Before the rush

Before the rush
by evan-pak

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Higher-end SLR cameras include a pentaprism to transmit the image from the lens to the viewfinder, whereas lower-end SLR cameras use a pentamirror to accomplish the same task.

  1. What are the differences between a pentaprism and a pentamirror?
  2. Why is a pentaprism preferable to a pentamirror in a SLR camera?
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The basic difference is that a pentaprism is a solid block of glass. This means it's fairly heavy. It has mirroring material on all but two sides, so the light enters one side, gets reflected around inside, and then comes out the one other un-mirrored side. This means there are only 2 air/glass interfaces involved: one where light enters the prism, and one where it leaves the prism. This keeps light loss to a minimum.

A pentamirror does pretty much the same thing, but it's composed of a number of separate mirrors, so there's air in the middle instead of a block of solid glass. This makes it considerably lighter and less expensive. It also means there's an air/glass interface for each individual mirror, so there's quite a bit more light loss.

As a result, a pentamirror is used almost exclusively in relatively low-end cameras where reduced expense and weight matter more than a relatively dim view through the viewfinder.

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And for a visual:





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+1 - Pentaprism is better, but your diagrams make pentamirror seem way cooler ;-) – Andrew Heath Jan 18 '12 at 8:38
Is there a reason why they have different shapes? – clabacchio Oct 16 '14 at 9:00
This is an addition to Nick Bedford's answer, and answers clabacchio's question. The extra right angle on top of the pentamirror illustration is "roof" of a roof pentaprism or roof pentamirror, which is actually what is used in a SLR. This reverses the image laterally, compensating for the reversal done by the lens. This is why the image in the viewfinder is correctly oriented. – user36866 Jan 8 '15 at 7:01

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