Simple photography lesson, all about aperture mode (AV mode)...
If you've read your camera manual and looked up photography online, you must of heard of aperture. If you have not, aperture simply controls the amount of light the lens allows to reach the digital sensor (how bright or dark your pictures will be, and the amount of the picture that will be in focus).
The most confusing thing about aperture is understanding that the higher the number (f-stop) the less amount of light that will reach your picture. The lower the number (f-stop) the more light that will reach your picture. It's kind of backwards in your thinking as you would normally think the larger number the more light, but thats just not how it works. So bigger number = less light, smaller number = more light. The number I refer to is called an f-stop.
If you put your camera mode dial into AV mode you'll be able to experiment with with the different settings and amount of light into your pictures. AV mode will set all your other settings for you (ISO, Shutter speed, etc... more about those setting later).
So the question is what aperture do you need with what picture settings. Many people like the look of the blurred background, to achieve this you will set your f-stop to a low number, the lower the number the more light there will be on your subject and the more blur you'll have in your background.
This picture was taken with the f-stop set at 2.8 (the lowest setting on the lens I was using):
You will notice how the background is blurred to the point that you can't even make out what it is. Now here is an example with the f-stop set at 22 (the highest for the lens I was using):
The focus is still on the subject, but you can see the trees in the background.
The next question is when would you use a high number f-stop. When shooting anything other then a subject directly in front of you, you would want a higher number f-stop. When shooting a landscape or a group of people it's imperative to use a high number f-stop to keep everything in focus and to showcase all the details.
I shot at each f-stop of the same subject so you could see the difference, there is only a subtle difference between some of the shots, but a big difference from the ones at the higher and the lower end of the scale.
Those were all shot in AV mode, which means my camera did all the other settings for me, I personally only shoot in M (manual mode) as I like to have complete control over my images, but AV mode is fine and really helps you learn and practice what different apertures will do to your images.
One final note, lenses are rated on focal length (the magnification distance) and their aperture value. If your lens states that it's a 70-200 5.6, that means your maximum focal length is 200mm, and the widest aperture is 5.6.
The smaller the aperture value the smaller the depth of field (more blur), and the larger the aperture value the larger the depth of field (less blur, more focus on the entire scene).
I hope this lesson was simple and gives you a better understanding of aperture, of course the best way to really wrap your had around it is to practice, practice, practice. Many point and shoot cameras have an AV setting as well, so your not limited if you don't own a dslr.