An optical viewfinder is the window used for framing and composing your picture. You peer through the optical viewfinder, which uses no power, to compose your image quickly. However, optical viewfinders have problems of their own, most notably the parallax error. Here, what you see through the viewfinder isn't exactly the same area that the camera lens records. The parallax error is caused by the difference in viewpoint between your camera's lens and the optical viewfinder - they don't have the same perspective and, therefore, don't show the same thing.
If the optical viewfinder happens to be mounted directly above the camera lens, its view will be a little higher than that of the lens itself. This becomes a problem chiefly when shooting pictures from relatively close distances (3 feet or less). Such a viewfinder tends to chop off the top of any subject that is close to the camera.
Optical viewfinders usually have a set of visible lines, called parallax correction lines (not to be confused with focus-area markings), which you can use to frame the picture. If you keep the subject matter below the correction lines, you can avoid chopping off heads. Of course, you get more of the lower part of your subject than you can view. Parallax becomes worse when the optical viewfinder is located above and to one side of the lens, as shown in Figure 1. In that case, a double set of parallax correction lines is needed to help you avoid chopping off both the top and the side of your subject.
In addition to parallax, here are other points about optical viewfinders:
- Magnification: The magnification of the optical viewfinders differs in many digital cameras. Put the camera up to your eye, and you might see a tiny image floating off in the distance, with the details of your scene barely discernible. Other cameras provide a big view that makes it easy to frame and compose your photo.
- Zoom: Believe it or not, some digital cameras with zoom lenses have optical viewfinders that don't zoom! Ideally, the image should match the view of your display and lens, but some cameras keep a fixed view and use indicator marks to show the picture area.
- Accurate viewpoint: The optical viewfinder may not show everything you're framing, with some image area clipped off the top, bottom, or sides.
- Diopter adjustment: Eyeglass wearers will want an optical viewfinder that offers adjustment for common prescriptions so the camera can be used without wearing glasses.
- Extended eyepoint: The eyepoint is the distance your eye can be from the viewfinder's window and still see the entire view. Some viewfinders mandate pressing your eye up tightly against the windows. Others let you back off a few millimeters, which is handy when you want to be able to see around the camera to monitor the whole scene (and not just what appears in the viewfinder) or need to wear your glasses while using the camera.
- Readouts: What information is available within and around the optical viewfinder? Some cameras show nothing but the unadorned image. Others have framing or parallax correction lines or perhaps a faint grid to help you line up the image. Perhaps some camera status indicators appear in the viewfinder, too, such as a flash-ready LED (light-emitting diode), within the field of view or located just outside the viewfinder window where it can be detected by the eye.
