SLR cameras show the image that is actually going to come on the sensor in the viewfinder. They do this by using a mirror in front of the sensor and the reflecting it to the viewfinder. The problem comes in the second reflection that is we need to get a 90 degree turn to get the image on the viewfinder here comes the pentaprism or the pentamirror

Lets see the principle behind this first, it will be easier to explain this using a pentaprism so we are going to talk in terms of a pentaprism and then explain the difference to this with regard to a pentamirror.

As this image shows a pentaprism does the job to turning the light 90 degrees. The internal reflection inside the prisms turns the light in 90 degrees. The image from the lens is reflected by the mirror which then falls on the prism which turns it by 90 degrees and sends it to the view finder. The operation of the prism is as simple as explained. Remember that our interest is to find out how a pentamirror and a pentaprism differ and why these two exist and which one to choose.

Now depending on the form factor of the sensor the amount of light that is reflected on to the viewfinder changes because a smaller form factors calls for less surface area and thus less total light to reflect. In simple words the reflecting mirror will be smaller the amount of light is directly proportional. Thus when using a smaller sized sensor the light we are having to deal with in the first place is less. The second thing we should understand is that reflection between mirrors that is mirror to air reflection has greater light loss, than a internal reflection in good quality material.

In a pentamirror reflection system an arrangement of mirrors does the job that the prism does, because here the reflection is on mirror with air as the medium the loss of light is greater. That is here a pentaprism is emulated by a series of properly arranged mirrors. These mirrors are however having air as a medium in between this reflective design does the job but cause a good amount of light lose.

Now why is light lose such a problem? When we are looking through the viewfinder especially in low light conditions if the image is not bright it will be difficult for us to determine properly the focus and other such details. Further more when using a smaller form factor sensor the amount of light is inherently low thus further reduction in brightness is experienced. Another problem would be difficulty in determining contrasts properly.

If brightness is so important why do we have pentamirror systems? This can be explained in just a single word “cost”. Mirror based design is very much cheaper than a prism based design because prisms are costly. The manufactures are always trying to reduce cost to make their products more mass marketable. They compromise on quality to do so. The interesting fact is that while there might be a 10 - 20% brightness loss , a pentamirror design has many important advantages over a pentaprism design.

The main advantage being that it is cheaper. The other advantage is that a pentamirror design is lighter, an SLR camera is a heavy piece of equipment adding to that the wieght of the lens and all make it quiet heavy , thus reduction in wieght is a good advantage.

Now pentaprism desings usually have a good viewing area of 95% or more, viewing area is the percentage of the area of the image , which is falling on the screen , that can be seen through the view finder. This is important because when the percetage is lower we dont see the edge of the image through the viewfinder.

Most of todays budget DSLR’s and Lower end prosumer DSLR’s use pentamirror to offer a better price and ligher bodies. If you are into low light photography or require a bright viewfinder then you should be looking at a model that has pentaprism. Overall the pentaprism is a sacrifice that you can make without much disapoinment for the advantage in price.

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