How does Auto-Focus Work?

Discussion in 'General Technical Discussion' started by PhotoByMark, Jan 4, 2006.

  1. Ok - sounds silly I know but, it seems that different cameras use different techniques for performing auto-focus.

    For example, my D70s and Olympus 2100UZ can focus very well through glass (like a fish tank). But.... My friend has a Canon Powershot A95 (I think?) (This is not a Canon Bashing thread - OK?:smile: ) but with his camera it is very hard to get it to focus through the glass.

    The point is here, I guess different camera's use different techniques to perform auto-focus? Perhaps P&S cameras use a less expensive focus technique than DSLRs?

    I do not know much on the different techniques available for digital camera focusing mechanisms and if someone can shed some light what the various methods that are used for auto-focus and perhaps a work-around so he can use it to take fish/coral photographs.

    I am making any sense here?
     
  2. Mark, I don't know if this will answer your question. But this article from Nikon is a good read.
     
  3. Yeah I think that kind of answers part of the questions.

    Phase detection vs Contrast detection.

    I suspect his camera uses Contrast detection? I still do not understand why focusing through glass would throw off his auto-focus on his camera.

    Thanks for the link.
     
  4. Interesting article, Phil. Thanks for pointing it out.

    Virginia
    aka beaucamera
     
  5. So I am still baffled.

    Contrast Detection cannot focus through glass??
    Anyone know?
     
  6. I can only speculate. If the glass is very milky it will reduce potential contrast in the scene. This makes finding the peak of a contrast curve very difficult. It may essentially have a wide flat top. If the glass has much structure itself(smudges, thickness,etc), you focus or attempt to focus on the glass itself. If the glass is inside the minimum range that your lens can focus, it may cause a very blurry image on the sensor and radically reduce overall contrast.
     
  7. It truly depends on how much light is being reflected by the glass as opposed to how much light is passing through the glass that is being reflected by the intended subject.
     
  8. Well, huh. These are reef tanks tremendous amount of light - much brighter than an average room light. Perhaps some camera's use even a different technique. Like I said with 3 cameras present, D70s, Olympus 2100UZ and the Canon A95. The first two can focus no problemo but, the A95 does not like to focus through the glass on the tank. Again, this is not a Canon bashing post! I only mention the brand and model in case somebody can help me understand why that camera does not work as well as others through glass.

    Thx, for everyone's help.
     
  9. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Is he in AiAF mode? If camera is not parallel to the glass (and usually it is not), he should try FlexiZone AF/AE mode.
     
  10. Ahh, interesting. I was waiting for the "Master" to chime in here!

    I do not even know what this means exactly but, it is enough to try to learn LOL. I will do some serious "Googling" to learn more about what you just told me.

    So Illiah - Do P&S cameras use some other focusing mechanism than Phase or Contrast detection?
     
  11. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Phase detection is quite difficult with P&S cameras, as it needs to place some elements in optical path (image splitter). With mirror box it is much easier - there is a semi-transparent "window" in the main mirror and a secondary mirror under it to reflect light to the AF sensor array.

    Modern Nikons do it this way:
    http://www.mir.com.my/rb/photography/hardwares/classics/NikonF5/focusing/index.htm

    P&S cameras use contrast-based AF, mainly. Some use ultrasound and infrared "active" autofocus.
     
  12. Thanks for all the info.
     
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