An analogy I like to use when describing how cameras work is the human eye.
We have a dark box (eye), fitted with a lens, that projects an image on a sensor (retina). The amount of light that is recorded on the sensor (retina) is controlled by the aperture opening (iris), the time that the shutter remains open (eye-lid) and the sensitivity (ISO) of the sensor (retina sensitivity).
Lets briefly analyze these elements.
The lens is perhaps the most significant element of the camera. Its quality directly affects the quality of the image. If it is cheap or it is defective, this shows directly in our photographs.
The lens also controls the focus distance, giving a sharp image of in-focus elements, and a less sharp image of elements that are further away or closer than the focus point.
Shutter and Shutter speed
The shutter acts exactly like the eye-lid. It prevents light from reaching the sensor (or film). To get an image, the camera briefly opens its shutter, so that enough light reaches the sensor. Imagine having your eyes closed, and then opening them briefly towards an object. The less bright the object, the more time you will have to look at it to get an adequate image. If instead you look at the sun, your retina will quickly get overwhelmed by light, and you will have to close your eyes to prevent damage.
The aperture opening is usually very close to the lens (like in the human eye) and it helps the camera further control the amount of light that reaches the sensor. Exactly like the iris.
During a bright day, our iris gets very small to reduce the amount of light reaching the retina. On the other hand, during the night, the iris opens wide to allow more light to pass through, and give an adequate image faster.
The aperture in cameras can have one major difference compared to the human eye. The actual opening can be big. This means that as the lens focuses to a certain distance, out of focus points scatter their light all over the sensor. And since this out of focus light has much glass to pass through, it gives blurry backgrounds and foregrounds. This is what causes that sweet creamy background in portraits. The effect that we call bokeh.
When the aperture is closed on the other hand, each elements is projected only on a specific location of the sensor. This makes the entire photograph (including background and foreground) appear in focus.
Sensor and Sensitivity (ISO)
The other important element that directly affects image quality is the sensor. The sensor works exactly like our retina. It contains photo-sensitive elements that actually “see” the image, and send it for processing and storage.
The sensor also has a sensitivity setting called ISO. This further controls how much light is required to produce a signal strong enough to be recorded as an image. To better understand how this works, lets think again about how our eyes work.
Imagine a bright summer’s day. The bright light forces your iris to close, but you still have to wear sun glasses to feel comfortable to do stuff like driving. What also happens is that the retina reduces its sensitivity trying to adapt to the intense light. You usually do not notice this until you go indoors. For example into a house with closed drapes. You need a couple of seconds until you can start seeing stuff properly inside the significantly darker room.
The opposite happens at night. For example when you turn off the lights before you go to sleep. At that point the room seems pitch-black. After a while, your retina starts to adapt and you start noticing light coming through the window shutters or your night-stand clock. And after a while you can easily see things inside the room.
At this point, you can also notice something interesting. You see things clearly, but if you try to notice the image you are seeing in more detail you will notice that there are many random spots. This resembles the digital noise that is produced when you set your camera in high ISO.
Now if you do something like turn on the lights again or check your mobile for messages, you will have to close your eyes because the light will be too intense for the increased sensitivity.
So how cameras work?
I will eventually write about these settings in detail in the Camera Settings category, using real photo examples, giving you directions on how to experiment yourself.