Through the use of the mirror results may have a focusing error in the distance setting, because the sharpness in the image plane of the film or the image sensor is determined, but with the help of a dial or a separate field sensor, which in the case of defects or substandard production is not geometrically realize exactly the image plane.
Typical examples are Rolleiflex and Mamiya C, where only the Rolleiflex medium format in two versions and one is made for Minox miniature format.
Canon EOS Rebel T4i
This type of camera has a number of advantages:
The viewfinder image is always visible and not obscured by the working aperture;
The recording sound is very quiet and
triggering the camera shutter causes virtually no vibration.
This is offset by some disadvantages:
Expensive lenses are not implemented for reasons of cost, since they would be doubly necessary;
It creates a parallax effect is especially noticeable in close-up or macro shots, since the optical axes of the two lenses are shifted against each other.