What is the difference between a pentaprism and a pentamirror?

  • 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?
  • 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.

    There are no mirror coatings on a pentaprism, they work based on total internal reflection.

    So, because the pentamirror has more loss of light than the pentaprism, does that necessarily mean photo quality is better with the prism?

    @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.

    @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).

    @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.

    "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: all else being equal, a pentaprism will be heavier than a pentamirror. But it's certainly true that the difference isn't particularly huge.

    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: with a pentamirror, an SRT-101 would be restricted to anchoring yachts under 30 feet...

  • And for a visual:





    +1 - Pentaprism is better, but your diagrams make pentamirror seem way cooler ;-)

    Is there a reason why they have different shapes?

    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.

    This doesn't really answer the question. And the diagrams from Wikipedia need attribution :(

    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.

  • 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)

    I've edited this answer to include both images. Once the edit is approved the other answer can be deleted.

    Next question; why are pentaprims pink?

    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

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Content dated before 7/24/2021 11:53 AM