Optical Viewfinder on a Digital Compact Camera
On a digital compact camera the viewfinder represents a simple optical system that zooms at the same time as the main lens and has an optical path that runs parallel to the camera’s main lens. Being so small, viewfinders have a problem represented by framing inaccuracy. Being positioned above the actual lens, the image that you see through the optical viewfinder is different from the image that the lens projects onto the sensor. This difference is most obvious at relatively small subject distances. Most of the time, the optical viewfinder allows you to see only a percentage (80 %to 90 %) of what the sensor will capture. If you want to have a more accurate framing, use the LCD instead.
LCD on a Digital Compact Camera (TTL)
Through the LCD an a digital compact camera you are able to see in real time what is projected onto the sensor by the lens and therefore avoids the above difference. This is also called TTL or “Through The Lens” viewing. If you need to frame accurately in very bright sunlight conditions, it is not recommended to use the LCD. In this case you should resort to the optical electronic viewfinder. The LCDs on almost all digital SLRs show only the image after it was taken and gives no live previews.
Optical Viewfinder on a Digital SLR Camera (TTL)
The optical viewfinder of a digital SLR shows what the lens will project on the sensor via a mirror and a prism and has therefore no parallax error. When you depress the shutter button, the mirror flips up so the lens can expose the sensor. Due to sensor limitations, the LCDs will only show the image after it was taken and gives no live preview. Some models have replaced the mirrors with a prism to solve this problem.
Electronic Viewfinder (EVF) on a Digital Compact Camera (TTL)
An electronic viewfinder (EVF) works the same way as a LCD on a digital compact camera and shows in real time what is projected onto the sensor by the lens. It is basically a small LCD (typically measuring 0.5″ diagonally and 235,000 pixels) with a lens in front of it, which allows you to frame more accurately, especially in bright sunlight. It simulates in an electronic way the effect of the (superior) optical TTL viewfinders found on digital SLRs and doesn’t suffer from parallax errors. Cameras with an EVF have an LCD as well, but no true optical viewfinder.