The concept of a "Properly exposed" picture is derived from the word expose. Back in the day when film was the only means of photography, the film was exposed to light via the lens to create the image later developed. However, the amount of light used in the exposure determines if the photograph is properly exposed or not.

In today's world, digital cameras are slowly taking over the photography realm. However, we still use older terms to describe key elements of a photograph.

In a nut shell.. To have a "Properly exposed" picture means the photographer "exposed" his "film" to the perfect amount of light needed to allow for the tones and colors of the subject to look right.

If you are a point and shoot kind of person and only use the "P" function or "program" function, then you need not read any further because you have told the camera to choose the proper settings to expose your picture correctly. There is no shame in doing this, only the possibilities for cool and unique photographs are limited.. However, if you would like to know the concept behind how to "make" a photograph rather than "take" a picture, please read on.

There are THREE things needed to create a properly exposed photograph. FILM SPEED, SHUTTER SPEED, and APERTURE. These three things are commonly referred to in a triangle fashion because each is just as important as the other.

Your camera's "Film Speed" will most likely be called "ISO". The most common speeds are 80, 100, 200, 400, 800, and 1600. However, some cameras are different and may include 50 and/or 3200... For the example below, "Film Speed" will be a bucket...

Usually "TV" on your camera, or simply "S". Shutter Speed is well... your shutter's speed. This determines how long light is let into your camera. There used to be set increments of shutter times. However, digital decided to break out and make up their own set of rules. The possibilities for shutter speeds are so vast that I am not going to list them all. Most compact cameras will have shutter speeds from 3 seconds up to as fast as 1/1000th of a second. Most SLRs however, will range from 30 seconds up to as fast as 1/4000th of a second, depending on which lens is on. Some also include an advanced setting which allows the photographer to manually control the shutter speed, allowing the shutter to be open indefinitely if wanted. While messing with these longer shutter speeds, I advise you to use a tripod if you plan on having an exposure last longer than 1/30th of a second. Most hands are not sturdy enough to produce a crisp picture if the shutter is open longer than that... For the example below "Shutter Speed" will be how long the water is on...

Usually "AV" on your camera or simply "A". Aperture is the size of the opening in your lens which light passes through. This opening size can be changed to allow more or less light in as needed. Like shutter speeds, the digital aperture capabilities are different from conventional aperture sizes. These sizes are adjusted by numbers ranging from about 2.8 up to 35 depending on how much money you spent on your camera and lens... The aperture is usually prefixed with an "f" so an aperture of "8" would appear as "f8" under the settings... For the example below, the "Aperture" will be the size of the opening on the hose...

Ok. Lets pretend you have a bucket. Lets call this bucket "Film Speed". Your goal is to fill this bucket perfectly to the rim using water. Lets call the water "light". To fill this bucket, you will need a hose. The bigger the hose, the more water which can flow through it. Lets call this hose "Aperture". Finally, you will need to control how long the water flows. Lets call this control "Shutter Speed". From what we know about standard "Film Speed" we know we will have 6 different sizes of buckets ranging from 80 to 1600. Something to keep in mind, in photography everything seems backwards. A film speed of 80 will actually be your biggest bucket, while 1600 will be your smallest. Next is the "Shutter Speed". Depending on how long it is determines how much water gets into the "bucket". When the bucket is filled perfectly to the rim, it is "PROPERLY EXPOSED". However, if the shutter speed is not long enough, the bucket will not get filled and cause the photo to be under exposed, or too dark. On the other hand, if the shutter is too long and lets in too much water, the bucket will overflow causing the photo to be over exposed, or too bright. And the final element is the size of the hose, or "Aperture". This is important because if you have a small opening in the hose, then your shutter will have to be longer to fill the bucket. Thus, on the other hand, if the opening is very large, then the shutter can be very fast because sufficient water can get in within a short amount of time. Again, the same as film speed, the smaller the number the bigger the aperture will be.

To change your film speed, you will need to go to your menu, settings, or functions depending on what camera your using. Your film speed will most likely stay constant during your shotting time. However, it is the easiest of the three to forget about. So if your photo is not turning out right, then double check your ISO.
Also, the aperture and shutter speed selections do not both have to be done by you. If you select the "AV" or "A" mode for aperture and change it according to the scale set in your camera, when you focus on your subject, the camera will choose the corresponding shutter speed for you to create a properly exposed photograph. And, if you were to choose an aperture which is outside the cameras limitations for shutter speed to be properly exposed, your camera will tell you by flashing the shutter speed number, or turning it red. The same goes for when you want to control the shutter speed, the aperture will be determined automatically.
On some cameras there is also the option to control everything. This function is usually an "M" for "manual". This will put you in charge of both the aperture and the shutter speed. However, most cameras will provide you with a light meter, so if the bar is in the middle of the two dots then the photo "should" be exposed properly.

At night there is not a lot of light available, so you know you will need a small bucket(800 or 1600). Especially if your subject is moving. Basketball games are especially hard to shoot photographs at because even though you think it is well lit, its not.
However, be warned. The higher number film speeds begin to reduce the quality of the photograph by leaving a grainy look when a color should be solid.

On the flip side, if you are taking pictures at high noon in the middle of summer, you will want to use a larger bucket or else your photo will be too bright. The good part though is that the quality of the photo is much better with a smaller number for film speed.

Get a tripod and go to a river or waterfall. Set the camera up and put it on a long shutter speed with a small number film speed. If done right, the water will have a really cool blurred glass effect, but be careful not to over expose... EXAMPLE

Put your subject in front of a bright background (sunsets are great for this). Then "properly expose" the background, causing the camera to make your subject become under exposed. This is commonly referred to as a silhouette. EXAMPLE

For this one you will need a tripod. On a clear night, go outside and focus on Polaris (the North Star). Then set a really long shutter speed, like 30 minutes. You will need to use the manual shutter technique here. When the photo is finally done, it will appear as if the stars are rotating around the North Star. And if you include the horizon in the shot, I think you will be pleased with the outcome. EXAMPLE is not my photograph.