The nuances of the structure of the mirror in modern central control rooms

The general principle of operation of a SLR camera has not changed for a very long time - a beam of light passes through the lens, is reflected through a mirror, is projected onto frosted glass, flips over in a pentaprism and we see the image in an optical viewfinder. When we take a picture, the mirror rises and a beam of light hits the matrix or film, thus forming the image that we saw through the viewfinder.

The nuances of the structure of the mirror in modern central control rooms

The nuances of the structure of the mirror in modern central control rooms

But in fact, in modern digital mirror cameras (CZK), everything is much more complicated. Modern cameras have a number of additional devices that few people realize:

  1. Metering sensor exposurewhich is at the pentaprism
  2. Pentaprism can be replaced by a pentaser
  3. Between the frosted glass and the pentamirror is a special transparent LCD display - in more detail here
  4. In modern central control centers in fact two mirrors
  5. The second mirror is suspended behind the main
  6. The main mirror has a special translucent window
  7. Between the mirror and the matrix there is a camera shutter
  8. Many cameras have a diaphragm control system in the mirror shaft
  9. The luminous flux is deflected by the second mirror down towards the autofocus system
  10. When you release the shutter, the second mirror hides, it is impossible to see
  11. The focusing system uses a sophisticated beam re-reflection technique to detect phase difference

It would seem that everything is simple, but you see how many additional difficulties. The picture below shows the general scheme of work of a modern central control center - it is clearly seen that the camera has both main mirror '121a' and additional mirror '121b'.

Principle of sight

Principle of sight

Modern CZKs have a special focusing system. The special module, which is located at the bottom of the main mirror, is responsible for focusing. Focus module listed as '132 AF sensor'. The light beam that passes through the lens is partially reflected at an angle of 45 degrees and directed to the '126' pentaprism, and partially passes through a special translucent window in the main mirror. The beam that went through the window of the main mirror reflected in an additional mirror and goes to the auto focus sensors. The auto focus sensor passes the beam through its special lenses, detects the phases and adjusts the focus.

The course of the beam in modern CZK

The course of the beam in modern CZK

When the picture is taken, the mirror rises, an additional mirror is superimposed on the main one, so that it is not visible. In this case, a ray of light through the open shutter hits the camera’s matrix. The photo above shows beam motion pattern for camera Nikon D2h.

General view of the mirror

General view of the mirror. A translucent window is not visible.

I almost never met such an extended description of the movement of rays in the chamber, and after the breakdown of my D700 decided to write this article. The mirror shown here was taken from the camera. Nikon D700, you can easily consider the structure of the primary and secondary mirrors.

Translucent window

Translucent window

If you look at the mirror directly, as shown in the figure above, it is impossible to notice the second mirror. The figure below shows the mirrors on the side, the fastening mechanism of the second mirror and the second mirror itself are clearly visible.

Second mirror

The second mirror is fixed behind the main one using a special mechanism

The figure below shows the front and rear view of the mirrors. An additional mirror and a translucent window in the main mirror are clearly visible behind.

Kind of mirrors

Kind of mirrors

Separately, I will focus on the fact that when the mirror is raised, the additional mirror slams. The additional mirror at the back is covered in black plastic, like the rest of the main mirror. In the raised position, such a system does not transmit light that comes from the optical viewfinder in the opposite direction (actually skips).

Visa mirrors in a raised state

Rear view of the mirror in the raised state. The second mirror closes the translucent window. The whole mirror does not let light through.

Conclusions:

Modern CZKs have a slightly different principle of operation than the old film SLR cameras. Basically, the difference is in availability second mirrorthat participates in the focusing system and transparent LCD display to project visual effects.

Like  - helping the project. Thanks for attention. Arkady Shapoval.

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Comments: 59, on the topic: The nuances of the structure of the mirror in modern Central Control Commission

  • Alexey

    interesting information, it turns out to be much more complicated than I thought until today. Thank you for your work Arkady!

