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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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    There are no mirror coatings on a pentaprism, they work based on total internal reflection. – Brandon Dube Apr 25 '17 at 22:18
  • So, because the pentamirror has more loss of light than the pentaprism, does that necessarily mean photo quality is better with the prism? – Philip Trepanier Jun 28 at 12:18
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    @BrandonDube: Having torn apart a few cameras, I can tell you with certainty that at least some do have mirror coatings on the outside. It may not do a whole lot, but it's definitely there in at least some cases. – Jerry Coffin Jun 28 at 13:53
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    @PhilipTrepanier: No. The pentaprism/mirror in only in the light path from the lens to the viewfinder, not in the path from the lens to the film/sensor. It has no direct effect on picture quality (as opposed to indirect effects, such as framing the picture better if you can see it better). – Jerry Coffin Jun 28 at 18:05
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    @PhilipTrepanier: the thing that is better with a pentaprism is the viewfinder. Depending on how much you care about good viewfinders (I am a viewfinder obsessive...) this may matter more or less to you. However pentaprisms are generally associated with higher-end cameras which may also have better image quality than cheaper ones, so to that extent they do correlate with better image quality, perhaps. – user82065 Jun 28 at 20:23
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And for a visual:

Pentamirror

Pentamirror

Pentaprism

Pentaprism

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    +1 - Pentaprism is better, but your diagrams make pentamirror seem way cooler ;-) – Drew Jan 18 '12 at 8:38
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    Is there a reason why they have different shapes? – clabacchio Oct 16 '14 at 9:00
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    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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    This doesn't really answer the question. And the diagrams from Wikipedia need attribution :( – mattdm Nov 6 at 13:32

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