Camera Parts: Viewfinder
The optical viewfinder is one of many camera parts that will vary depending on the model and type of camera. The main notable difference is between digital compact cameras and DSLRs. Each viewfinder has its own method of allowing the image to be seen while taking a shot.
Digital Compact Viewfinder
A digital compact camera will have less accuracy when framing due to the internal optical system and position of the viewfinder being slightly above the actual lens. The viewfinder is usually quite small and parallel to the main lens, which causes the "parallax error." The LCD screen can be used to avoid this effect, or there are some cameras that have guidelines within the viewfinder to counter the problem as well.
Digital SLR Viewfinder
DSLRs do not have the viewfinder concerns that are problematic with
the digital compact camera. The optical viewfinder is based on a mirror
and prism system that projects light on to the sensor. With the DSLR
camera, you are actually looking "through the lens" or TTL once the
shutter button has been depressed (this causes the lens to be exposed to
the sensor). In some cases, the optical viewfinder will be the only way
to take photos with a DSLR, although some newer models have
incorporated live-view mode on the LCD.