Pentaprism vs Pentamirror – Which is Better?

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The pentaprism is located above the reflex mirror in a DSLR camera; immediately in front of the viewfinder.

A pentamirror is cheaper and lighter — a pentaprism is more expensive. Does that mean that a pentaprism is better? Is it worth the extra cost?

On this page I’ll discuss:

  1. What pentaprisms and pentamirrors do
  2. How are pentaprisms and pentamirrors different from each other
  3. There’s a video of me taking apart two SLR cameras and removing the pentaprism and pentamirror
  4. Is a pentaprism better?
  5. Answers to Questions Sent-In by Readers

Pentaprism vs Pentamirror — The Movie!

Would you like to see exactly what a pentaprism looks like? And how it’s different to a pentamirror?

In this video I take apart two cameras. One has a pentaprism, the other a pentamirror. You get to see what they are made of and how they are different.

Watch this video

UPDATE: After making this video, I weighed them again using the grams setting on the scales to make the measurement more accurate. The pentaprism is 43 grams. The pentamirror is 13 grams. When we take in to account that, on the model shown in the video, the pentamirror also incorporates the focusing screen and viewfinder eyepiece, the pentamirror is around ¼ of the weight of the pentamirror.
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What Do They Do?

In the context of photography: pentaprisms and pentamirrors are normally found in the viewfinder system of SLR and DSLR cameras.

Pentamirrors and pentaprisms perform exactly the same job. Their design reflects the light coming through the lens in such a way as to ensure that the image we see through our camera’s viewfinder is the right way around — both up-and-down and left-to-right.

The way they do this is almost identical. They are very similar in shape. Some of their surfaces are mirrored (coated in a special compound that turns the surfaces in to mirrors) to reflect light in a particular way.

For Your Information
The word penta- comes from an Ancient Greek word that means five. In our case it refers to there being five sides to the prism (or mirror assembly) when viewed from a certain angle

How are Pentaprisms and Pentamirrors Different?

As you can see in the video above pentaprisms are made from high quality glass and pentamirrors are hollow constructions with some of the walls having reflective surfaces.

Pentamirrors are cheaper to make as they can be made from thin walls of plastic. Due to the extra cost in manufacture, pentaprisms tend to be found in higher end cameras. Putting a pentaprism in to a very cheap camera would probably add too much to the selling price and reduce its commercial viability.

For Your Information
We should really call a pentaprism a Roof Pentaprism because its shape is slightly different to that of a standard pentaprism.

Which is Better?

The image that you see in the viewfinder of a DSLR is a little darker in a camera that uses a pentamirror — making manual focusing harder in low light. If the design of the pentamirror and its housing is such that moisture and/or dust can enter the chamber (see the video), then the mirrored surfaces may degrade over time and dust could become a permanent annoyance.

That said, a pentamirror is lighter and cheaper. This makes the cameras that use them more affordable and lighter to carry.

It’s important to remember that the final photograph will not be affected at all by whether you have a pentaprism or pentamirror (unless your focusing is off!). So which is better depends: if you can’t afford a DSLR that has a pentaprism but can afford one that uses a pentamirror, you could argue a pentamirror is better. If you plan to use manual focusing a lot then you may find it easier to focus using a pentaprism especially in dim conditions.

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Reader’s Questions

Do you have a question? Scroll down to the comments section and type your question in there. We’ll be notified immediately and get to work on the answer. Also, other readers will see your question and may give you their own personal viewpoint.

Question: Which manufacturers use pentaprisms and which use pentamirrors?

The issue with a pentaprism and pentamirror is not that one manufacturer chooses to use them and another doesn’t. Camera designers can reduce the production costs of a model by choosing to use a pentamirror. So most manufacturers will choose to use a pentamirror in some of their models and pentaprisms in others.

Question: How will I know if the model I want uses a pentaprism or pentamirror?

Check the sales literature or manufacturer’s website. This information will almost certainly be found there.

Question: Can I tell by looking through the viewfinder?

Well, funnily enough, not necessarily! In theory a pentamirror is darker but design improve all the time and a good pentamirror may out perform a bad pentaprism. You’ll also be most likely to be looking through the viewfinder in a well lit store — where the differences are less obvious.

Question: The camera I like/can afford has a pentamirror. Should that put me off buying it?

No.

Personally I think it’s more important that a beginner photographer gets out and starts taking as many photographs as possible—as soon as possible. Regular practice contributes towards your ‘10,000 hours’ (as I discuss in this article) and is extremely important.

When I started taking photography seriously I didn’t have a lot of money. The camera I bought was much cheaper than those owned by most of my friends at high school. It was heavy and loud and had hardly any advanced features. Using it I got the highest grade in my photography class. During the next year or so I worked a part-time job to save for a better camera. I took that one to college with me. That camera had a pentaprism.

Your buying choice will depend on a whole bunch of factors. You have to decide which features are important to you. But don’t delay buying a camera just because you want to wait until you can afford one that that has a pentaprism. Get out and take photos. Lots of photos. Now!

Question: Does the choice affect the quality of the final image?

No. The pentaprism/pentamirror is only used while you are viewing the image. As soon as you press the shutter release the reflex mirror will change position and the viewfinder will go dark. At that moment neither the pentaprism nor pentamirror is being used. The light entering the camera will travel towards the light sensitive surface (either the digital image sensor or the film) and, while the shutter is open, the image will be captured. So which ‘penta’ you have has no effect on image quality because it’s not being used at that moment.

Question: Does a pentaprism work by Total Internal Reflection?

No! All of the surfaces in a pentaprism that need to reflect the image are mirrored. In order to work by Total Internal Reflection these surfaces would need to be above a critical angle which would be incompatible with building a compact pentaprism for use inside an SLR camera.

Do You Have a Question?

If your question hasn’t been answered in this article, please send it in so I can consider it for inclusion. Alternatively, leave a comment in the box at the bottom of this page so that other Photographer Insider Info viewers can give their opinion.

Something Important to Remember

There comes a point where the camera is less important than the photographer. When I first started out, some of my best images were taken using cheap equipment. Don’t let unimportant choices worry you too much. If you don’t have much money a camera with a pentamirror will take much better photos than not having a camera at all! Ultimately just buy the best camera you can afford and get out there taking photos. As this article explains, that is far more important to your photography than worrying too much about the exact design of your camera.

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This article’s thumbnail illustration is by wikipedia user Basilicofresco. Shared under Creative Commons License CC BY 3.0 (Last checked: 2013-02-13)

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