You don’t have to know how a digital camera works to be able to take pictures. But if you want to take pictures that you’ll enjoy looking at for years to come, it helps to know what’s going on when you push the shutter button. I’ll run through some of the basics here — the manual that came with your camera should explain in more detail anything that you want to know more about.
Every digital camera, from a point-and-shoot to a professional DSLR (go here for more information about camera types), shares the same fundamental operations. The heart of a digital camera is its sensor, a light-sensitive surface that measures the light that falls on it so the camera can create a picture. Your eyes can adjust to a wide range of light (think dawn, to noon, to dusk, to night) and still get a clear picture of your surroundings, but a camera’s sensor needs some help to get a picture that works. In general you will want a “Goldilocks picture — not too dark, not too bright, but just right. And there are three basic controls that determine how bright a picture your camera gives you.
The first control is the shutter speed, which determines how long the sensor will be exposed to the light. Shutter speed is generally measured in fractions of a second, so a speed of 1/100 is one-hundredth of a second. The second control is the aperture, which determines how much light is reaching the sensor. Think of the aperture as something like the diameter of a hose that waters a garden: the bigger the diameter, the greater the amount of water that gets through the hose. Apertures are measured with numbers labeled f-stops, with higher f-stops indicating a lower aperture. I’m not going to explain all of the math involved, just recite that multiplying an f-stop by 1.4 cuts the light reaching the sensor by 1/2; multiplying an aperture by 2 cuts the light by 1/4. The aperture and shutter speed working together determine the amount of light that hits the sensor. So an aperture of f4.0 and a shutter speed of 1/125 of a second delivers the same amount of light to the sensor as an aperture of f2.0 (four times as much light as f4.0) and a shutter speed of 1/500 of a second (one-fourth as much time as 1/125). (If your garden hose is bigger, you can deliver the same amount of water to your lawn in less time.)
Why would you care what your shutter speed or aperture is? If you’re taking pictures of a mountain, or something that’s sitting still, your shutter speed may not be so important. But if the subject of your picture involves motion — four-year olds enjoying a play date in the yard, or a basketball game — and your shutter speed is too slow (1/30 of a second, rather than 1/250), the people in your picture will be blurred. Another advantage of a faster shutter speed is that it’s difficult to hold a camera steady at slow shutter speeds, so even stationary objects can be blurred. As for aperture, a wide aperture (small f-stop) lets in more light but makes it more difficult to keep everything in a picture in focus.
So maybe you want a fast shutter speed for your picture (no blurring) and narrow aperture (high f-stop) so everything’s in focus, but what if there’s not enough light reaching the sensor and your picture comes out dark? That’s where the third control, sensitivity or ISO, comes in. High ISO numbers convert a limited amount of light hitting the sensor into the Goldilocks picture that you want. An ISO of 100 (generally the lowest sensitivity on a digital camera) works when there’s lots of light; an ISO of 200 delivers the same brightness level for the picture with half the amount of light. If you reduce your shutter speed by half, you double the ISO and still get the same brightness in your picture. But higher ISO’s are a mixed blessing — you can take a sharp, focussed picture when there’s less light available, but there will be “noise”, speckles, if you look at the picture closely (or make a large print). A picture with speckles may be better than no picture at all, but there is a trade-off for raising the ISO (and many point-and-shoot cameras won’t offer an ISO above 800 or 1600).
Another help with dark pictures is of course the flash on the camera. A camera’s flash can be quite useful for pictures of nearby (5-15 feet) subjects but doesn’t help much at all for subjects that are further away. So it’s at best a partial solution.
All of this business about shutter speed, aperture, ISO and flash is a lot to keep straight, which is why point-and-shoot cameras try to make it simpler. Most “Scene Modes” on a camera are nothing more than pre-set combinations of shutter speed, aperture and ISO, perhaps with some flash thrown in. The camera manufacturers invest a good deal of effort in programming the scene modes so they work in as many situations as possible. But if you’re trying a scene mode and just can’t get that Goldilocks exposure, you may need to adjust your camera’s settings to get the light right. And there may be pictures you want to take that don’t fit into “Scene Modes” at all — playing with the aperture, for example, to keep the portrait in a picture sharp while the background is pleasantly blurred.