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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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2 Answers 2

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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@JayLancePhotography, please accept this answer. It clearly and completely answers the question. –  DragonLord May 10 '11 at 14:44
    
@DragonLord: Or you'll come burn down my house with your dragon-y fire breath? Lol. ;-) –  Jay Lance Photography May 11 '11 at 3:28
    
Of course not... LOL –  DragonLord May 11 '11 at 12:12
    
Why hasn't this been accepted yet? –  Andrew Heath Jan 18 '12 at 8:38
    
@AndrewHeath: at least according to his profile, Jay Lance hasn't been around here since last August. –  Jerry Coffin Jan 18 '12 at 10:09
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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 ;-) –  Andrew Heath Jan 18 '12 at 8:38
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