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DSLR diehards.

Discussion in 'AP Magazine Feedback & Suggestions' started by Learning, Sep 8, 2021.

  1. EightBitTony

    EightBitTony Well-Known Member

    Maybe, I only know how the Canon's work :)
  2. Learning

    Learning Ethelred the Ill-Named

    It is not closed loop because a DSLR confirms focus at the focussing unit plane not the image plane.
    EightBitTony likes this.
  3. Petrochemist

    Petrochemist Well-Known Member

    Some of the lenses I've used with out electronics have been screw drive AF ones with very limited recognition only contacts (PKA variants, which have electrical contacts but no electronics instead just using conductivity with the lens body to encode max aperture etc.) Others have been focused via an autofocus teleconverter where the teleconverter has a screw drive AF but most of the lenses used with it being mechanilcal only models that predate even screw drive AF.

    In addition there's the Techart pro - where the adapter has electronics itself but the various lenses connected to it often don't - but that's only on mirrorless so perhaps not relevant.

    I believe the discussion as a whole was not just about AF micro adjustment but about mirrorlesss/dslr focusing & cameras in general, but even if not the differences in manual focusing can highlight the different requirements for AF.
    Mirrorless cameras use the main sensor for focusing while DSLRs (when not in liveview) use either a separate AF system or the viewfinder ground glass both of which will be slightly out as manufacturing tolerances do not allow mass production to the necessary tolerances. The same manufacturing tolerances apply to lenses which is why SLRs can need adjustment for specific lenses. I've also never felt the need to apply AF adjustment on my DSLRs - perhaps because I've only used really fast lenses (f/1.4 & faster) on mirrorless.
    Last edited: Sep 19, 2021
  4. GeoffR

    GeoffR Well-Known Member

    It is a closed loop between the focus sensor and the lens, when the mirror moves the loop is opened and the focusing stops. Just because the loop doesn't include the image sensor doesn't stop it from being a closed loop.
    A DSLR focusing system doesn't need to know where the lens is either. All systems suffer from inertia meaning that once the focusing system starts to move it will continue to do so unless it is stopped either intentionally or by friction. However, I was unaware that Canon cameras ignored micro-adjustments in live view which implies that there must be some braking applied by the system to ensure accurate focusing. It also means that I have no idea for what error micro-adjustments/fine tuning actually corrects.
  5. Chester AP

    Chester AP Well-Known Member

    AdrianSadlier likes this.
  6. EightBitTony

    EightBitTony Well-Known Member

    You're really overthinking this.

    With a DSLR using the mirror, the focusing system focuses the lens using light from the mirror. Then the mirror lifts up, and the image falls onto the sensor. Any tolerance issues between the sensor position, the focusing system position and the maths involved means that while the focussing system will think the image is in focus, the image on the sensor will not be. Micro-adjustments allow you to tell the focussing system that it needs to forward or backward focus to match the position of the sensor.

    With a DSLR in live view, or a mirrorless camera, the image sensor is used to focus the image, so there can't be any tolerance differences in position when it goes to take the image. The image landing on the sensor from the lens is either in focus or not, for both the pixels used to achieve focus and the pixels used to record the image.
    IvorETower and Petrochemist like this.

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