When you are to the point that you'd like to control your photos more, it helps to have a basic understanding of what shutter speed/aperture/ISO all do and how they relate to each other.

Basics:

Shutter Speed: This is how fast the shutter is when taking pictures. The faster the shutter speed, the more you'll stop motion. The slower it is, the more likely you'll have some "softness" in your image.

A good rule of thumb is to have at least 1/125 as your slowest shutter speed when taking pictures of someone moving slightly (turning a head, walking, etc).

If someone is still and YOU are still (a sleeping baby, for example, or a very still adult), you may be able to get as slow as 1/40 with no shake... but I can't! LOL!

If you are taking photos of someone running or a child playing, you'll need something faster - 1/250 or sometimes even faster - if you want to completely stop the motion.

Another thing to keep in mind is that with a longer lens, you'll also need a faster shutter speed. Generally speaking, for a relatively still subject, it should be 1/focal length. So if you have a 300mm lens (not using IS (image stabilization)), you'll need 1/300 as a minimum speed to make up for camera shake.

Aperture (f-stop): This is how wide open or how closed the lens will go when taking a photo. When it's wide open (a smaller number), more light will get in, but less of your image will be in focus. The smaller the opening (larger the number), the less light, but the more that will be in focus.

But a better way to remember: The larger the f-stop number, the more that's in focus. The smaller the number, the less that's in focus. 22 is a has a LOT in focus, 4.5 has only a small amount, 1.4 has almost nothing except the subject in focus.

How they are related: Before we go to ISO, it's important to understand how aperture and shutter speed are related. And how to decide on a good one.

Basically, the larger the f-stop, the slower the shutter speed. The smaller the f-stop, the higher the shutter speed.

But how do you decide on what it should be?

The place I start is to figure out what's more important to me - how much is in focus or whether or not I want to stop the movement. For a portrait of a child, for example, that's at rest, I will want to make sure that I keep the background out of focus to keep the child IN focus and more prominent. So I'll set my f-stop to the lowest number possible - we'll say 4.5. (And this is obviously using manual.) Then I can look through my viewfinder and see if I'm over or under exposed with my shutter speed.... I adjust it so that my viewfinder meter is in the middle and look to make sure that my shutter speed is faster than 1/125 and start shooting!

But wait! I am not READY for manual!
Easy! You should have an aperture priority or a shutter priority or both on your camera. Decide what's most important and set it on that.

For example, if you need to have a lot in focus, but don't care about the shutter speed because you're on a tripod, set your camera to Aperture Priority ((AV on your Canon camera) and then set your aperture to the highest it can go (22, for example). Then click away! if you have movement, try using your camera on a timer or use your remote to set it off - it was probably pressing the button (if there's not wind.)

Or if you're taking a photo of a child, set your aperture to 4.0, for example, and start clicking! This will automatically adjust your shutter speed for the available light! Just make sure it's higher than 1/125 and you should be good to go!

Don't care about how much is in focus, but really want a certain shutter speed? Set it to shutter priority (TV on your Canon camera) and start clicking! Just watch out that your aperture number isn't flashing - when it is, that tells you that your photo will be too dark.

But I've got a serious problem! When I use those modes, my images are dark or they are soft and blurry! What can I do about that?
That's where ISO steps in. The lower the ISO, the more light you need. The higher the ISO, the less you need. But the higher the ISO, the grainier your image will be.

So, if you are trying to shoot inside and you're having problems, try bumping your ISO and see if that helps! With the newer cameras, you can actually bump it up to 800, 1600 and sometimes even 3200 and get acceptable photos! (My 10D couldn't go above 800 or it looked horrible.)

There is a ratio, to help you figure out how to set your ISO/shutter speed/aperture.

Using your in camera meter

-2 | | -1 | | 0 | | 1 | | 2
...|..........| |..........

When you look through your viewfinder, you should see something like the numbers and lines above. Different cameras look different, but will probably be similar. The red line below the 0 shows you if you are over exposed or under exposed. If it's to the left of the 0, it's under exposed (or towards the negative side). If it's to the right (positive side), it's over exposed.

When you point your camera at something and you're in an automatic mode (aperture priority, shutter priority or fully automatic), your camera will take a photo trying to keep the image as well exposed as it can.

Just remember that for every line (or stop) over or under exposed, you can do one "click" on your camera to rectify it. In other words, if you're over exposed by one like (where the green line is), raise your shutter speed by one click (or raise your aperture by one click, or lower your ISO by one click) and you should be perfectly exposed!

To rectify the pink line, which is 5 stops under exposed, you can lower your shutter speed by that many clicks, lower your aperture number or raise your ISO - or any combination of those to get those 5 stops (clicks) back.

Questions? Please ask in this thread! I'll fix the main post, if necessary, or I'll just answer in the threads. Also feel free to post here for feedback! (And I'll try to add example photos sometime before 2009.)