Exposure Definition – Why are my photos underexposed or overexposed?
Let’s first have a look at what the shutter is and how it works. On good old film cameras, when taking a photograph, the shutter (a little device that blocks light from entering the camera) is removed for a very short interval and the film is exposed to light. We are talking about very short spans of time, just a 1/100th or even a 1/1000th of a second.
Over or Under Baking the Pie
If light falls on the film for too short a time span the photo is underexposed (below left). If the film gets exposed for too long, the film will be over saturated with light and the film will be overexposed (below right).
In modern day DSLRs, the same philosophy for shutter is used. There are two shutter curtains. When the button is pressed to take the picture, the first curtain goes up and image sensor behind the curtain gets exposed to light coming from the aperture opening of the lens. After some time a second curtain comes from underneath and closes the shutter to stop the exposure time. This combination of two curtains is used to ensure that each part of the sensor is exposed to light for exactly the same time. The time for which the image sensor remains exposed to light, is called shutter speed. Fortunately if you set your digital camera to automatic mode, the camera can calculate how much time the shutter should remain open to generate the correct exposure. A small on-board computer with many tricks up its sleeve tackles the problem of exposing your images just right.
There are occasions where you might not be happy with the exposure as set by the camera. In instances like this you might want to use the Exposure compensation feature on your camera. With this feature, if you feel that the photos are a bit too dark, you can set the slider to a positive number.
In the image above (second line from the top), I have set exposure compensation to +2. For dark photos, the camera will leave the shutter open a bit longer for the sensor to expose the image somewhat brighter. Conversely, for a photo that is too bright, a minus value will decrease brightness. Exposure compensation can be set on most cameras even in auto or program mode, and gives you as photographer a degree of control over the camera’s automatic settings.
To Sum Up
To explain the concept of exposure I have simplified things a bit. Exposure is not just solely about the length of time the shutter is open to let light in. A couple of other variables also come into play to explain why some photos you take might end up soft or blurry.
Have a look at our next article to see why this happens.