What does the pentaprism or pentamirror do in an SLR viewfinder?
Asked 3/13/2014
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In an SLR, light from the lens reflects off the main mirror onto the focusing screen, then passes through a pentaprism or pentamirror to the eyepiece. At first glance, it seems like a simple two-mirror periscope arrangement could send the image to the viewfinder with fewer reflections and possibly better brightness.
Why do SLRs use a pentaprism or pentamirror instead of a simpler pair of mirrors? What optical problem is the prism/mirror assembly solving?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
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It's important to realize that you don't actually look directly through the lens with an SLR! If you did, a periscope style arrangement would do just fine.
What you are actually doing is looking at an image projected onto the focussing screen by the lens. This image is flipped left/right and up/down by the lens, and then up/down again by the main mirror.
This leaves the image flipped left-right. If you get chance to play with a medium format camera with waist level viewfinder (which amounts to just looking at the focussing screen through a tunnel) you'll see the image flipped left/right, which makes it difficult to use if you're not used to it.
To avoid this difficulty and present the user with a normal looking view in the viewfinder without the left/right inversion requires a roof pentaprism/pentamirror. A roof pentaprism is where one of the surfaces is split like a vaulted roof in order to laterally flip the image. Note that there are only three (not five!) reflections inside the prism, it's only pentagonal as one of the unused faces is truncated to save weight.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
12y ago
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Generated from our catalog & community — verify before relying on it.
The pentaprism/pentamirror is mainly there to correct the image orientation.
In an SLR, you are not viewing directly through the lens; you are viewing the image formed on the focusing screen. That image is inverted by the lens, and the main reflex mirror flips it again in one axis. The result at the focusing screen is still reversed left-to-right.
A simple two-mirror “periscope” would relay that image to your eye, but it would still appear laterally reversed. That makes composition and tracking awkward. A roof pentaprism or pentamirror re-reverses the image so the viewfinder appears upright and correctly oriented, matching the scene.
You can see the difference on cameras with waist-level finders, where looking directly at the focusing screen gives a left-right reversed image.
So the extra optical complexity is not just to redirect the light path — it is to provide a properly oriented viewfinder image that is much easier to use.
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