The first step to understanding digital photography is to understand exposure. The holy trinity — as some photographers refer to it — of exposure are shutter speed, aperture size, and the sensitivity of the digital camera's sensor to light.
To capture a photograph, you really need four things: a subject you want to photograph, a camera, light, and someone to press the shutter button (that would be you, dear reader). Your subject reflects light and casts a shadow. When you press the shutter button, your camera records everything in the frame and renders an image.
If you just point the camera and shoot without visualizing the end result, you still end up with a picture. In most instances, however, the image won’t be something you’d want to frame and hang on your wall.
The shutter speed is the amount of time the shutter stays open. A typical digital camera has shutter speed ranges from 1/8000 of a second to 30 seconds. The aperture is the size of the hole through which light is admitted to the sensor.
The size of the aperture is referred to by a value known as the f-stop. The concept of the f-stop confuses some beginning photographs. A large aperture is designated by a small f-stop number, and a small aperture is designated by a large f-stop number. An aperture with a value of f/2.8 lets gobs of light into the camera, and an aperture with a value of f/22 lets only a small amount of light into the camera.
To properly expose an image for a given amount of ambient light, sufficient light must reach the sensor to produce a distribution of pixels from dark to light. The darkest parts of the subject in your frame are the shadows and the brightest parts of the subject are the highlights.
If too little light reaches the sensor, the image is underexposed; details in the shadow areas become black and all detail is lost. If too much light reaches the sensor, the image is overexposed; details in the brightest areas are blown out to pure white and all detail is lost.
For a given amount of light, there are many combinations of shutter speed and f-stop that will produce a properly exposed image. You can let a little bit of light reach the sensor (small aperture, large f-stop number), which means the shutter needs to remain open longer (slow shutter speed).
Or you can let a copious amount of light into the camera (large aperture, small f-stop number), which means the shutter will remain open for a short duration (fast shutter speed). The camera's light-metering device measures the amount of light and then does all the math and comes up with a shutter speed/f-stop combination that will properly expose the image.
The combination of shutter speed and aperture is known as the Exposure Value. There are myriad shutter speed and aperture combinations for a given Exposure Value. But will the combination the camera chooses when you shoot in Auto mode yield the type of image you envision? If it doesn’t, then you take matters into your own hands by switching from automatic to a different shooting mode.