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Film requires a certain amount of light hitting it in order to expose the image properly. Too much light and it is overexposed or very light. Too little and it is dark or black in extreme cases. There are 2 ways to control how much light hits the film - aperture and shutter speed.
The aperture is opening in the lens, like the pupil of the eye. When a person gets light shined into their eyes, the pupil closes, much the same as the way an aperture does. This controls the amount of light entering the eye, or in photography, the camera. The wider the aperture, the more light is let in, the smaller it is, the less is let in.
Your shutter is a component of the camera that is similar to the eyelid of the eye. It opens and closes at different speeds to allow the light to pass through the aperture. If the aperture is wide, the shutter will not need to be open very long in order to let in a certain pre-determined amount of light. Conversely if the aperture is small, the shutter will need to be open long enough so that enough light passes through the lens and gets to the film.
Good so far?
Depending on what film speed (ISO) you are using, you will adjust your settings accordingly. To build on the previous explanation a little, films are rated from 25 to 6400 with 25 film being the least sensitive to light, meaning that it needs MORE light to get a good exposure. This is why you use, say, 100 speed in bright sunshine or high light conditions. As you go up the ISO scale, the films become more sensitive to light and do not need as much light in order to expose the film properly. As a result, you can take good pictures on overcast days, in shade, or inside, even in low light theatre situations. Keep in mind though that the higher the ISO, the more grainy your pictures will be, and this gets more noticeable at larger sizes like 5x7, 8x10 and up.
Your aperture and shutter work together to determine how much light to let in. If the aperture is wide, the shutter can move really quick and still let in the SAME amount of light as if the aperture was small and the shutter was slow. The problem with slow shutter speeds is that since they are open for so long waiting for all that light to enter, you are liable to get camera shake from handholding (your heart beats, you jiggle a little, or your subject moves) and your picture is ruined (or maybe it looks better that way )
When you make the aperture wider, the shutter can go faster. When you make the aperture smaller, the shutter has to slow down to let enough light in. It is a balancing act between the two and as you go up in one, you go down in the other. Two steps up in one setting means two steps down in the other.
How to know what to set it at? Practice! This is where photography becomes not only an art, but a science. Digital is great cause you get to learn what your settings should be without wasting film. You get to a point where you can go outside, look at the sky, view the light and say "hmmm today looks like a f5.6 at 125 day" where F is the aperture referred to as F-stops, and 125 is your shutter speed. 125 refers to 1/125th of a second so 150 is faster than 125 cause 150 is 1/150th of a second, or 1/2000th of a second etc...
No one can tell you what your settings will be at a given moment without seeing and evaluating the light at that exact moment, and seeing the light falling on your subject. If a cloud moves, it all changes. See? A science!
If you're eager to start using your camera, one way is to start with your camera on "P" or "AUTO" which means the camera does the work for you. A built in light-meter (in-camera) scans the scene and computes what it feels would be the right settings to get good exposure. The cameras today are fairly good at this, but they can make mistakes. Take note of how the settings change up and down under various conditions. Take 36 pictures and note your aperture and shutter speeds - some digicams record this information with the picture for later reference!
You can also set your camera to Aperture Priority meaning that YOU set the aperture, and the camera will set the appropriate shutter speed.
You can also use Shutter Priority where YOU select the shutter and the camera sets the aperture. This is best when you need to capture fast action - like a running toddler, or an athlete in motion, and you want the fastest shutter possible. You might want to set it to 1/500th of a second, but in doing so it will need to have a wide enough aperture to match and some lenses don't have apertures as wide as what would be needed at that speed, so you would dial down the shutter until you find an aperture you DO have....sadly it might not be enough to give you the speed you need, but this is another topic altogether - lenses and their speeds.
Apertures are such that the higher the F number, the smaller the opening. Backwards hunh? The wider the aperture, the smaller the number will be.
For example look at your lens, and it will tell you the focal length of the lens, followed by it's aperture:
50mm 1:1.8
Means it is a 50mm lens with a maximum aperture of 1.8
If it says 70-210 3.5-5.6 you have a zoom lens. 3.5 is the maximum aperture when you are zoomed OUT, and 5.6 is the maximum when you are zoomed IN.
Well....that was a lot of information! I hope you have a better understanding! Best thing to do is get a book on photography basics and learn about how altering shutter speed and aperture in various pictures can create interesting effects - this is how the art comes back into photography
Hope this helps a little. I know it took me a while to get a grasp on it, and I still find myself thinking out loud sometimes to be sure I have it all right.
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