The amount of light entering the camera when you push the button is called it’s exposure. If the amount of light is more than required, the image will be over exposed. It will be more bright than the actual scene. On the other hand, if the amount of light is less than required, the image will be under exposed. It will be more dark than the actual scene.
There are two ways you can control the exposure in a DSLR: 1. by changing aperture or 2. by changing shutter speed.
Simplest way to understand is by thinking about the iris of eye. It expands when we are in a dark room and it contracts when we are in bright light. Similarly the aperture of lens can increased to let more light in or contracted to stop the light. The diagram below shows the change in aperture and relative change in size of hole opening to let the light in.
Note the text on side of each aperture settings: f/1.4, f/2 etc. they are called the F-stop numbers. They refer to the RATIO between the focal length of the lens and the aperture. Since it is a ratio, the maximum aperture attained by a lens varies. We will discuss this later. Important thing to remember here is that smaller the F-stop number the larger the aperture.
As we change the aperture from lower f-stop to next f-stop, the amount of light entering the camera doubles. So amount of light entering at f-stop f/2.8 is double than f/4 and so on.
2. Shutter Speed:
Again going by previous analouge of the way our eyes adjust to light, have you noticed when we enter a very bright place we blink a lot to compensate for that brightness. In reality we are cutting off the amount of light entering our eyes. Similarly, we can change the exposure by changing the time for which shutter of the camera will be open to let the light in.
Shutter speeds are measured in seconds and fractions of a second. Usually the f-stops for the shutter speed is given as:
- 8 seconds
- 4 seconds
- 2 seconds
- 1 second
- 1/2 second
- 1/4 second
- 1/8 second
- 1/15 second
- 1/30 second
and so on. Each of these settings is clearly half/double the length of time of its immediate neighbours. Hence yet again, each f-stop is changing in the amount of light by factor of 2.
As we discussed, each f-stop changes the amount of light by half or double. You can do that either by changing the aperture or shutter speed. This leads us to the idea of equivalent exposure. Let me explain this by giving you an example.
Let’s say the “correct” exposure for a particular setting is f/5.6 aperture at 1/125 second. You can get the smae exposure by increasing the aperture by one f-stop and increasing the shutter speed by one f-stop. Hence, exposure at f/5.6 aperture at 1/125 second = exposure at f/4 at 1/250 second = exposure at f/8 at 1/60 second and henceforth. These different combination of aperture and shutter speeds that give same exposure are called EQUIVALENT EXPOSURE.
So now the question is what is the advantage of changing aperture versus shutter speed.
Changing shutter speed: This is straightforward. As you decrease your shutter speed or prolong the time of exposure, it become increasingly difficult to hold the camera steady for that amount of time (time of exposure). Result: Blurry images. You can compensate a bit by changing the ISO speed to a higher speed but that tend to introduce more noise in the image. Hence it seems more advantageous to increase the aperture.
Changing Aperture: As we increase the aperture, the depth of field decreases. The depth of field (DOF) is the portion of a scene that appears sharp in the image. To see the change in DOF with increasing aperture click here.
Hence it is a trade-off between DOF and prolonged exposure. As we discussed earlier, f-stop of aperture depends on the focal length of the lens. As the focal length of the lens increases, the maximum aperture of the lens decreases. Hence, the lenses with shorter focal length are faster than the lenses with longer focal length as their maximum aperture is larger and hence their equivalent exposure can be obtained at a higher speed or lower exposure time.
Only one thing remains now. How is the exposure measured? Usually the SLRs have an in-build light meter which measures the amount of light reflected from the object of interest (shown in diagram below). Advance or professional photographers use external light meters for the same purpose. The external light meters measure the incident light and hence are more accurate.
(from: Cambridge in colour)
That’s all for today. Will be back with more.
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