You know from our article about how the DSLR works we published earlier that in order to see the photo through the viewfinder, the light has to be reflected by the mirror in the camera body and then sent upwards to the pentaprism, reversing it upside down and laterally, to send it to the viewfinder.
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However, there are a lot of DSLRs on the market that use a pentamirror instead of pentaprism, causing a lot of confusion among the beginner photographers because of the gobsmacking difference in price. In this article we are going to explain you the key differences between a pentaprism and pentamirror and tell you which one is better.
Both the pentaprism (up) and pentamirror (down), which should be called roof pentaprism and roof pentamirror because they are located inside of the top part of your camera, perform the same function of sending the light to the viewfinder. Why is it then that two cameras, one having a pentaprism and the other one a pentamirror, are so different in price? There are two main differences between them.
Firstly, the pentaprism is a whole piece of solid glass that is heavy and durable whilst the pentamirror is usually made of plastic that is coated with a special reflecting material. The pentamirror is much lighter and less durable and it’s said that with time it may become dusty or even lose its shape, which doesn’t happen with the pentaprism.
Secondly, DSLR viewfinders that use a pentamirror usually have worse image quality and much less brightness. That may be important if you shoot using manual focus or in low-light photography.
That’s about it. As you can see, these differences are not important at all if you are not into professional photography. For beginner photographers it may be especially unimportant because it won’t affect the quality of your photos but only the quality of the image you see through the viewfinder.
Obviously, if you are a professional photographer who earns their living on it, the pentaprism would be a better option because it’s more durable and gives you a sharper and brighter image in the viewfinder. But if you’ve just started to take fancy in photography, several hundred dollars of difference between the two may be crucial for you because you’d better spend them on another lens or photo accessories. Good luck!
We hope that this article was of help to you and we’ll be glad to answer your questions in comments. Feel free to shoot us a message!
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Penta prisms are also known as pentagonal prisms and pentagonal dichroic prisms. Penta prisms are a kind of optical reflective prisms. Penta prisms are optical prisms with five reflective surfaces that can deflect incident light by 90°. The incoming light is reflected twice in the prism, changing the direction by 90°, not only will not be inverted, nor will it change the image's handedness. An ordinary right angle prism will invert the image and change the handedness of the image.
Link to optec
Penta prisms are extremely useful in calibration systems because they can regulate right angles very accurately, and separately regulate the angle of incidence. The ray enters from one side, and after two reflections of 270 degrees inside the prism, it exits on the other side adjacent to it.
At present, pentaprisms are widely used. In SLR cameras, roof-type pentaprisms are commonly used optical elements and double pentaprisms are used in building construction surveys. The two pentaprisms are joined top to top and a plumb line to mark the correct angle.
The five-sided mirror uses specular reflection, and the five-sided prism uses total reflection. The specular reflection uses a silver-plated surface, and there must be a loss of light.
The total reflection has no light loss at all. Therefore, the penta prism must be brighter than the pentamirror. Generally, the penta prism can only achieve 80% of the brightness of the penta prism. This is the only advantage of the penta prism.
When light travels in the same medium, it travels in a straight line. Only when one medium reaches the surface of another medium, will there be a transmission, reflection, and refraction phenomena. When the light is projected on a certain mirror surface of the pentamirror viewfinder, it first enters the glass medium from the air medium. And then it returns to the glass medium from the silver-plated reflective surface, and finally passes through the glass medium and then is transmitted back into the air medium.
This process needs to be looped five times to complete the image light path rotation when the pentamirror viewfinder is framing the view. Even if the glass transmittance of the mirror is high, some of the lights will inevitably be absorbed when passing through the glass. Therefore, after multiple absorptions, the brightness of the viewfinder will inevitably decrease. Although the reflected light path of the pentaprism viewfinder is the same, because it is always transmitted in the same medium, the viewing brightness is relatively much higher.
The advantage of using a pentamirror as the viewfinder of an SLR camera is that it helps reduce the weight of the camera, and the cost is relatively low compared with a pentaprism, so some popular SLR cameras are often used. The disadvantage of this viewfinder is that the viewing brightness is low, and the stability is relatively poor, so severe vibration should be avoided as much as possible. It is for these reasons that most mid-range and above SLR cameras still take the solid pentaprism viewfinder.
For more information, please visit Penta Prisms.