Stopped down focus

Discussion in 'Digital SLR' started by AaronW, Jun 23, 2006.

  1. AaronW

    ben brugman Guest

    Because in the 'IR assist' mode it is only used as a light source with
    a pattern build in. Although light is neccesary for the focus system
    the 'IR assist' is not an essential part of the workings of the focus
    system.

    The method I described uses IR as an integral part of the focussing.
    The IR was triangulated, killing the IR beam would disable the
    focussing system of those camera's.

     
    ben brugman, Jul 7, 2006
    #21
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  2. AaronW

    ben brugman Guest

    It's not easily. Most sensors are not capable of detecting an angle.
    Although triangulation is done, this is not done by sensors which
    detect an angle, but with a row of sensors, where the detection is
    done by registring on which of the sensors the centre of the beam
    falls and working out the angle.
    Or on older camera's the beam of the camera sweeps, the sweeping
    mechanism is connected to the focus mechanism. The sweep
    is stopped when the sensor detects the beam.
    600 nm is not 6m.
    A nanometer 10-9 meter, or 0.000000001 meter

    Detection of this scale is difficult, and not usefull for AF.

    Ben
     
    ben brugman, Jul 7, 2006
    #22
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  3. AaronW

    ben brugman Guest

    It was done by triangulation. There are no cheap commercial
    detectors available which receive enough light to detect a signal
    with an accuracy of only nanoseconds. The sensors are just not
    sensitive enough to detect light in such a short time.
    At the time there where no cheap commercial electronics to work
    fast enough to do the timing. So the method described of timing
    light was (and still is) not feasable for consumer products.
    Look at the price of a laser gun ten years ago to get an estimate
    of how much that technologie would cost.
    (The lasergun emits so much laserlight (not just ir) that this
    technologie would not be good for the health of the persons
    being taken pictures of, if this technologie would be used).

    ben
     
    ben brugman, Jul 7, 2006
    #23
  4. AaronW

    ben brugman Guest

    Speedometercamera's with lasergun technologie use that technologie.
     
    ben brugman, Jul 7, 2006
    #24
  5. Ben,
    Thanks. I thought so.
    But does a police laser gun work like this? I'd have thought it would
    use the doppler shift to measure speed (actually I'm sure). On the
    other hand, I know that there are laser guns that measure distance
    (used eg by the artillery), so these presumably do work by measuring
    the elapsed time. Do you know if this is so?
     
    achilleaslazarides, Jul 7, 2006
    #25
  6. Well, this is what I said. Also, what I wrote is "any integer multiple
    of 600nm (eg 6m)". Well, 6m is an integer multiple of 6nm (as is 6km,
    for that matter). And the reason I give for not being able to use
    interference between two beams for distance measuring at this scale is
    valid.

    You are actually the third person in this thread to "explain" to me
    that 1nm=0.000000001m (when reading what I wrote would indicate that I
    do know this, I would think); I imagine it will make continuation of
    any discussion rather difficult if we all start doubting if the others
    have any clue what they are talking about.

    Anyway, obviously nobody here reads carefully what the others say
    before answering, so the whole thing is a waste of time for all of us.
     
    achilleaslazarides, Jul 7, 2006
    #26
  7. AaronW

    J. Clarke Guest

    I think you'll find that they use doppler, not interval--they don't care how
    far away the subject is, just how fast it is going.
     
    J. Clarke, Jul 7, 2006
    #27
  8. AaronW

    acl Guest

    I thought so too, but it seems that the laser guns (as opposed to radar
    guns) simply measure the distance (by measuring the elapsed time) many
    times.
     
    acl, Jul 7, 2006
    #28
  9. AaronW

    ben brugman Guest

    I stand corrected, I suppose that the doppler shift is the method used.
    ben
     
    ben brugman, Jul 7, 2006
    #29
  10. AaronW

    ben brugman Guest

    I think you are correct and that I was wrong.
    Ben
     
    ben brugman, Jul 7, 2006
    #30
  11. AaronW

    ben brugman Guest

    If I was the third person to make the same "explaination" than you
    probably were not very clear on the matter.

    You also wrote
    "1) two sensors, one emits, the other detects; given the reception
    angle, you know the distance"

    Sensors doe not emit, that is not there function, combined with the
    fact that this is an English language newsgroup, I assumed (wrongly)
    that you didn't understand the nanometer, because of being part
    of the metric system. (If Nasa can make that mistake, most Americans
    can make that mistake).
    I did read carefully, but because not information was supplied I made
    some wrong assumtions.

    So sorry for that.

    ben
     
    ben brugman, Jul 7, 2006
    #31
  12. AaronW

    acl Guest

    No, it seems that, for laser guns, it is not doppler (it's doppler for
    radar guns). I remember the BBC ran a program where they demonstrated
    how such a gun can result in a nonzero reading off a wall (the user
    pulled the triger and scanned along the wall). Had it worked using the
    doppler effect, it would have returned zero; it didn't because it works
    by measuring the distance many times. So you were correct.
     
    acl, Jul 7, 2006
    #32
  13. AaronW

    acl Guest

    Maybe I was and maybe not. I don't really see how I can "unclearly" say
    eg "seven nanometres", but what do I know? Anyway, all sorts of
    miscommunications may occur with a text-only medium.
     
    acl, Jul 7, 2006
    #33
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