The "correct" exposure for a photograph is determined by the sensitivity of the medium used.
So what does exposure mean in plain English?
Dreyer definition: Exposure is the amount of light that is “captured” during the taking of a photograph.
The exposure of a photograph can either be:
1) Not enough ~ Under exposed
2) Enough ~ Correct Exposed
3) Too much ~ Over exposed
The following image shows from left to right, under exposure, correct exposure and over exposure.
Exposure is influenced by the following 3 factors, that we will discuss in more detail in future topics:
1) Shutter Speed
2) Aperture
3) Film Speed
I would like to explain the concept of exposure by means of the following pictures:
(I tried my best to make this look like the side view of a camera)
Our goal is to get enough light on the sensor (or film in the old days) to have a correct exposure. We can control the amount of light by means of the Aperture and the Shutter speed.
Shutter Speed
What will happen if we keep the aperture and the films speed constant, while changing the time that the sensor is exposed to light?
Aperture
What will happen if we keep the Shutter speed and the films speed constant, while changing the aperture?
Film Speed
What will happen if we keep the Shutter speed and the Aperture constant, while changing the Film Speed? Note that the amount of light needed change as the film speed change. If the films speed goes down, the more light is needed. If the film speed increase less light is needed. Films speed is noted in ISO.
Quick Start
So how do we apply this to photography? When we take pictures usually we want to achieve a correct exposure. We can do this by adjusting the film speed, shutter speed and aperture, or we can leave this for the camera to adjust while we focus on capturing the moment.
Until we better understand the influence and other effects that Film Speed (ISO), shutter speed (sec) and Aperture (f~stop) have on our pictures we can set our camera on Program “P” mode or the Full Auto (Green) mode. In Program mode (P) the camera will do all the changes to achieve a correct exposure.
Where do we find program mode? You can use either the “P” setting or in the case of using a Canon you can use the green mode. More on these in later topics. For more information refer to your Camera User Manual.
Homework Assignment:
Refer to your Camera User Manual to see how to make use of your camera’s Exposure Compensation function.
Choose a subject that you can photograph.
1. Take a picture without setting the exposure compensation on your camera. This should be correctly exposed.
Aperture
What will happen if we keep the Shutter speed and the films speed constant, while changing the aperture?
Film Speed
What will happen if we keep the Shutter speed and the Aperture constant, while changing the Film Speed? Note that the amount of light needed change as the film speed change. If the films speed goes down, the more light is needed. If the film speed increase less light is needed. Films speed is noted in ISO.
Quick Start
So how do we apply this to photography? When we take pictures usually we want to achieve a correct exposure. We can do this by adjusting the film speed, shutter speed and aperture, or we can leave this for the camera to adjust while we focus on capturing the moment.
Until we better understand the influence and other effects that Film Speed (ISO), shutter speed (sec) and Aperture (f~stop) have on our pictures we can set our camera on Program “P” mode or the Full Auto (Green) mode. In Program mode (P) the camera will do all the changes to achieve a correct exposure.
Where do we find program mode? You can use either the “P” setting or in the case of using a Canon you can use the green mode. More on these in later topics. For more information refer to your Camera User Manual.
Homework Assignment:
Refer to your Camera User Manual to see how to make use of your camera’s Exposure Compensation function.
Choose a subject that you can photograph.
1. Take a picture without setting the exposure compensation on your camera. This should be correctly exposed.
2. Now adjust the exposure compensation to a negative value. Take a picture of the same subject from exactly the same spot again. Compare this picture to the first one. What do you notice? This should be darker, or under exposed.
3. Now adjust the exposure compensation to a positive value. Take a picture of the same subject from exactly the same spot again. Compare this picture to the first one. What do you notice? This should be lighter than both the previous pictures, or over exposed.
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