    • Arkady Shapoval

      It's my pleasure

  • Igor

    Hello,
    Thanks for the interesting article!
    The only IMHO when shooting is better to sharpen the entire detail, I want to consider not only its part

    • Arkady Shapoval

      ok, we’ll close the diaphragm harder.

  • Alexander

    Hello, Arkady, I sympathize with the breakdown of your D700, what happened to him?

    • Arkady Shapoval

      The mirror control mechanism has broken.

      • Alexander

        I hope that it will be possible to repair quickly and cheaply, creative success to you and thanks for the excellent articles

        • Arkady Shapoval

          Repair turned out to be expensive and very long.

      • Anatoly

        So it prompted you and prompted this review to be done.
        Cognitive, thank you.

        • Arkady Shapoval

          Yes, the bad also has a lot of good :)

  • Siberian

    Informative!
    “The secondary mirror in the back is covered in black plastic, like the rest of the primary mirror. In the raised position, such a system does not allow light coming from the optical viewfinder in the opposite direction. ” So stray light through the viewfinder is a myth? I mean that in the internet there are tips that when shooting with a tripod, especially at long exposures, it is better to cover the viewfinder.

    • Arkady Shapoval

      When shooting with a tripod, the exposure metering is incorrect, since the light from the eyepiece affects the measurement, therefore there is a special shutter for the eyepiece for this. My d700 has a special aperture for the eyepiece.

      • Siberian

        Logically, I didn’t think about exposure metering, since I almost always look at the eyepiece with a tripod and it turns out to be covered. Although of course the situations in the shooting are different.
        The tips for “closing the eyepiece” were all about long exposure exposure. In particular, this is often found in articles on infrared photography and shooting “frozen” water and sky.

    • Eugene

      all the same, in practice, parasitic light from the realm of myths.
      I explain why so with examples.

      when shooting handheld through the viewfinder (VI) it is clear that we valiantly obscure the light by ourselves.
      when shooting with a tripod during the day, the sun from the VI side - again we block the light with ourselves, the rest will not play a significant role.
      when shooting with a tripod during the day, the sun from the side of the lens - all the more light will not get into the VI (exception - you reflect sunlight in a snow-white T-shirt)
      shooting from a tripod in the evening and generally in low light - where the difference in shutter speed is measured in seconds, talking about the influence of light is generally ridiculous.
      I'm not talking about the night - exposure metering is generally useless, and shooting parameters can be set by eye +/- for a few minutes.

      Cases when I HAD to make the camera seriously make a mistake with shutter speed (at least three stops, initially the bright areas were irrevocably illuminated):
      - light reflection in the VI with foil and white glossy paper, a laminated badge (which turned out to be at hand)
      - LED flashlight purposefully shone in the VI (4 diodes with 10 cm)
      - shooting from a tripod, the lens is aimed at the ground, the weather is sunny (less critical case, lightroom extended the overall overexposure almost without loss)

      Conclusion - in the general case, the effect is microscopic, but it is better to protect it from the obviously strong VI light, although closing the shutter tightly is also too much.

  • VALENTINE

    I think that the CZK will die out like dinosaurs or mammoths, giving way to mirrorless mirrors with interchangeable optics. PENTAX K-01 is an example.

    • Alexander

      not sure about your words, since mirrorless cameras have a completely different focusing system, much less accurate and slower

      • Siberian

        Sony now has the SLT line. There are certainly many shortcomings, but technology does not stand still. I think this is a more promising direction for professional cameras, although for many NEX mirrorless enthusiasts they cover all needs.

        • Arkady Shapoval

          SLT is another matter.

          • Siberian

            I put my comment in a slightly wrong place :). In general, I wanted to say that the Central Concert Hall will not die out like mammoths, they mutate, yes, but they will not die out :). Although in 20-30 years ...

          • Arkady Shapoval

            We will see. Everything is changing very quickly.

