Light and the DSLR



The easiest way to understand how light behaves is to think of light as if it were water. When there is very little light you get "dripping." A drop of water is analogous to a photon of light.



When you have a lot of light you get a "stream." A stream of water in analogous to a wave of light.


And when you constrict the opening thru which it's passing (for a camera it's the lens' aperture) you get a spray pattern of drops.


Now imagine spraying your yard with a large fan pattern of water; it takes a little bit of time for the area to become evenly coated. This is because the drops (photons) are random. This randomness is called "photon shot noise" in photography. The less water that is available, the longer it takes to coat the area.


Now imagine the lawn is covered with containers to collect the drops. These represent the pixels of the camera's sensor. And imagine having the spray of water on for only a fraction of a second. Due to the randomness of the drops in the spray pattern some containers will have collected more drops than others, and some may not have caught any. The larger the container used (larger pixels/ lower MegaPixel (MP) count) the more likely every container will collect a more even quantity of drops. This randomness and uneven collection of drops is the cause of almost everything "bad" in photography (noise, loss of dynamic range, loss of color information, etc., etc.). A "high MP" camera is using teacups and a "low MP" camera is using buckets.

When you have bright light it is like trying to capture drops from a large stream of water. When you have less light it is like trying to capture drops from a very wide spray pattern. This is why you can use faster shutter speeds and higher ISO's in bright light with less issues than you can in darker situations. If you capture a fraction of a second from a stream of water, you have still collected more water than you would from a light spray pattern.


So that's the basics of how light behaves with a little about the camera's sensor/pixel size. Now let's talk about how the camera controls the light.


The camera acts exactly like your eyes do in this respect. In fact, the terminology is very similar.


With the eye the amount of light let in is controlled by the size of the pupil. The size of the pupil is controlled by the iris. And how long light is let in is controlled by how long your eyelids are open. Once the light hits the back of the eye it is converted into a usable signal. If you were to look in a mirror and then turn on the lights you would see the pupil of your eye automatically reduce in size. It does this in order to keep the amount of light entering the same as the room gets brighter. And your camera will do exactly the same thing in some of the automatic modes.

With the camera the amount of light let in is controlled by the aperture which is also called "the entrance pupil." The size of the aperture is controlled by the diaphram blades which are also called "the iris." And how long light is let in is controlled by how long the shutter is open. Once the light hits the sensor it is converted into a usable signal. The more light collected, and the more evenly it is collected, the higher the quality of the signal generated.

The only difference is where the shutter is placed


That's it! You'll notice I didn't include ISO and that's because ISO doesn't have anything to do with the actual collection of light. With film ISO was the film's sensitivity to light and your camera's sensor has that too. And just like film, the sensor's sensitivity to light is not variable (it's called the "base" or "native" ISO). Other ISO's are amplification of the previously generated signal very much like the volume control is for an audio signal in a radio. Just like the radio with fairly crappy speakers, if you turn up the volume too far the sound starts to break up and the output song is severely degraded. And just like the radio, the quality of the signal being amplified has a huge impact.

The actual reasons a digital image starts to degrade with higher ISO's is more complex (having to do with analog vs digital amplification, number of stages of amplification, etc) but the gist of it is the same. 

And there you have it... how a DSLR works in 766 words.

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