The exposure time (or also called shutter speed) is probably another very intuitive shooting parameter. It is measured in seconds, and represents the amount of time the sensor will be exposed to the light coming from the scene.
How to read it?
In the example below, there is the exposure time on the bottom left side of the viewfinder. The number 250 represents the exposure time in fractions of seconds (i.e. 1/250 seconds). This is not intuitive at the beginning but you will get familiar with it easily with some practice.
Most of the exposure times you will use as a photographer will be shorter than a second. That’s why the fraction notation is so convenient. Even some bibliography describes this parameter as shutter speed. However there are some cases when exposure time can be as high as many seconds. For those cases a quotation mark (“) will follow the number indicating how it should be read.
Some examples for exposure times shown in the viewfinder:
- 1” : 1 second
- 1/10 : 0.1 seconds
- 1/100 : 0.01 seconds
- 1/500 : 0.002 seconds
Importance
The exposure time is very important because it allows you to control the exposure value (the longer the exposure time, the higher the exposure value).
To keep in mind
This is a rule that I find very interesting. One stop of light is lost (or gained) as a consequence of halving (or doubling) any of the three main factors that control the amount of light: exposure time, aperture, or ISO. As an example: if you have EV -1, you can either: double the exposure time, double the aperture, or double the ISO. On the other hand, if you have EV +1, you can either: halve the exposure time, halve the aperture, or halve the ISO.
Play with it!
I strongly recommend you to play a bit. Use this fantastic online simulator: CameraSim. Put it in manual mode. Use the following reference settings: ISO 800, f/9 and shutter (exposure time) 1/400 seconds.
- Leave the exposure time at 1/400 seconds. Take a picture. It should look fine as the EV is 0. Try to remember how the pinwheel looks like for exposure time 1/400 seconds.
- Keep the manual mode. Increase the exposure time to 1/100 seconds and take a picture. Try to explain why it is so white. Now decrease it to 1/1000 and take a picture. Try to explain why it is so dark. Now repeat the exercise but pay attention to the pinwheel. Is it sharp when taking the picture with 1/100 seconds of exposure time? Why so?
- Keep the manual mode. Play a bit with the exposure time to verify that halving it leads to decreasing the EV by one stop of light.