It's good from time to time to review the basics of how a camera works and the adjustments you have at your disposal. So many people go around taking pictures with their cameras set on automatic. Modern digital cameras do a fantastic job in point-and-shoot mode, but there is a whole world of creative opportunity for those who take a few minutes to explore the features.
There's even a formula that describes the whole process: H=Et. Where H is the luminous exposure, E is the image-plane luminance and t is the exposure time in seconds. Perhaps a more practical explanation might help.
Aperture, Exposure, ISO
Think of those three elements as three legs on a stool. When all three are working in harmony, you have solid support. If one of them is a little off, it throws the whole stool out of whack. By balancing these three components in different ways, you can achieve remarkably different results in your photographs.
ISO
Formerly called ASA, represents the light sensitivity of the recording media. Whether it's film or a digital imaging chip, the trade off here is image quality. Regardless of the recording media, the less light the image capture device needs, the more artifacts in the resulting picture.
In modern digital cameras that degradation of image quality comes from the camera's on board computer trying to fill in and adjust an image with less than optimal lighting. To compensate for low light the camera increases the sensitivity of the imaging chip, introducing a whole host of visual artifacts collectively known as "noise."
The more light, the lower ISO the camera can select. The camera's computer is designed to select the ISO that will yield the clearest possible image for the available light. Somewhere in the neighborhood of 20 percent below full daylight, your camera's imaging chip will be operating at maximum efficiency. To get the proper amount of light, your camera can select a higher ISO or trigger the on-board flash.
Aperture or f-stop
The second exposure control for photo lighting is your aperture. The aperture sits between your lens and imaging surface. This works the opposite of ISO. The smaller the number, called an f-stop, the more light is getting through the lens.
When you hear photographers talking about a "fast" lens, they mean a lens that has the ability to select very low f-stops.
What changes is the focusing depth of field (DoF). At lower f-stops, your camera has a narrower depth of field. At higher f-stops your camera has a very wide DoF. So if you want the background in focus, think higher f-stops (which are smaller openings) and if you want to throw the background out of focus, think lower f-stops (wider openings).
Shutter Speed
The shutter on most modern SLR's is actually more like plates that leave the imaging surface exposed briefly, sometimes called "curtains." The leading surface moving out of the way is sometimes called the "first curtain" and the trailing "second curtain" cuts the light off to the imaging chip.
That's why most flash units have specific sync speed. At very fast shutter speeds there's not enough time between the first and second curtains for the flash to fire without one of the curtains being in the way. It also explains the feature some flash units have labeled 1st or 2nd curtain sync. You can select to have the flash fire in sync at the opening of the shutter, or just before it closes.
The shutter priority feature of your camera lets you choose the shutter speed. Faster to freeze objects moving fast, and slower speeds to create motion blur.
Experiment with the different modes your camera offers. You'll find the range of possibilities quite exciting.
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