      • Andrei

        A bit wrong - in mirrorless cameras, focusing is more accurate, there is no such insanity as back focus and front focus. But the focusing speed is lower today, but phase sensors are already built into the matrix, so the speed will be exactly the same

  • Aleks de Kairoy

    good article, Arkash, thanks.

    • Arkady Shapoval

      It's my pleasure :)

  • Kostya

    Dyakoy for a garnish article. I’m talking to Vipadkovo myself, having shown a Dodatkovo mirror and not knowing why.

  • Gene jb

    I think that over time, phase focusing will be built right into the matrix and there will be a mirrorless with phase focus. Yes, a lot of things can be built into the matrix. There are two-layer matrices with a wide DD. or foveon. I used it like a Canon sx20, so watching the TV through the viewfinder is not very pleasant.
    As for covering the viewfinder at long exposures - yes, my screen does not turn off and the exposure metering is lying. you need to either turn off the screen or close the viewfinder. but usually, of course, I shoot on M and it doesn't matter what the metering shows there.

  • Gene jb

    Wikipedia quote: Phase autofocus
    The principle of phase autofocus (patent).

    Phase autofocus is used in modern SLRs and some compact digital cameras. Its main element is special sensors, the number of which in professional models can reach several tens. Sensors in the DSLR receive fragments of the light flux from different points of the frame using special mirrors. Also, phase detection autofocus sensors can be built directly into the matrix of a digital camera (Fujifilm Hybrid Focus technology) [1].

    • Alexander

      Thanks for the information, I didn’t know this, but the plus to the DSLRs is their large size and weight, it’s easier to get rid of the shake

      • Gene jb

        vice versa harder. the mirror rattles when shooting.

  • levani

    on my d40 the mirror claps so much that it’s scary)))

  • Anna

    Thank you very much!

  • Paul

    Thank you so much for the information. I did not know how the focus occurs. I have a D300 and I used to think that to focus from a camera to an object, the distance is measured (by ultrasound or infrared rays) and then the motor (invert) rotates the focus on the lens to the desired value depending on the distance. And the principle of focusing in contrast, I can not even imagine how it happens.

  • Kiba

    Arkady, tell me, on what run did the mirror mechanism die?

  • Alex

    An additional advantage of DSLRs, in my opinion, is that the matrix in the normal mode is idle for most of the time, therefore it heats up less, and therefore the thermal component of the noise decreases.
    In the mirrorless matrix, the matrix works constantly.
    Given the pixel density of modern matrices, this can significantly degrade image quality, which, in turn, will require more sophisticated noise removal algorithms.

  • Anatoly

    Just a miracle article. Especially for me - a techie.

  • Vladislav

    "Modern CZK have a slightly different principle of operation than old film SLR cameras."

    Not very old film cameras with phase AF also have an additional mirror (for example, I have a Nikon F80 film camera) :-)

  • Alexander

    Arkady hello.
    How much I read about SLR and mirrorless cameras (I use NIKON D610). I can’t understand why in a modern camera MIRROR ?!
    After all, you can immediately display the image from the matrix in the viewfinder. Why is this mechanism needed?
    Or a fashion tribute to PROF level? Then in budgets why a mirror?

    • Jury

      They do just that in mirrorless cameras, only the viewfinder is no longer optical, but electronic, different focusing, energy consumption, etc. This is another class of devices that occupy their niche in the market. Help article https://radojuva.com.ua/2015/11/ovi-i-evi-ovf-and-evf/

      • Alexander

        Thank you Yuri

  • Alexey

    Hello, an additional mirror does not rise on Canon 1200D? What is the reason?

    • anonym

      Baba Vanga will be in touch soon ...

  • Dana Furniture

    Focus points and systems for Canon EOS cameras. Phase focusing system for modern digital SLR cameras. You can see about the structure of the mirror in the section 'Nuances of the structure of the mirror in modern CZK'.

