Light, Exposure and Composition are the basic elements of any good photograph. Light and composition will be covered later.

The correct exposure is not always the correct exposure. What?

The correct exposure is not always the desired exposure.

We want want some photos correct, some very dark, some very light and everything in between.

Knowing how to get the "correct" exposure (or as correct as possible given the conditions) gives you the skills to make your exposure anything you want it to be, i.e. the desired exposure.

• ISO - how sensitive is the camera to light. Noted as 100, 200, 400 etc. The same numbers that were used for film speed. Set ISO first.
  
  
• Shutter speed - how long is the shutter open allowing light to come into the camera? Often noted in seconds (1") or fractions of a second (1/60)
  
  
• Aperture size- how large is the opening that light passes through on its way into the camera? Noted as an f-stop number: f2.8, f4, f5.6. These are fractions too. They are calculated as the ration of the focal length (ex: 50mm lens) to the diameter of the opening. So, f/2 would be an aperature with a 25mm diameter since 50/25 = 2. Don't let this confuse you, just know that they are fractions and fractions with smaller numbers on the bottom are bigger.
  
  

The exposure triangle...

A-S-I: Aperture Size - Shutter Speed - ISO Sensor Sensitivity

All 3 Settings Interact, if you change one the exposure will change...

The Sunny 16 Rule is a good example of how Shutter -Aperture - ISO work together. It states that if you set the Aperture to f/16, and take the reciprocal number of the ISO for your shutter speed, you will be pretty close to a realistic exposure if the sun is shining on your subject. So: ISO of 200, f/16, and 1/200 second shutter time. OR ISO of 1600, f/16, and 1/1500 second shutter time. Sometimes you have to choose the shutter time that is closest.

The Overcast 8 Rule is similar...It states that if you set the Aperture to f/8, and take the reciprocal number of the ISO for your shutter speed, you will be pretty close to a realistic exposure if it is an overcast day. So, IS0 400, f/8, and 1/400 second shutter time. OR So, IS0 800, f/8, and 1/800 second shutter time. These rules are just starting points. If it is mid-winter the sun will be less bright. If it is near the end of  the day on an overcast day it will be darker as well.

Exposure Modes (Shooting Modes - CANON)

Manual M - You control all 3 points of the exposure triangle. This gives you the most control but you need the highest level of understanding and experience to use it effectively and quickly. Many experienced photographers think of it as the easier one to use in lots of situations.

Program P - You set the ISO, the camera's computer sets the shutter and aperture. This is thought of as the Automatic Mode but you still control the sensitivity. Great choice for snapshots but rarely used for serious work unless there just isn't time to make the other settings.

Shutter Priority S or Tv - You set the ISO and the shutter, the camera sets the aperture. You decide that shutter time is a priority because you want to stop or blur the action in a scene. Fast times stop the action, long times blur it.

Aperture A or Av - You set the ISO and the aperture, the camera sets the shutter. You decide that the aperture is your priority because you are more interested in the Depth of Field (DOF) idea than the shutter time. Often used when shooting from a tripod as the shutter times can get very long and the camera would shake if you were hand holding it. DOF is the area of a picture that looks acceptably sharp in front of and behind the focused subject. Small apertures expand the DOF, and large ones shrink it.

Complete AUTO - shown with different marks depending upon brand/model if available. Often the Green Auto choice. The camera controls everything and common adjustments may not even be possible.

Scene modes - shown with different marks depending upon brand/model if available. The camera controls everything and common adjustments may not even be possible. You won't need these once you know how and why to make these settings with the other modes.

Fast shutter time (1/1000 of a second) versus a long time (1/5 oF a second).

Large aperture (f/2.8) versus a small aperture (f/16)

Depth of Field Illustration Video

https://youtu.be/c2IYt4qBSqI

Histograms

2 stops under, 1 stop under, 0, 1 stop over, 2 stops over

Histogram showing extreme over-exposure, blown out highlights

Histogram showing extreme under-exposure, blocked up shadows

Histogram showing only middle grays, this image would look very "flat"

Bracketing

Your camera may have bracketing controls. These let you easily shoot a range of shots above and below the "correct" exposure. Some are easy to set, some not but see "compensation" below.

Lock

Your camera may have a dedicated Exposure lock or it may be tied to the focus lock. This would be used with A, S (Tv), or P modes. Point at an area you want to meter for exposure like a gray card, lock that exposure and re-compose.

Compensation

These controls are for manually bracketing your exposures or just nudging them one way or the other. Full, 1/2, or 1/3 stops up or down. Over exposure or under. Plus exposure or minus exposure.

There may be controls on the camera for plus or minus exposure adjustments. Look for a button that looks like this. You press it, then turn your control dial to increase the light in the image (+) or decrease the light (-).

Exposure adjustments example...This shows a normal exposure in the middle. Minus one stop on the left and plus one stop on the right.

Some Helpful Links

• Good explanation of why good exposure doesn't work correctly sometimes: http://www.athentech.com/Videos/exposure-correction
• Aperture, f stops: http://en.wikipedia.org/wiki/F-number
• Shutter Speed: http://en.wikipedia.org/wiki/Shutter_speeds
• Why shutter and f-stop numbers are confusing: http://www.earthboundlight.com/phototips/why-shutter-speed-and-aperture-are-upside-down.html
• ISO:http://en.wikipedia.org/wiki/Film_speed#Digital and http://forums.cnet.com/7723-7593_102-363839.html
• Use the chart...Download chart (the interactive feature only works with Mac Preview).
• Camera Exposure Simulator: http://www.andersenimages.com/tutorials/exposure-simulator/
• My Digital Photography School article on learning to set your ISO: http://digital-photography-school.com/use-program-mode-set-iso/

When shooting:

What would be your normal?

You may want to do some careful tests with your camera body to determine if it meters "correctly." Sometimes they are slightly off.