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How things work - AF and Shutters

Discussion in 'Olympus Cameras' started by marcr1230, Jan 24, 2014.

  1. marcr1230

    marcr1230 Mu-43 Regular

    149
    Nov 28, 2013
    I'm curious if anyone can explain AF in mirror less vs DSLR

    My understanding is that with olympus E-M1, there are dedicated
    AF pixels on the sensor

    I assume for contrast based AF - you can read these pixels quickly
    The more "grey" there is the less focused, the more of high and low values
    Found - the better the focus, when the lens changes focus, the fewer the mid values

    When no better result is found - you are in focus
    Is this about right?

    Now - another related question - why do we need a mechanical shutter?
    While the camera is focusing, the sensor is exposed. If we can read pixels in real time for focus, can't we electronically flush the sensor, start a clock , and read the sensor after a brief interval

    Is the shutter needed to start the sensor from "empty" and close the door after exposure while the sensor is read and the values recorded?

    I hope this makes sense

    For DSLR is the AF sensor in the prism? Or behind the mirror (partially silvered?)

    Once the DSLR AF sensor determines best focus - the shutter is opened and closed etc and the picture recorded

    Is focus faster on DSLRs because they have a dedicated AF sensor instead of a dual purpose image+AF sensor?

    Is the theoretically more accurate focus possible on mirrorless due to the focus being read on the same sensor that will record the image? I suppose it's possible that a dedicated AF sensor is slightly off in optical distance and doesn't precisely match the picture recording sensor

    Anyway I hope I was clear - if anyone has time and patience to explain and correct,
    Much appreciated
     
  2. marcr1230

    marcr1230 Mu-43 Regular

    149
    Nov 28, 2013
    I found a good explanation about CDAF -
    http://photographylife.com/how-phase-detection-autofocus-works

    Seems mostly like I thought
    Not sure though why if you can have PDAF pixels
    Built into the image sensor - why can't you theoretically focus just as fast as a DSLR

    Interesting to note that PDAF is similar to rangefinder focusing and the system
    Can know when it is back or front focused - that's primarily why it's faster than CDAF

    Now to look for the answer to the shutter question
     
  3. speedandstyle

    speedandstyle Mu-43 Hall of Famer

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