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Why the Mirror?

Discussion in 'General Technical Discussion' started by Samer, May 24, 2007.

  1. Samer


    Sep 19, 2006
    Jupiter, FL
    A collegue of mine asked a simple question; why does a digital SLR need a mirror?

    I kinda gave him some arm waving guess about not wanting the sensor to be on all the time, a friend suggested the battery draining if you put a display in front of the viewfinder to show what the sensor sees.

    The collegue's theory is that the manufacturers didn't want to redesign anything so they just replaced the film with a sensor and kept all the old hardware. I think it's been a long enough time that the current mirror setup must still be superior.

    Does anyone know the real technical advantages of the current mirror setup besides battery savings?
  2. jaymc

    jaymc Guest

    The 3 main advantages of "through the lens" (ttl) are:
    1 - you get a big, bright image that represents what you'll see in the final picture.
    2 - no squinting to compose your image on an LCD viewfinder that washes out in bright sunlight.
    3 - much easier to see if in sharp focus.
  3. Apart from the valid point of battery drain, those three points are the main reason why I do NOT want them to change their design.
    (You may tell your collegue so.)

    I may add another point:
    SLRs in contrast to P&S's with just their main sensor allow for autofocus measurement using "phase detection". This allows the AF system not only to determine whether the subject is sharp or not, but also in which direction to move the lens (give there is at least some contrast in the scene to start with). This is extremely useful for focus tracking.

    Nikon has an article describing that here: http://www.nikon.co.jp/main/eng/portfolio/about/technology/nikon_technology/caf/index.htm
  4. Chris101


    Feb 2, 2005
    They have cameras that have electronic viewfinders like this one that do not have mirrors. A lot of us feel that the slight lag between the picture on the evf and reality is a disadvantage when shooting action. So we prefer the live view of an optical (not electronic) viewfinder. To view through the lens optically requires a mirror.
  5. Also, when you see through the same lens as the sensor. what you see is what you get (WYSIWYG). In non SLR and TTL when you see through a different viewfinder, you get parallax problems with close-up subjects. The mirror redirects the view to you. When lifted lets light go directly to the sensor (or film).

    Last edited by a moderator: May 25, 2007
  6. Alex, this is correct, but I thought he was referring to digital P&S's which are TTL also.
  7. His question starts: "A colleague of mine asked a simple question; why does a digital SLR need a mirror?"

    I understood SLR. Maybe you are correct on TTL P&S. Ok.
  8. Samer


    Sep 19, 2006
    Jupiter, FL
    Yeah, the WSYWIG is not an issue when using the sensor information for the viewfinder display. So I don't think that would be a valid argument.

    Thanks for the replies. I agree that the clarity and quality of the actual light must always be better than an attempt to recreate it.

    To clarify, I am perfectly pleased with my SLR, I just got a bit stumped by the question.
  9. Yet your colleague asked a quite legitimate question that, I think at least, camera manufacturers are trying to deal with.

    The display from the EVF is not yet the same as the optical display, but one day it will be the same. At that point, why bother with the mirror.

    Some of the design of DSLR's deals with acculturation more than a strict engineering for the most efficent design approach. The CoolPix 900 (the tilt swivel one) was a radical rethinking, of camera design. What is surviving though, is the more 'traditional' design approach associated with the APS film size (cigarette pack size). Some of the features of these designs have been resisted in the DSLR space, although the recent Canon offering with 'Live View' may herald greater acceptance of EVF in the DSLR world.

    I loved the tilt swivel viewfinder on my C5700 and would warmly embrace it on a D200 or D2X, or any other Nikon DSLR. Lot'sa folks would sniff at it though.
  10. Yet another brilliant move on the part of Canon to further woo pros.

    The next time you see one of those crowd scenes with shooters holding their cameras over their heads shooting blind, you'll understand just what significant feature it is.
  11. Woody,

    I agree with you completely. I've never understood why some would 'sniff' at it.
  12. photoshooter

    photoshooter Guest

    I agree, but there are other advantages as well, you can tether the camera to a computer, with live view, the computer is your viewfinder.
    When the remote is available, it will help you accomplish the same thing.
    With the software they supply, you can even focus with the live view, in addition to all the other things you can do.
    Nikon has Capture control pro, it is a great tool, but the addition of being able to see the shot.
    Well just think of the possibilities,, when using remote cameras, awesome for the sportsshooter.

    Nikon are you reading this.
  13. Photoshooter,

    As I understand the original poster, the question way why a mirror. I agree that tethering to computer is great, but at the same time, when I'm shooting at the Santa Claus Parade in Toronto, powering up a notebook is far less useful than a tilt swivel LCD or EVF. Live view, so that I can view the shot functionally is more powerful in many cases than a bright view finder.
  14. jcovert

    jcovert Guest

    Hey, looks like we have a little LiveView fan club going on in here! (of which I'm also a member).

    Maybe one day they'll make a TTL camera with no mirror, just the sensor moves into place. ;)  (an engineer I am not.)

    Here's a question I've always had...how do they get ANYTHING (like a mirror or shutter) to move at 1/2000 of a second in order to make exposures like that? How do they cut the light off that quick?
  15. So, on the wish list is: DSLR with 1. regular TTL View Finder , 2. LCD with dual function as 1. view finder <pivot/tilt possible > and 2. camera control display functions. The image sensor drives the LCD for: 1. view finder functions as well as 2. capturing the image. This way I would have the choice between 1. instant slr action and 2. p/s behavior <good & bad>.
  16. gvk


    Jun 17, 2005
    Mystic, CT
    Mirrors and shutters do not have to move all that fast even for a shutter speed of 1/8000 and a rate of 5 to 8 fps. The speeds involved can be roughly estimated from the camera's specifications. Nikon specifies the shutter lag at 37 ms for a D2X and 50 ms for a D200. A significant part of that delay, probably more than about 30 ms, is the time it takes for the mirror to get out of the way. For a 5 or 8 fps frame rate a picture is taken every 200 or 125 ms. The mirror must also return, and still leave some interval between shots when the viewfinder not blacked out. Mirror blackout is quoted as 80 ms for the D2X and 105 ms for the D200. Assuming 30 ms for the mirror to rotate 45 degrees, and a mirror height of 30 mm (measured on my old F2 film SLR, the D2X mirror is a bit smaller, around 25 mm in height) gives an average velocity for the mirror's edge of about 0.8 m/s. Allowing for acceleration, deceleration and damping, the peak velocity is probably around 1 to 1.5 m/s, no more than a fast walking pace.

    Also, remember that SLRs and DSLRs use a focal plane shutter consisting of two blades or curtains that at higher shutter speeds only expose a narrow strip of the film or sensor at any given moment. At the highest flash sync speed (1/250, or 4 ms for an F6, D2X or D200), the entire shutter must be open for the millisecond or so of the flash duration. Allowing for some tolerance, this leaves about 2.5 ms for each curtain to traverse the frame. The curtains on Nikon's recent shutters move vertically. So for film each shutter curtain has to move 24 mm in 2.5 ms, or about 10 m/s. A DSLR shutter could be about 2/3 this speed due to the smaller DX sensor. This about an order of magnitude faster than the mirror moves, but thin shutter blades have a lot less mass, and thus are more easily accelerated and stopped. For higher shutter speeds, the curtains do not move faster, however, the rear curtain follows the front more closely resulting in a narrower exposed strip.
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