[john0213]

i will chimp in to fill up some more gaps. below is what i've posted on photography-on-the.net so i might as well copy this over.

Nothing gets better than to experiment all the setting yourself. By just reading all the theory, you will only be able to know it then forget it, the actual usage is the part that will stamp these knowledge to your mind.

When using M mode, take extra attention on where the exposure meter sits. the indicator should give you a pretty good idea how bright you want your image to be.

for all the crop factor user having 450D, 40D etc, try to stick with ISO below 800 first. Unless you know exactly what you are doing, don't bump it up to 1600 because the quality of image can be discouraging for beginner.

Many people says that ISO 1600-3200 is completely fine, but also be aware that many of these users have the full frame 5D, the ISO resolving ability is a lot stronger for the full frame format.

Like the OP said, your subject determines the priority of the three factor, however, always consider adjusting the ISO last. Use it as an accomendating tools for giving you the proper exposure as Aperture and Shutter Speed determines the theme of the image.

If the focus object is moving really fast, you might need to consider your shutter speed as priority since the prevention of motion blur dominates the clarity of the picture. For example, when taking picture of a bee flying on flower, due to the bee constantly moving, experience tells me that i would need a very fast shutter speed to freeze the moment, so somewhere around 1/500 should be save. with that in mind, i then determine how much of a depth of field i would like my picture to have. I want the surrounding to blur but not by too much so 4.0 seems good. However, when i set my camera to this setting, my ev meter shows me that i am underexposed.

so, now i have two solutions.

1. I can either bump up the iso from 100 to 400 to give me that extra light.

or

2. i can stop up my aperture to 2.8 if it does not severely affect the theme of the image i was looking for.

Vice versa, this above procedure can also be used on Aperture.

On the example of a sunset landscape shot. The surrounding is getting dark and my light source isn't exactly ideal at this time of the day. I know that i can't have a 2.8 aperture because i want my whole scene to be in focus. Through experience I know I will need f/11 so I went ahead and set the aperture to f/11. According to the aperture, i then set my shutter speed. Due to that my camera was on a tripod, handshake was of no concern for me so ISO can stay at 100.

these are just some of the example on setting the theme of the picture. I suggest anyone seriously about photography to really shoot more. the more you shoot, the more you will what setting to go for each theme.