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The viewfinder is named after its function: It helps you find the view that you want to photograph. A plain viewfinder does nothing but give you a view. It can be as simple as two frames that you line up to locate the image that the camera will picture. Most digital cameras elaborate on the basic viewfinder design.
Optical viewfinders use a combination of lenses that work like a telescope, giving you a view at a glance so you don't have to line up frames. The lenses are small and separate from the lens in the camera used to take the actual picture. The least expensive digital cameras use simple optical viewfinders.
Many cameras have reflex viewfinders, which use the camera lens as part of the viewfinder. Through the use of a mirror or prisms, you can see the actual image seen by the lens. A mirror reflects the lens image up into the viewfinder, the reflection being the camera's reflex. The most expensive professional digital cameras use true reflex viewfinders (as do a few lesser expensive models).
A reflex viewfinder allows you to see the effects of zooms. Moreover, the reflex viewfinder gives you a detailed color image without the need for using a power-hungry (and battery-draining) LCD panel. But because you don't see how the camera's electronics are affecting the image, you can only guess at how the camera's electronics are dealing with the image.
Digital cameras add video viewfinders or LCD panels that allow you to see exactly the image seen by the camera lens after it's processed by the camera's circuitry. In addition to letting you preview the image before you capture it, the LCD panel also lets you review the images you've already captured.
Big-screen or view-screen viewfinders give you a larger LCD display meant to be watched like a television set instead of a pirate's telescope. It gives the best possible view of the image before and after recording. Using big-screen viewfinders can seem unnatural to anyone accustomed to the peepshow method of previewing images in other camera formats.
Some digital cameras give you what looks to be a conventional optical viewfinder at the back of the camera—you peer through it with one eye—but put a small LCD panel at the other end to create an electronic view finder (EVF). The EVF gives you a preview like an LCD panel at the back of the camera but is smaller and dimmer (it doesn't have to overpower daylight as does the panel on the back of the camera), so it uses substantially less power than the big panel on the back.
Some digital cameras switch their EVFs to black-and-white in dimly lit situations to help you focus more easily. Note, too, that the frame rate of the EVF, often slower than conventional video systems, may distort smooth motion. It's something you have to get used to.
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