ISO is set to auto). Hence the name shutter speed priority.
Motion blur
If you photograph moving subjects and exposure time is long enough to record motion, it will appear on the picture as motion blur.
On the picture above, cars and people appear blurry, while buildings are sharp. Exposure time was long enough to record motion. An exposure that is long enough to record motion is called a long exposure. The faster and closer your subject and the longer your focal length, the shorter must be exposure time if you don’t want to record motion.
When photographing still subjects, nothing changes. An exposure that is long enough to record motion is still called a long exposure, but as the subject doesn’t move, it’s the photographer's motion that the camera will record. The more the photographer moves, the shorter must be exposure time not to capture his movement. Usually, the frontier between a long and a short exposure time is a ratio of 1 to the focal length used. For instance, if a 50mm lens is used, an exposure will be long if longer that 1/50th of a second. With more stability, it's possible to take longer exposures without moving. On most DSLR cameras, you can lock the mirror so it doesn’t flip into the camera when taking a picture. Also, use a tripod and a remote shutter release button to secure your camera.
