I love photography because it is half art, half science. The art half you can acquire through practice, experimentation, and taking inspiration from other people's work. When it comes to learning the mechanics of how to take a good photo, you might read articles I've written, like "Getting Started in Digital Photography" and "Master Your Camera's Depth of Field." Or you can fiddle with your camera controls and see how they affect the exposure, even before you take the photo.
You no doubt have heard that a properly exposed photo happens when you combine the right shutter speed and aperture. If you leave the shutter open for a short time, for example, you need to open the aperture relatively wider so it admits more light. Close the aperture down, and you'll have to leave the shutter open longer to keep the overall amount of light the same.
If you set the camera on Auto or Program mode, it handles those details on its own. On Aperture Priority, you can control the size of the aperture, and the camera sets a matching shutter speed. The opposite is true in Shutter Priority Mode.
And then there are a few curve balls: Changing the ISO makes the camera relatively more or less sensitive to light, so a higher ISO allows you to use a faster shutter speed/aperture combination in less light. And your camera's Exposure Compensation control lets you over- or underexpose a photo based on the currently selected exposure settings.
The LCD as a Laboratory
Even if you've heard all that before, you might wish there was an easy way to experiment without taking and comparing photos with different settings. And there is: The display on the back of your camera is like a photography lab, just waiting for you to experiment (without any smelly chemicals). The LCD, after all, is not just for seeing the photos you've just taken. It can preview the photo you're about to take, and that means it can simulate the various exposure settings quite accurately.
There's one big caveat. If you have a digital SLR, its LCD might not give you a "live view" of the photo you're about to take. Most digital SLRs use the LCD as a playback screen only, though some models have a live view mode. Check your user guide if you're not sure.
See for Yourself
Ready? The easiest way to experiment with your camera's exposure controls is to set it down on a table, put the camera in Manual exposure mode, and watch the LCD to see how changing the shutter speed and aperture change the brightness of the image. Start, for example, with a shutter speed of 1/10 second and an aperture of f/4. You might see something like the dim image on the left.
Now spin the shutter speed dial. As you lengthen the shutter speed, the image gets brighter, as you see here on the right.
Shorten the shutter speed, and it gets darker. In the image on the left I made it too short.
Now switch gears and change the aperture--as you make the f-number bigger, the image gets darker. That's because a big f-number corresponds to a smaller aperture opening. Make the f-number smaller, and the image brightens.
While you're exploring the effect of exposure settings, be sure to change the ISO. As you'd expect, a bigger ISO value will brighten the screen. This shows you how you can increase the ISO to take a low-light picture at a reasonable shutter speed, so you don't need a tripod.
Check Out the Histogram
I should point out that you can't necessarily look at the LCD and know if the scene is properly exposed. While the image might look good, the LCD itself might be set too bright or dark, which will affect how the scene appears on the screen . Many digital cameras allow you to turn on a histogram display while previewing your photos, and that's a better indication of whether the photo is about to be properly exposed.
First, review the basics of interpreting histograms; then you can use the same technique we've been using to read the histogram display on the LCD. If you overexpose a scene with too much light from the aperture and/or shutter, you'll see something like this, in which the curve bunches up on the right side of the graph.
And if the scene is too dark, the curve moves to the left side instead. If you use the camera controls (shutter, aperture, and ISO) to position the curve so that it doesn't crawl off either extreme, you'll see something like this.