The Pentaprism And Pentamirror

The Pentaprism And Pentamirror

We have seen how one of the characteristics of the SLR cameras is the presence of an optical viewfinder, or the ability to observe the scene through the lens and then get a view of all identical to what will be captured by the camera sensor when shooting image. In order to allow the optical viewfinder, positioned high above the lens axis, to observe the scene, SLR cameras are equipped with two “structures” designed to re-direct the light: the mirror placed in front of the sensor ( on the axis of the lens) and the pentaprism.

A pentaprism is a five-sided reflecting prism used in the optical viewfinders of the cameras, in particular the SLR akusewa. It is an object made with the purpose to deflect a beam of light with a constant 90 °, even if the incoming beam is not perfectly at 90 ° relative to the prism. The light beam is reflected two times inside the prism: the second reflection serves to flip the image in turn inverted by the first reflection. This way we can observe, through the optical viewfinder, the image “straight” and not reversed, what would happen if instead of a pentaprism had a normal mirror.

To obtain the two reflections, only two faces of the prism are machined to mirror while the other three are coated with a non-reflective material to reduce spurious reflections.
The pentaprism, however, is not unique but is flanked by a slightly different version, called pentamirror.

The main difference is that the pentaprism is a single, solid piece of glass. It is consequently relatively heavy. This piece of glass has all reflective surfaces with the exception of two sides, namely the input and the output of the light (reflected by the mirrors placed on the other sides). The reflections inside the prism are only two: this in order to minimize the loss of light due precisely to the reflections.

a pentamirror is, visually, very similar to a pentaprism but it is not a single block of glass but it is different mirrors joined together to form a pentagon. The difference is that then the prism is filled with glass while the pentamirror is full of air. This allows the pentamirror to be much lighter but also much cheaper. The disadvantage is that the reflections you are always two but there are also two interactions glass / air which results in a greater loss of light compared to the pentaprism.