This first week of classes, I want you to Read your Camera Manual.

You will NEED to know how to set your camera to MANUAL exposure mode- not an auto or "scene" mode. You will be expected to demonstrate how to set your camera's sensitivity to light- called variously "film speed," "ISO," "exposure index (EI), or just "speed." This variable is expressed, for example, as "ISO 100" or  "ISO 50." A setting of  ISO 100 is twice as sensitive to light as an ISO 50 setting.ISO 200 is twice as sensitive as ISI 100.ISO 50 is one-fourth as sensitive as ISO 200.Doubling or halving the ISO doubles or halves the sensitivity to light.The light sensitive element within the camera body, the "image sensor" is kept in complete darkness, until the moment of "exposure" as a "shutter" quickly opens and closes, allowing the image forming light through an opening in the lens.

Your camera's image sensor requires a specific range of light to produce a correctly exposured image. Too little light and the image is dark or "underexposed." Too much light and the image is too bright- overexposed.

Once you set the ISO, you can use the in-camera light meter to deternine the correct "exposure." Check your manual to understand the metering system's display. Also find out how to display the "histogram."

Beside ISO (exposure index), there are other controls which are used to set the camera for the correct amount of light for the specific scene displayed within the camera's frame.

You will use the "shutter speed" dial to set how long the image sensor is "exposed" to light through the lens. This value is set with a time value: usually fractions of a second, but often minutes, hours or even, in extreme situations, days.
The shutter's settings are usually expressed as, for example "one-one-hundred-twenty-fifth of a second" and written as "1/125 sec."
Changing the shutter to 1/60 sec lets twice as much onto the sensor as 1/125 sec.

Doubling the time doubles the light. Halving the time halves the light.

In order to adjust the exposure, the camera offers a second- complimentry- device built into the lens called the "lens opening," "diaphram," "aperature," or "f/stop." It is a series of curved blades which operate together to make a variable opening for the light to stream through the lens (for a period of time controlled by the shutter). The opening is expressed as, for example, "f/8."  The numeric values of  these f/stops represent a ratio between the actual size of the hole, and the "focal length" of the lens-essentially the magnifying (or reducing) value of the lens. This ratio produces a confusing situation where the larger lens openings have small numbers.
The base f/stops-hold overs from the film age- includes this of "full" range of stops:

   f/1.4, f/2, f/2.8, f/4, f/5.6, f/8/, f/11, f/16/, f/22, f/32

BIGGER OPENING                                           SMALLER
more light                                                       less light

Each full stop allows either twice or one-half the amount of light to reach the sensor.

As an example, an opening of f/8 allows twice as much light to pass through the lens as f/11.
f/2 allows half as much light as /1.4

When we discuss exposure, we must describe these three factors. For example:
On a bright sunny day, with the sun behind the camera (light is falling on the subject from the front-referred to as "front-lit,"  and a sensitivity of ISO 100, the correct exposure would be expressed as-

   "ISO100, 1/125 sec @ f/16"

The beauty of this exposure system is that you can vary your choice of f/stops and shutter speed, and ISO as necessary to achieve correct exposure in any light.

    ISO100, 1/250 sec @f/11

allows the same amount of light to the sensor because of the fact that each of these variables, in full steps, either doubles or halves the light quantity.

The bottom line is that you must find the means by which you set these  controls, then learn to read the meter to determine the correct exposure, then set these controls on your camera.

Letting the camera's autoexposure system make these decisions will not work when we begin working with off-camera flash in the studio. You must learn f/stops and shutter speed!
There are very good reasons for manually selecting your exposure. We will deal with the logic behind these selections next.