I helped offset the costs of college by doing some professional photography and video production. I enjoy photography, but try not to give myself too much credit for my photography skills because if you've got the right equipment, taking great pictures can be relatively simple.
When I got my first SLR camera, some of the things that that often confused me were the definition of and relationship between ISO, aperture, and shutter speed. I knew that these items were related and affected the overall quality of pictures, but I didn't
really understand how they worked until I read
this article a few years ago (or some derivative of it). The author compares taking a picture to filling a bucket with water. In order to have a photo that is neither washed out nor too dark, you will need to fill that bucket with water, ensuring it is full to the top and not overflowing. In his analogy, the water represents
light, the amount of time the faucet is left on represents the
shutter speed (how long the light is exposed to the film/sensor), the strength to which the faucet is adjusted represents the
aperture, and the size of the bucket represents the
ISO. This article effectively describes the relationships. I, however, have an example of my own.
Photography, after all, is simply the capturing of light and manipulating it so that it is directed to a special surface (film or digital) that can 'remember' it. Do this for a moment: look at a bright light, wide-eyed, for half a second (not the sun -- you'll go blind) and then close your eyes quickly. Do you see how that light has burned a faint image into your eye? Your eye was just acting like a piece of film (or a digital sensor), capturing the light. Now look into that same bright light for about two seconds; close your eyes. Does that same burned image look a little brighter now? I'll bet it does. Because your eyes (which are similar to a camera's shutter) were open longer, they let more light into your eye (the sensor), thereby creating a brighter image.
To change the amount of light in our 'photograph,' we just adjusted our 'shutter speed.' Now let's adjust our 'aperture.' Look into the same bright light for about two seconds, but squint your eyes just a bit. Close your eyes, and I'll bet that the image that remains on your eyelids is more faint than it was when your eyes were wide open for the same amount of time.
This is because by adjusting the aperture (the degree to which our eyelids are open), we're putting a bottleneck on the amount of light that we're letting into our eyes. This is why we squint naturally when it's very bright outside. With a camera, if too much light is let in, the photo will get washed out. The aperture is the camera's 'squinting' mechanism. As humans, we're not worried about our photos getting washed out -- we're worried about damaging our eyes. When it's a very bright day or when someone shines a light in our face, we naturally react by adjusting our eyelids to let in an optimal amount of light -- kind of like a camera's automatic mode. ISO is a little more difficult to describe with this example and will be discussed later.
When taking a photo, a photographer (or camera automatic mode) needs to adjust the aperture and shutter speed to allow the optimal amount of light to be exposed to the film (in the digital world, the 'film' exists in the form of a CMOS light sensor). Aperture and shutter speed have a supplementary relationship; too much or too little of one can be made up by adjusting the other (think of it as a balance -- like the ones you see on flags in courtrooms). This is a key principle in photography. If your aperture is set very low (the eye is squinted), you can still get a nice picture if you let the shutter stay open for a little bit longer. If it's bright outside and your aperture is set high (the eyes are wide open, looking straight at the sun), the photo will be washed out unless the shutter speed is very fast (looking only at the image for a small fraction of a second).
Though aperture and shutter speed are supplementary, the artistic effects that can be achieved by adjusting them are different. Depending on the physical environment and artistic intent of the photographer, he or she may choose a slow or fast shutter speed or a high or low aperture. By doing this, the photographer will need to make up for any extremes on the other side of the balance. Here is an example:
I took this picture a couple of years ago in Montana. Do you see how the water is kind of blurry? I achieved this affect by holding the camera very still and letting the shutter stay open for .4 seconds. .4 seconds may not sound like a long time, but since the water was moving so fast, it captured a lot of movement for the amount of time the shutter was open. Now, if I hadn't held the camera completely still, the rest of the things in the photograph, like the rocks and bushes, would look blurry. But by holding the camera still, I was able to capture the movement of the only thing that moved naturally.

Because I took the picture of the waterfall at two o'clock in the afternoon, it was very bright outside. If I had left the shutter open for that long without adjusting the aperture, the photo would be washed out. Check out this example from a trip to Puerto Rico when I tried to capture the moving creek running over my feet mid-day. I captured the moving water by leaving the shutter open for almost a second, but because I didn't 'squint' the camera's eye enough, my image was washed out (a.k.a 'overexposed').
