What’s the camera mirror for?

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Experienced and enthusiastic photographers know what the mirror does in a DSLR. But as we are here to debunk terminology…


In a SLR (Single Lens Reflex) camera using film, and a DSLR (Digital Single Lens Reflex) using an image sensor instead of film, the image enters the lens and is bounced off the mirror, then bounced around a 5-sided glass prism called a pentaprism (named so because it has 5 sides) before projecting through the optical viewfinder. See the very simplified drawing.

mirror and pentaprism
OK, I know the pentaprism looks awfully complex – wouldn’t having a second mirror work? Well, no because the image would be upside down; the pentaprism reflects the image like a mirror but also flips the image so it is the right way up.
This arrangement of mirror and pentaprism is the reason why the camera is called a (Digital) Single Lens Reflex. Simply, the photographer sees exactly what is coming through the lens, hende the term TTL or ‘through the lens’. On cameras where the viewfinder lens is separate from the photographing lens (as was often the case with older compact cameras) there was some “parallax” errors where those two images were not exactly the same.

When the photographer presses the shutter release button (to take a photo), the mirror flips up and the shutter snaps open to expose the image onto the sensor. Yes – the viewfinder will go dark/blank when the mirror is up. Also, it is not possible to have a live LCD screen on the back of the camera (at the same time as retaining the optical view off the mirror) because the sensor does not see the image until the photo is taken.

There is a simple elegance to this – although it’s not a simple mechanism, and DSLRs have that familiar lump on the top housing the pentaprism.

Mirrorless cameras

Simply, these cameras do away with the mirror and the pentaprism. And the optical viewfinder is replaced by a small electronic screen in the viewfinder. All of the sudden the camera is lighter, small and mechanically simpler.
You can also have a live LCD screen on the back.

So, let’s go mirrorless!

Sure, for some cameras that is a great idea and many compact cameras do that (as do smartphones). But there are some advantages to retaining a mirror:

  • The optical viewfinder shows exactly what will be photographed – and not an electronic representation at a lower resolution, and thus far has not been truly representative of the image.
  • Less power is needed because the main sensor is not “on”, so extending battery life.
  • The sensor is dark until exposed to the image. This probably leads to crisper images than if the sensor was permanently lit.
  • You can see the image when the camera is powered off, and there is no lag or contrast/brightness adjustments going in as in an electronic viewfinder.


On the positive side, mirrorless cameras offer a lot, such as:

  • The mirrorless cameras are smaller and lighter although the weight factor may be offset by needing bigger batteries and they tend to consume more power.
  • Under certain condition the electronic viewfinder is way better – such as low light shooting where an optical viewfinder would be too dark.
  • It is also true that pointing a mirrored SLR at the sun can be very, very bad for the human eye and a mirrorless design provides separation.
  • Active information about the image can be displayed in the viewfinder, such as exposure, focus points, and this may also help tracking of moving objects.
    This is less camera judder and they are quieter (if you turn off all the annoying bleeping that accompanies every action!).
  • They will be easier to clean and fix.
  • They are cheaper to make.


At the precise moment in time, I would probably buy a “traditional” mirrored camera. Most mirrorless cameras do not “look and feel” like and SLR, and seasoned photographers might be reluctant to take that route just yet. Although offerings like the CANON EOS M5 and Fujifilm X-T2 hit the mark in that respect.
But the manufacturers are working hard on mirrorless and will make leaps and bounds. I can predict I will (OK, probably) migrate to mirrorless DSLR. As a Sir Humphrey might say “when the time is right”.

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