SLR cameras display the image that is actually going to come out on the sensor in the viewfinder. They do this by using a mirror in front of the sensor and reflecting it back into the viewfinder. The problem comes in the second reflection that it is necessary to turn 90 degrees to get the image in the viewfinder, here comes the pentaprism or the pentamirror
Let’s look at the principle behind it first, it will be easier to explain this using a pentaprism, so let’s talk in terms of a pentaprism and then explain the difference from a pentamirror.
As this image shows, a pentaprism does the work of turning the light 90 degrees. The internal reflection within the prisms rotates the light by 90 degrees. The image from the lens is reflected by the mirror which then falls on the prism which rotates it 90 degrees and sends it to the viewfinder. 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 back to the viewfinder changes because a smaller form factor requires less surface area and therefore less total light to reflect. In simple words, the reflective mirror will be smaller, the amount of light is directly proportional. Therefore, when using a smaller sensor, the light we have to deal with in the first place is less. The second thing we must understand is that the reflection between mirrors, that is, the reflection of the mirror in the air, has a greater loss of light than an internal reflection in a good quality material.
In a pentamirror reflection system, an arrangement of mirrors does the work that the prism does, because here the reflection is on a mirror with air as the medium, the loss of light is greater. That is to say, here a pentaprism is emulated by means of a series of correctly arranged mirrors. However, these mirrors have air as the medium between this reflective design which does the job but causes a fair amount of light loss.
Now, why does light lose such a problem? When looking through the viewfinder, especially in low light conditions, if the image is not bright, it will be difficult for us to correctly determine focus and other similar details. Also, when using a smaller form factor sensor, the amount of light is inherently low, so you experience more glare reduction. Another problem would be the difficulty to determine the contrasts correctly.
If brightness is so important, why do we have pentamirror systems? This can be explained with a single word “cost”. The mirror-based design is much cheaper than a prism-based design because prisms are expensive. Manufacturers are always trying to cut costs to make their products more mass marketable. They compromise on quality to do so. The interesting fact is that while there may be a 10-20% loss in brightness, a pentamirror design has many important advantages over a pentaprism design.
The main advantage is that it is cheaper. The other advantage is that a pentamirror design is lighter, an SLR camera is heavy equipment which adds to the weight of the lens and makes it all quiet so the weight reduction is a nice bonus.
Now pentaprism designs usually have a good viewing area of 95% or more, viewing area is the percentage of the image area, falling on the screen, that can be seen through the viewfinder. This is important because when the percentage is lower we don’t see the edge of the image through the viewfinder.
Most of today’s budget DSLRs and low power DSLRs use pentamirror to offer better value and lighter bodies. If you like low-light photography or need a bright viewfinder, you should look for a model that has a pentaprism. Overall, the pentaprism is a sacrifice you can make without much disappointment for the price advantage.