Adjusting the aperture for artistic value will affect what's called the depth of field (how much of what you see is in focus). Look at this picture of the hermit crab in Puerto Rico. Do you see how some things are out of focus? This is an intentional artistic modification to focus on only the subject. By zooming in and setting the aperture to as open as possible (eye as wide open as it can be), I was able to achieve this effect. But because my aperture setting was so wide open, I needed to set my shutter speed accordingly (which many camera's can do automatically).
Because of the definable supplemental relationship between aperture and shutter speed, digital SLR cameras have modes that will allow the photographer to set either the aperture or shutter speed and have the other variable set automatically. So if I'm taking pictures of a waterfall and want to leave the shutter open for a full second, I can set my shutter speed to 1" and let the camera adjust the aperture to whatever is appropriate for the given amount of light in the current environment. This setting will depend on how much light is present (what time of day, etc).
This relationship is also the reason that it's easier to take fast-moving pictures during the day when it's bright outside. Let's say we're taking an action photo of a boy hitting a baseball, wanting to capture him mid-swing. To stop his fast movements, we'll need to have a very fast shutter speed (something like 1/1000 of a second). But if our speed is that fast, we'll need the aperture to be set to be as open as it can be. If it is a bright day and there's lots of natural light around, then enough light should get into the camera during the brief instant that the shutter is open to give us a nice picture. But if we're trying to take the picture at night with a standard lens, with the shutter open for such a brief period of time, not enough light will get into the camera, even with the aperture set to be as open as it can be (unless you have a very expensive lens that lets lots of light in) to make a nice picture. In a dark environment, even with the aperture set to be as open as possible, the shutter will have to stay open longer to let enough light in, creating a blurry picture if the subject is moving. Therefore, when shooting at night, it is easier to take nice pictures of things that are still. Because photography is all about capturing light, you've got a lot more flexibility if you shoot during the day with lots of it at your disposal. You can always hide natural light from the camera; you can't add it -- that's why flashes exist!
ISO is also related to aperture and shutter speed. Essentially, ISO is sort of like the resolution of the photograph. By adjusting the ISO (also called film speed), you're affecting the amount of light required to make a nice photograph. The higher your ISO, the less light is required to take an optimal picture. By changing your ISO setting, you're effectively changing the capacity of the photo, which will also affect the amount of detail that you're able to capture. Ideally, if everyone took pictures of things sitting still in bright light, we could use a very low ISO all the time. That would provide the highest quality shots. But in reality, we shoot things that move and we shoot things in the dark. By telling the camera to use a higher ISO, we're basically saying "Just take the picture. I know there's not much light, so you don't have to give me as much detail." If given 'permission' to deliver a slightly more grainy photo, a camera can take photos that it normally wouldn't be able to with a lower, higher-quality ISO.
Consider taking a photo of friends sitting around a campfire. There is not very much natural light, and no matter how big you set the aperture (the lens has physical limits -- the bigger the lens, the bigger the aperture can be), the shutter is still open too long to take a non-blurry picture. So what do I do? I set the ISO to the highest value that I can. That means that the camera will require less light to take picture, but as a trade-off will deliver a more grainy picture. Because I don't have a lot of light, I realize that I'm either going to get a high resolution blurry picture, or a low-resolution crisp picture.
It takes a long time to really understand the relationships between the ISO, shutter speed, and aperture. But if you practice with a digital SLR camera and adjust one variable at a time, you'll see how it can affect the look and feel your photos and the moments you're trying to caputre. Once you get a feel for which types of changes create which types of effects, you'll be taking beautiful pictures in no time.
Good rules of thumb:
1. Always use the lowest ISO setting that you can to get crisp, non-blurry pictures. ISO 200 is a good setting to use on bright days (100 is even better if your subjects are relatively still). At night, if you don't use a flash, you might need to set it as high as 800, 1600, or 3200, depending on the capabilities of your camera.
2. To make artistically 'blurry' pictures, adjust the shutter speed to be open for longer. If it's bright outside, you (or the camera) will need to adjust the aperture to make sure that not too much light gets in.
3. To adjust the depth of field, try zooming in and adjusting your aperture. Aperture is measured in the terms of a fraction (f/5, f/2.5), with f as the numerator (standing for focal length) and the number as the denominator. The lower your number, the wider open the aperture.