E = A + S

Exposure is the most important photographic concept to understand.  We've all heard Einstein's famous equation: E = MC² which states that matter and energy are basically two forms of the same thing, but for photographers, E = A + S. Get this one in your brain, and your skills as a photographer will improve.

Exposure = Aperture + Shutter Speed

Here are some practical definitions:

Exposure: The total amount of light that hits the sensor (or film) for a given picture. If too much light hits the sensor, then the picture will be overexposed (too bright). Too little light and it will look underexposed (too dark).

Aperture: The size of the hole that allows light to shine through a lens. Aperture is referred to by what photographers call f-stops. Typical f-stops are f2.8, f3.5, f4, f5.6, f8, f11, f16 The smaller the number, the larger the hole. For example, f2.8 is a large hole and f20 is a small hole. Each f-stop lets in twice the light of the previous f-stop.

Shutter Speed: How long the camera's shutter is open. This is easy to understand. If light hits a camera's sensor for a very short period of time, the exposure will be less than if light hits the sensor for a longer period of time.

Aperture + Shutter Speed: These two settings effect the amount of light hitting the sensor and for how long the light hits the sensor. To achieve the correct exposure these two settings much always balance out. (The Equation balances just like any other math equation--yes, this is math in real life).  So if the lens is opened wider (like f2.8 verses f8) then the shutter speed must be shorter to achieve a correct exposure. Stop down the lens to f8 (as compared with f2.8) in the same lighting conditions and the shutter speed must be increased for the same exposure. Photographers are always keeping this balance in mind.

The Exciting Part! 

Depth of Field: The effect of changing the aperture (lens opening) is called depth of field. This is the first thing I consider when taking a picture. Do I want everything in focus (as in a landscape shot) or do I want the background to be blurry (as in a portrait).

Most portraits work best with a blurry background. That means I want a very shallow depth of field. The eyes may be in focus, but their nose may not be. To achieve this, use a wide open aperture (f2.8 or wider). You can think of it this way:  small f-number yields a small depth of field.

Most landscape shots work best with near and far objects in focus. To achieve this, choose a larger f-stop like f11 or f16 (some lenses close down even further to f20, f22, or even smaller). Remember: larger f-number yields a very large depth of field. 

It's counter intuitive because a larger number means a smaller hole in the lens for light, but a larger depth of field (more things are in focus). Just remember: small number (large hold) yields small depth of field and larger number (small hole) yields a larger depth of field. 


Shutter Speed: The effect of changing the shutter speed controls motion. Sometimes you want things to be blurry (like moving water). Sometimes, you want motion to be frozen in time (like a sports shot).  Shutter speeds are measured in fractions of a second. When the camera shows a 60 on the screen, that means 1/60th of a second. If the camera shows a 500, that means 1/500th of a second. Most cameras go up to 1/2000th of a second. What do you think the shutter speed was for this picture?

It was about 1/80th of a second. Pretty slow right? But I wanted the background and the cyclist's feet to look blurry to show motion. For this picture I used a technique called panning in which I moved the camera to follow the cyclist. This created a nicely blurred background which communicates motion. It's a lot more exciting picture than if I had taken it at 1/1000th of a second.

This pelican was about to land and I wanted to freeze the action. The shutter speed here was more like 1/800th. A slower shutter speed would have given me a blurry wings or even a blurry bird.

Think about the picture you are wanting to take. Is it bright outside? That means you will require a faster shutter speed to achieve the correct exposure. Are you taking a portrait with a shallow depth of field (like at f2.8). That's a wide open lens and a lot of light is coming in. Therefore, you will use a faster shutter speed to achieve a correct exposure. But if you are taking a landscape shot at dusk then the settings would be very different. First off, you would want to select a larger f-stop number like f16 to get a deep depth of field. But that's a very small hole for light to enter the camera. If it's already dim outside, that means you would need a very long shutter speed and it might require a tripod. Every landscape photographer carries a tripod for just this reason.

This link shows examples of the effects of fast and slow shutter speeds and large and small aperture settings. Notice how the depth of field changes (aperture) and how motion is either blurred or frozen in time (shutter speed).

http://forums.dpreview.com/forums/read.asp?forum=1022&message=29686375

You're on your way to becoming a much better photographer.