Well the manual that came with your camera for sure. Eventually you should read it cover to cover. As for websites...let me just Google a few real quick. Jwedehase's link looks like a great one.
I wrote this myself on these forums and coincidentally just found it.
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Thread Unknown?
(unknown)...........Oh goodness without understand that, everything I said might as well been Chinese. Not to let all the previous typing be wasted, I will try to redeem myself.
Exposure (how your camera sees light and how "bright" the picture turns out) is affected equally by three main powers. Aperture, Shutter speed, and ISO
1) APERTURE (the most powerful…and confusing)
Think of a garden hose...a very fat one with an adjustable nozzle on the end. Instead of water, it deals with light. The maximum Aperture of the hose is its thickness. The adjustable aperture is how large of a nozzle setting you have. The very largest hose that exists in theory is f/1 (though an f/0.7 was used to shoot one movie). As you can see, this number is a fraction. If I replace the “f†with “1†for demonstrational purposes, 1/1 is bigger than 1/2.8 or ¼. Here is the mathematical formula:
(stolen from Wikipedia)
Don't worry about memorizing that fraction because you won't actually need to bust out a pen and paper to figure the answer out every time. "f" represents the Focal length (35mm, 50mm, 100mm...) "N" is the number we place under the "f" (f/2.8, f/4, f/5.6...). But notice the square root in that equation. Each setting increases light by a square root of 2, and each increment (called a "stop") DOUBLES the light of the increment before it. The "stops" are: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. Many cameras, such as my 40D can measure these stops in 1/3s (f/2.8, f/3.2, f/3.5, f/4...)
The square root of 2 is about ~ 1.414214
f/1 x 1.414214 = f/1.4
f/1.4 x 1.414214 = f/2
f/2 x 1.414214 = f/2.8
f/2.8x 1.414214 = f/4
and so on...the main thing to understand is that each stop doubles the one before it.
The fraction f/4 does change with the focal length.
When you see something, your eyes are actually interpreting light bouncing off of it. Light fades over distance and you have to increase the sensitivity to the light depending on distance. A 400mm lens set at f/4 has a diameter of 100mm. A 100mm lens, also set at f/4, has a diameter of 25mm. This is why those big telephoto lenses are so…BIG! Not just showing off, physics says they have to be.
Every lens has a unique amount of stops it can go to. My 24-70 is an f/2.8 maximum f/22 minimum. My 70-200 is a f/2.8 max f/32 min. The Canon Xti kit lens 18-55mm is a f/3.5-f/5.6 max and f/22-f/38 min. When it 18-55 is zoomed all the way out to 18mm (wide) it can go to f/3.5. When the 18-55 is set to 55mm, it can only go to f/5.6. Many cheaper lenses have this annoying drawback. Your PowerShot can go from f/2.8-f/4.9 max and f/32-f/45 min. This is why I told you to zoom your camera all the way out, so that you can get f/2.8.
One thing about aperture that doesn’t have to do with light (well at least not in the manner I’ve been talking about): The larger the aperture (f/1) the LESS of the picture will be in focus. This is why you see the front and back of pictures blurred out with only a small selection in focus. A favorite of portraits, open the nearest magazine (golf digest for example) and look at Tiger Woods. See how his big silly face is all smiles, you can even count the dimples in his chin. Look at the tree behind him…it looks like a kindergartener drew it. The leaves are 1 big green blob and the trunk is a blurry brown line. I bet you never even noticed before, and that is the whole idea. Tiger is the only thing in focus, so your eyes don’t even bother to look at the cute baby squirrels just born in that big green blob. The photographer probably used f/2.8 or even f/1.4. If he used f/22, you could see those baby squirrels and Tiger wouldn’t be nearly as attention grabbing. How far away the photographer was from Tiger, and how far away the cute baby squirrels were behind him also plays a role. The closer the lens is to the subject, the smaller the focus (known as Depth of Field or DOF). If the photographer backs up, then you could see some of the grass 2/3 behind the focus point and 1/3 in front.
2) Shutter Speed
Shutter speed is a far easier subject to understand. Remember the nozzle on your light hose? Well it has a handle bar. The shutter speed is how long the handle is pressed, measured in seconds. In your camera, your shutter closes and takes a picture. Depending on the speed of the shutter you can let more or less light in. These are also measured in stops. Each is twice as large as the last: 30 seconds, 15 seconds, 8 seconds, 4, 2, 1 second, ½ of a second, ¼, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, and 1/8000 of a second are typical ranges in DSLRs. A shutter speed used in these time frames will include EVERYTHING that happens within that time in the picture. So if you have a tang bouncing off the walls and your shutter speed is 1 second, everywhere that tang goes within that second will show up in the picture as a blurry streak. Likewise, however much YOU move the camera around in that second will merge the edges of everything together in a blurry mess. This is called camera shake, enter the tripod. For general picture taking, I wouldn’t recommend less than 1/30 without a tripod. For fast moving fish, 1/125 but preferably 1/250 shutter speeds should freeze them in place. The faster it moves, the faster the shutter speed.
These numbers also effect or are affected by Aperture. Say you are at f/5.6 @ 1/60 but you want a 1/250 shutter speed. Move the Aperture *down one stop* to f/4 and the shutter speed also moves a stop to 1/125. Move another stop to f/2.8 and the shutter speed moves to f/250. How much LIGHT you have depends whether f/5.6 = 1/60 or if there is *twice as much light* f/5.6 = 1/125. Whatever the light is, moving from f/5.6 to f/4 always doubles the light the camera sees, and hence doubles the shutter speed to bring it back down.
3) ISO
ISO is the the 3rd edge of the triangle, and the cheater’s way out. This is how sensitive your camera is to light. If you are at f/5.6 @ 1/60 but really want that background in focus…and need a higher shutter speed more, moving your ISO will fix your problem.
ISO 100, f/5.6, 1/60
ISO 200, move to f/8, 1/60 OR f/5.6, 1/125
ISO 400, move to f/11, 1/60 OR f/5.6 1/250 OR f/8, 1/125.
My camera goes from ISO 100-3200 in 1/3 increments
Aperture determines how much of the shot is in focus. Shutter speed determines how long the subject is recorded, and ISO determines how much interference you see as little black or colored dots (two different types of “noiseâ€Â) Turn your TV to a channel to playboy. See all the “snow†covering the hot chick? That is the video version of high noise. Turn your radio to a channel that doesn’t exist…it’s the SAME THING just audible. “Film grain†is the still picture version of the phenomenon called “noiseâ€Â. Different cameras handle noise better than others. You may not want to venture all the way to ISO 400 because you get spots all over the place. I can’t even tell the difference from ISO 100 to ISO 400 on my camera.
Your Powershot’s range is 80, 100, 200, 400, or auto. Unfortunately it will choose the aperture and shutter speed for you every time, so this has all been kind of a mute point. At least (I hope) you now understand the basics.
