The shooting triangle.
The best way to think of photography is to look at its root. It means literally, writing with light. So when ever you pick up your camera to take a photo -think of light.
Photography is -- capturing the light of the moment on film (or recording it onto a digital camera’s sensor). Virtually everything involved with a camera revolves around manipulating the available light to produce a photograph, and there are three main terms that interconnect with this manipulation: ISO, Aperture, and Shutter Speed.
ISO, APERTURE, AND SHUTTER SPEED
When you pick up your camera to take a photo, the first thing you need to consider is how much available light you have and the speed of your ISO setting. ISO stands for International Organization of Standards, but you really don’t need to remember that. What you do need to know is that
ISO is the setting on your camera that tells your camera how quickly your digital image sensor will capture the light that it’s exposed to.
The lower your ISO setting (100 speed, for example), the slower the exposure of light onto the image sensor will be. The faster the ISO (1000 speed, for example) the faster the exposure of light onto the sensor. In other words, for lower ISO settings, you will need more light to correctly expose each image.
Because a low ISO uses more light, the resulting image of a stationary object will be very sharp and clear. The higher the ISO setting, the less available light you will need, but keep in mind that the resulting image will be have more grain (or in the case of digital cameras, more digital noise).
The benefit of digital SLR cameras, as well as some point and shoot, is that you can adjust your ISO setting for each shot you take. As a general rule, for bright outdoor shots use the lowest ISO setting you have. I have 100 on mine, but many point and shoots only go down too 200 or 400 speed. When you move to a setting with less light, such as when you go indoors, you can quickly increase your ISO to 800, 1000, or higher to keep on taking photos. But remember, the higher the ISO the more "noise". The photo of Mac below is what I mean by noisy, if you look at the shaded part of his face he will see a lot of grain, or noise when talking about a digital photo. Its the downfall of high ISO but without a high ISO there wouldnt be enough light to take the photo.

Once you’ve gotten the idea behind ISO, then we need to move on to Aperture.
Sounds technical doesn’t it? It’s really not that scary. Once you understand these terms you’ll wonder why you didn’t learn them sooner. You can do a lot of creative shooting once you understand the basics.
Aperture refers to the opening that allows light to enter your lens. F-stops control the amount of light that enters the lens. It’s really that simple!
The tricky part is understanding the concept of a small aperture and a large aperture. It’s a little backwards. The smaller your f-stop, the larger your lens opens up. So, the larger your f-stop, the smaller your lens will open. Still with me? You’ll want to know this in order to control your depth of field, which controls the amount of blur you see in your photos. A small aperture makes everything in focus. A large aperture makes only the subject you’re focused on in focus.
Just remember:
Small f-stop =large opening= only the subject you’re focused on in focus Large f-stop = small opening = everything in focus To recap: The aperture is
- the opening in your lens right in front of where it connects to your camera that either opens to allow light to pour through the lens or closes to constrict the flow of light.
- The aperture acts like the iris of your eye in that it can control the volume of light provided to give you an accurate image. Most point and shoot cameras have a fixed aperture (meaning you can’t change it), but virtually all bridge and digital SLR cameras have adjustable apertures.
- Apertures are referred to by their f/stop (a technical number obtained by taking the focal length of the lens and dividing it by the aperture of the lens). For example, you’ll see numbers like f/1.8 or f/5.6 used to describe a lens. In addition to indicating the aperture, f/stops represent the lens speed and indicate how quickly light will enter the lens and focus onto the film or digital sensor.
- Here’s the important part -- the lower the f/ number the faster the lens will capture light onto the film or sensor. For example, if a lens is listed with an f/stop of f/2.8, then that is as wide as the aperture on the lens will open, as well as being the lowest f/stop allowed by that lens. At the lens’s lowest f/stop, nothing will stop the light from entering the lens and being captured by the camera. Light will flow through the lens like water that’s been unleashed from a dam. This is very nice in low light situations since low f/stops allow you to take pictures that you normally wouldn’t think possible, but there is a drawback.
- Because nothing is slowing the light from entering the camera, your lens will only focus on a small portion of the image and leave a large portion of the image blurred. This can be very nice or very frustrating. With larger f/stops like f/22, the aperture on the lens closes down to the size of a pinhole and the light very slowly enters the camera. Because the light flow is restricted, the film or image sensor has much more time to capture that light and you will get images where almost everything in the image is in focus. By the way, if you’ve heard the term “depth of field,” this is what it is referring to – the portion of your image that is in focus.
- A large depth of field would have more in focus and would use a larger f/stop. A shallow depth of field would have less in focus and use a smaller f/stop.
Here are 3 examples of the difference the f/stop or aperture opening makes to the back ground.
In the first photo, with this lens the lowest number (therefore the biggest opening) I can go to is f/5.6 you can see how blurry the background is.

Then my changing the number to a higher f/stop therefore the hole is getting smaller. I get more of the background noticeable. Its becoming more infocus.

now at an f/stop of 36, much more of the background is noticeable. (although because I was not focusing right, nothing is really in focus) but you can see the differences in the backgrounds, right?

After you’ve set your ISO and considered your lens aperture, it’s time to move on to Shutter Speed. Now you’re probably beginning to figure out that if your lens aperture is at f/22 and light is slowly trickling into your camera, then anything moving would be a blur. The same could be said for instances when you have your ISO set at 100 and you’re trying to capture someone in motion. This is where your shutter speed comes into play with your f/stop and ISO settings. Your shutter speed is how quickly the shutter, or the little door that opens in front of your film or image sensor, operates. It can range from several minutes to 1/1600 of a second or more. When you set your shutter speed on your camera, you’ll see numbers like 60, 120, 250 and so on. These actually represent 1/60th of a second, 1/120th of a second and so on. Generally speaking, if you are holding your camera and it is not mounted on a tripod, you should keep your shutter speed at 1/60th of a second or faster. To capture quickly moving objects your shutter speed should be around 1/1600th or faster.
When you combine the ISO setting, the aperture (or f/stop) setting and the correct shutter speed, you are putting together the pieces of the camera puzzle that create your
exposure.Actually, after I wrote all this out (mainly from the photography tips on twopeasinabucket.com) I saw these great articles on everything, in fact they have learning video links too. Check them out.
http://digital-photography-school.com/blog/learning-exposure-in-digital-photography/http://digital-photography-school.com/blog/iso-settings/http://digital-photography-school.com/blog/shutter-speed/http://digital-photography-school.com/blog/aperture/