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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?
Jay Lance Photography
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3 Answers3

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The basic difference is that a pentaprism is a solid block of glass. This means it's fairly heavy. Light enters one side, gets reflected around inside, and then comes out one other 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.

Jerry Coffin
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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
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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 '19 at 13:53
  • 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 '19 at 12:18
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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 '19 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. –  Jun 28 '19 at 20:23
  • "Lighter" might actually be marketing BS to upsell the lower production costs of pentamirrors - If you ever held a bare full frame pentaprism in your hand, they are not AS heavy as they are made up to be. – rackandboneman Nov 12 '20 at 21:18
  • @rackandboneman: all else being equal, a pentaprism will be heavier than a pentamirror. But it's certainly true that the difference isn't particularly huge. – Jerry Coffin Nov 13 '20 at 03:19
  • Yeah, but it's not like would be at risk to lose a K10D or SRT-101 to buoyancy in thick air if these had a pentamirror :) – rackandboneman Nov 14 '20 at 20:02
  • @rackandboneman: with a pentamirror, an SRT-101 would be restricted to anchoring yachts under 30 feet... – Jerry Coffin Nov 14 '20 at 21:29
  • The Mamiya C series TLR system included the pentamirror porrofinders in addition to Pentaprism finders. The weight difference at 6x6 was non-trivial on an already heavy camera. The mirror finder got a light meter earlier as well. The wide fixed apertures of typical viewing lenses made light loss less critical. And the fresnel standard view screen also helped. All for the sake of suggesting that high end camera systems have also used pentamirrors for high end camera reasons….and TLR cameras were already committed to high quality mirrors. – Bob Macaroni McStevens Jun 16 '21 at 22:58
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And for a visual:

Pentamirror

Pentamirror

Pentaprism

Pentaprism

Dai
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Nick Bedford
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    +1 - Pentaprism is better, but your diagrams make pentamirror seem way cooler ;-) – Drew Jan 18 '12 at 08:38
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    Is there a reason why they have different shapes? – clabacchio Oct 16 '14 at 09:00
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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. –  Jan 08 '15 at 07:01
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    This doesn't really answer the question. And the diagrams from Wikipedia need attribution :( – mattdm Nov 06 '19 at 13:32
  • This answer is misleading, because it shows a roof pentamirror and a regular (non-roof) pentaprism. All pentaprisms and pentamirrors in SLRs and DSLRs are roof-type. – scottbb Apr 03 '20 at 18:36
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an actual Pentaprism I have form an old SLR an actual Pentaprism I have form an old SLR

Actually, real Pentaprisms look more like the drawing of the Pentamirror shown above.

The Pentaprism drawing is just a geometrical illustration to show the light path and how it is reflected.

These are photos of an actual Pentaprism I have form an old SLR, (click for full size)

uhoh
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Ibra Spanish
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  • I've edited this answer to include both images. Once the edit is approved the other answer can be deleted. – uhoh Apr 02 '20 at 09:30
  • Next question; why are pentaprims pink? – uhoh Apr 02 '20 at 09:31
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    Sorry about the image quality. It is not PINK, it is white glass. The reason it looks pink is due to the lighting in the room and a photograph taken with mobile phone (cell phone) without any white balance correction (the reason why setting white balance is soooo important for JPG images). With RAW images you can fix that later on – Ibra Spanish Apr 03 '20 at 17:53
  • You can fix white balance problems without RAW, you just need the proper software. – Mark Ransom Jun 16 '21 at 03:55
  • Mark, I wish, but that's not the case. See this answer for detailed information. Essentially WB done on raw file is pre-processing that operates on plenty of data. WB on JPEG is post-processing with stripped (and essentially different) data. The implications are described in link. – Krzysiu Sep 06 '21 at 18:25