Aperture and Shutter Control

A camera uses two things to control how much light reaches the camera's photo sensors ... and one additional thing to manage the sensitivity of that sensor.

 

When your eye is exposed to light, its pupils may contract or dilate depending on how much light is present. This helps your retina (your "photo sensor") receive a regulated amount of light. Notice that if you do not have enough light it is difficult to make out any details in what you see. Notice also that if you have too much light, you have difficulty keeping your eyes open and might only be able to see brightness ... just before you go blind (damaging your retina).

 

A camera works in a very similar way. The lens has an aperture (like your pupil) that can open and close thereby regulating the amount of light reaching the camera's sensor.

Another capability your camera has is the shutter. That is the actual opening in the camera lens that allows the light to enter.

 

Together, the aperture (how open or closed the pupil is) and shutter (how long the pupil stays open, or how long your eyelid stays open to allow light to hit your retina) control how much light enters your camera.

 

Aperture size is annotated as f/n, where n is a value such as 1.4, 2, 16, or 32. There are a number of values, but know that the smaller the number, the larger the aperture. Because of the f/n notation, the term usually used for aperture is "f-stop".

That third thing I spoke of is a way to manage the sensitivity of your camera's sensor. Our eyes don't have anything like this, but we can use something called set up by the ISO (International Standards Organization) tohelp us with this. This organization developed standards to define what sensitivty should mean and related that sensitivity to a sliding scale ... referred to as the ISO number. You will see such numbers as 50, 100,

200, 320, 400, 640, 800, and more all the way up to 102,400 (in some cameras). I'll say more about what those mean in detail and why you would want to change the sensitivity of your camera. Right now, just know that there is something to help us manage that sensitivity.

So ... why do I care about these things? In one word: Exposure. When we capture an image, we want the exposure to reflect what we are seeing (or to reflect what we want to see). As I open the aperture, I let more light in allowing me opportunities for more detail until I reach the point where I let too much light in and I can't make out light areas from the very light areas ... or the worst case, the entire picture is white.

 

If don't let enough light in (i.e., aperture is too small or the shutter speed is too fast, or both), then the image I captured will be too dark.

 

Our cameras help us with this. If we put the camera into aperture priority mode (meaning we set the aperture), the camera will select the correct shutter speed to achieve that "perfect" exposure. Likewise, if we put the camera into shutter priority mode (meaning we set the shutter speed), the camera will select the correct aperture for that "perfect" exposure.

 

In many cases, the camera's sensor sensitivity (ISO number) is automatically set making the camera not only choose an aperture or shutter, but also the ISO. In the "normal" point-and-shoot camera, we allow the camera to make all of those decisions for us. All we need to do is point the camera in the right direction and let the camera choose the aperture, shutter, and ISO to get us that "perfect" exposure.

 

Great! But it doesn't always give us what we want. More on that in the class ... as well as how we know if our exposure is "perfect" ... and how ISO can help and hurt us.

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