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Question on contrast detection AF

Discussion in 'Panasonic Cameras' started by agentlossing, Jul 10, 2013.

  1. agentlossing

    agentlossing Mu-43 Hall of Famer

    Jun 26, 2013
    Andrew Lossing
    So I have this notion about how contrast detection AF works, but I have no idea whether I'm correct. Does the camera body utilize the sensor data exclusively in determining focus? And with a live view sensor, does adjusting exposure settings actually affect the quality of light that enters the sensor, or just the result your LCD screen feeds back to you? I'm wondering whether, say, low-light focusing ability could actually be improved by brightening the exposure, thereby improving the sensor's ability to detect contrast. I doubt it, but I don't know these kind of systems as well as I do the older DSLR focus systems.
     
  2. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    The light getting to the sensor and thereby the CDAF system would be determined by the size of your aperture. If using a native lens then the aperture will be automatically regulated to reach optimal performance for the LCD. With a DSLR it was just left wide open until the time of shooting, but with a non-reflex camera sometimes wide open lets in too much light for the LiveView system to give a proper view, so the lens is stopped down by the camera to provide a better view. That is why moving a Micro Four-Thirds camera around and pointing it at light sources will give you that motorized sound from the camera constantly adjusting the aperture, depending on your lens.
     
  3. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Yes, the sensor data is used exclusively to determine focus. There are no other AF sensors in the body. (There are some cameras that combine both an in-body PDAF sensor and CDAF, but that's different.)

    As far as exposure, if you're using a native lens for the system, focus will normally happen at full aperture, so it doesn't matter what your exposure is set to. The shutter and aperture are fully open, so the sensor is getting the maximum amount of light possible.

    If you're using a manual or adapted lens, then AF may take place at shooting aperture. But that isn't as important with CDAF as with PDAF. PDAF AF systems won't focus below a designed in aperture limit because the aperture affects the angle of light rays hitting the sensor. CDAF sensors don't care about that. As long as there's enough light hitting the sensor, they can AF. And the latest systems can AF in very dim light, indeed.

    What may affect focusing accuracy is the picture mode set (e.g. vibrant vs. portrait vs. B&W and other such choices). There is conjecture that (at least some) cameras use the output of the JPEG conversion for focusing, and choosing a higher contrast setting will improve AF. I don't know that there is any hard evidence to support this.
     
  4. agentlossing

    agentlossing Mu-43 Hall of Famer

    Jun 26, 2013
    Andrew Lossing
    I guess my real question is, can increasing ISO or opening up the aperture actually affect how well the CDAF works, since the LCD is trying to show you an accurate representation of the scene under your current settings? Or is the camera showing you a modified feed that isn't what the sensor is actually seeing? Come to think of it, shutter speed is one setting that wouldn't directly affect the live view sensor, yet the LCD changes to represent the kind of image that would result. So maybe I just answered my own question :/
     
  5. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    I think I already answered the question. The AF happens off the sensor, not the LCD. Nothing that affects only the LCD image can affect AF. And the sensor always gets the maximum amount of light available, because the shutter and aperture are both completely open.

    So (1) ISO has no impact on the amount of light actually reaching the sensor, and therefore no impact on AF. And (2), the lens aperture is always wide open during AF (if you're using a native system lens), regardless of any other setting.