A camera is a box which lets you record an image projected onto a light sensitive material. That could be film, or it could be a digital sensor. To make things simple, we willassume a digital sensor.
The camera has two ways to control the light coming into it: shutter speed and ISO.
// Some digital cameras do not use a mechanical shutter. The application of the concept, however, is the same whether or not a physical shutter is used.
Until the moment the sensor is exposed to light, it is concealed by a shutter. The shutter is a piece of metal or cloth that covers the sensor until the moment the shutter button is pushed. When that happens, the shutter opens for the fraction of a second set by the shutter speed dial, exposing the sensor and making the image.
The ISO setting controls the sensitivity of the sensor. Just like with film, making the sensor more sensitive means it will have more noise (or “grain,” in film parlance).
To resolve light and focus it on a particular spot, the camera needs a lens. The lens also controls the amount of light reaching the sensor through the aperture. The aperture is the size of the hole created when a series of blades are engaged in the lens.
The size of the aperture will affect the sharpness of the image. This is known as depth of field. A wider aperture (e.g., f/1.4) will have a narrower depth of field, resulting in anything in front of or behind the focus point being blurred. A small aperture (e.g., f/16) will have a deeper depth of field, resulting in more of the foreground and background being in focus.
—Recasting what you know —
The shutter speed will affect motion within the image. With a slower shutter speed moving objects will blur. With a fast shutter speed, moving things will be frozen.
The ISO is the light sensitivity of the sensor. A low ISO will result in the cleanest image with the most range of highlights and shadows. A high ISO will have more noise and less dynamic range.
The aperture is part of the lens that determines how much light passes to the sensor. In combination with the shutter speed and ISO, the aperture is the third piece of the puzzle that results in an exposure. The size of the aperture affects the depth of field.