I am getting ready to teach a photography class later in the spring to folks who are new to the use of a DSLR camera. At first, it can be kind of intimidating, with so many dials and menus and things to fuss with that many just set their camera to fully automatic and let it do all the thinking. After all, it was an expensive camera, it should be able to take great pictures on its own ... right?
But not so fast! All this picture-taking stuff doesn't have to be super complicated! Nope ... not if we begin with a grasp of something called the "exposure triangle" as a foundation for all photography. You see there are only three things that you have to think about to have a properly exposed image; they are the size of the lens opening (aperture), the length of time light is allowed into the camera (shutter speed), and finally the sensitivity of the electrical sensor (ISO). Each of these variables has to be in balance with the other two ... but here is the neat part ... your expensive camera is going to help you do the balancing act, no matter what way (mode) you chose to take a picture. And here's best part ... as you progress in your photography skills, you will learn that the artistic qualities of your images have everything to do with how you combine the shutter, aperture and ISO.
So let's take two examples, a perfect exposure will need just the right amount of light. If I want to take a picture of the kids playing soccer, I might choose a fast shutter speed like 1/1000 of a second to "freeze" the action. My decision to allow light into the camera for such a short interval will now require me to choose a larger lens opening so that my light grab in that instant is just the right amount. Or maybe I want to photograph a pier, and I want as much of it in focus as possible. In fact, choosing a small lens opening has the effect of helping us to do that, but our choice again requires us to compensate by lengthening how long the shutter stays open so that again, the correct quantity of light comes in. We may need a 10 second (slow) shutter to get the job done. Makes perfect sense right!
In real life as you begin to use your camera, you will usually set two of the three variables of the exposure triangle and then your camera will figure out what the remaining one needs to be (thank goodness you're thinking). Typically you will set the camera's internal sensitivity (ISO) to 200 if things are brightly lit or perhaps as high as 1600 if the lighting is poor. I suggest you also turn your camera to "aperture priority" and predetermine the size of the lens opening you want to use. Begin with a value like 5.6 which is a medium opening for general picture taking. Your camera will lock in that value and automatically determine how fast the shutter needs to fire for a proper exposure.
Now that you have been introduced to the workings of the exposure triangle, Google it and add to your knowledge, especially as it pertains to how these three variables can be combined to accomplish some interesting visual effects. As you take more and more pictures, the values for shutter speed, aperture, and ISO will have greater relative meaning to you but today is starting point to learn and grow from.