  • Andrei

    I haven’t read all the comments, but I’ll tell you about the myth of the viewfinder, it’s enough to take apart this thing once to understand that it’s not how it cannot affect the metering)))) the sensors are installed so that they can catch backlight only if it is purposefully illuminated by a searchlight

  • Igor

    Arkady, good afternoon! I take this opportunity to design you as a professional with a mirror for experiments. I’m in the USA right now, I bought a D200 in perfect condition on ebay, it was sold as not working or for spare parts, it turned out there was simply no battery, but I had no time to get it, I shot 642 frames, I bought it for $ 53. The device was stored without a front cover and a mirror slightly dusty, does not affect the image in the viewfinder, nor does it affect the quality of the photo. The network has a lot of information about cleaning the sensor, but little and it is contradictory about the cleaning of the mirror. If it does not, tell me. how can I wipe the mirror and is it worth it? Sincerely, Igor

  • Konstantin

    I have a problem, back focus. Canon 650D. It seems from the moment of purchase. This shortcoming seemed to not immediately notice, I am an amateur and sinned on kit 18-55. And when gradually my requirements for photo quality began to increase, I bought a fix 50mm stm. Here Also, the sharpness was not perfect. I checked on the “target” and made sure of the lack of a camera. The warranty passed. The service, for just picking up the carcass, immediately invoices 3000 rubles plus repairs. I think my hands are growing from that place. I want fix this jamb myself. I haven’t decided yet where to go: move the phase sensors closer, or the matrix half a screw further. Who has experience, tell me.

    • Ivan

      With this approach, you will finally break the camera.

    • Arkady Shapoval

      Find another service, do not climb yourself

    • BB

      If your arms are straight and not scary, then you can. Personally, on Nikon D5100, I tightened the AF sensor adjusting screws (for me, it's not so bad as skewing the sensor). I twisted all three screws by the same number of revolutions, the first time there was a 'flight', I turned it back, it became good. I checked it with two lenses 50 / 1.4 and 58 / 2.0 (I took 50 / 1.4 from a friend) After this, everything worked well, back / front was not observed on other optics

      • Ivan

        You at least know that there is an autofocus sensor. Konstantin wrote about phase sensors in general.

        • BB

          Um, maybe I don’t understand something, but the AF module is the phase sensors (fixed, roughly speaking, on one plate that has the ability to 'adjust' the position). When I climbed into the camera, I also had only a general idea ... If a person wants to try, let him try at his own peril and risk. In principle, with straight hands to completely ditch the apparatus - it still needs to be tried.

          • Ivan

            Here's a typical DLC AF sensor (in this case, a Canon EOS 50D):

          • Ivan

            And this is what's inside (a single photosensitive design):

          • Ivan

            I can't say about Canon, but in Nikon's service manual, the AF sensor is not adjusted in its seat. Only the system of mirrors and the position of the matrix relative to the bayonet are regulated.

            • Michael

              The EOS300D was precisely regulated, even in the instructions like recommendations were. What's on the new ones - I don’t know

              • Ivan

                See the correspondence below, I have already admitted my mistake.

        • BB

          ... and the local 'service' (in a city of a million inhabitants) for adjusting the front / back focus then broke some bad sum, and without giving a guarantee that it would be better. Of. there is no Nikon service in the city. I send it to another city to clean the matrix altogether - even taking into account the delivery, it comes out cheaper than from the locals (and again from the locals without narantia, which will be better after cleaning).
          In Ukraine, I can recommend the 'Photoparts' service in Nikolaev - they do it efficiently, relatively quickly and at an adequate price. I repaired their camcorder and lens, cleaned the matrices of cameras.

      • Ivan

        And this is from the service manual for your D5100. It says to tighten three screws, but not hard, and then each

        • Ivan

          turn back two turns:

        • BB

          I no longer remember where I was twisting, but I did not completely twist, and I twisted all three the same way. The position of the sensor is just adjustable, and this is easier to do than twisting the eccentrics of the mirrors, and even more so 'knocking down' the plane of the matrix relative to the optical axis, there is much less chance of screwing up something

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English-version of this article https://radojuva.com/en/2012/12/second-mirror/