Calculating subject distance

Discussion in 'Digital SLR' started by Geoff, Mar 5, 2006.

  1. Geoff

    Geoff Guest

    Just a note to my fellow photographers. If you're ever taking a
    photograph of a helicopter and it comes so close you could almost touch
    it, cover up the camera and run! Don't do what I did yesterday and, like
    a total idiot, keep snapping the damn thing as it gets lower and lower,
    and drifts nearer and nearer to the car park. What was the reason I
    didn't even consider the down-wash until the thing was right over my
    head? It wasn't fun being directly beneath some large rotors that were
    that close. Luckily my lovely D70s didn't get injured, it was only my
    pride as I tried to shield myself, probably looking pretty stupid.

    Anyway, does anyone know of a technique to calculate how far the subject
    is away? I'd like to do this because I don't believe the helicopter
    should have been that low over a car park. I had my 300mm zoom set to
    maximum (don't ask why I did when the thing was so close - I think I was
    just panicking a bit so wasn't thinking straight! I have a nice shot of
    the rear seat passenger with his mouth open as if to say, "Oh my God
    there's someone there, get out the way!"). I guess maybe if I knew the
    size of the helicopter (I could guesstimate that) then I could look at
    the part of the helicopter in the frame, then aim the camera at
    something the same size on full zoom at a similar angle from me, then
    measure it's distance away? Is there a more scientific approach? I guess
    I could Google this one but I kinda wanted to mention what happened to
    you all so you can have a good laugh at my stupidity. :)

    Geoff, Mar 5, 2006
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  2. Geoff

    Alan Browne Guest

    Get your head out of the viewfinder. Helicopters can be very dangerous
    to people on the ground, and getting grit blown at you is only part of it.

    Alan Browne, Mar 5, 2006
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  3. Geoff

    Geoff Guest

    Nice photos Alan, particularly the red helicopter. I would never have
    stood under the helicopter intentionally but it did fairly quickly drift
    over towards me. Once I've got my nerve up to go back, I certainly won't
    be parking in that spot again. :)

    Geoff, Mar 5, 2006
  4. Particularly when they open up with the 20mm chain gun or shoot off a
    couple of Hellfires.


    "A combat photographer should be able to make you see the
    color of blood in black and white"

    David Douglas Duncan
    Speaking on why in Vietnam
    he worked only in black and white
    John A. Stovall, Mar 5, 2006
  5. SNIP
    It's not too hard to calculate. All you need is a known size of an
    object and the size of its image projected on the sensor (no.of pixels
    times sensel pitch). Multiply the Object/Image ratio (is the
    reciprocal of the magnification factor) with the focal length, and
    that's the distance. Assuming you don't need accuracy down to the
    inch, but rather to plus or minus a foot, you'll be close enough.

    Bart van der Wolf, Mar 5, 2006
  6. Geoff

    Jeremy Nixon Guest

    When trying to think of ways to answer the question "How far away from me
    is that helicopter?" is this really the first thing that comes to mind?
    Can you really not just look at the thing and know how far away it is?
    I guess I'm just a little confused as to how this could be difficult.
    Jeremy Nixon, Mar 5, 2006
  7. Geoff

    Geoff Guest

    When there is a noisy helicopter right above your head, with incredible
    wind being generated by the rotors which is about to hit you, it's not
    too easy to think about how far away it is. It just happened once and
    caught me off-guard and scared the cr*p out of me, and at the time the
    last thought I had was how far away it was. I just knew it was damn
    close. lol.
    Geoff, Mar 5, 2006
  8. i hope he does not drive a car...
    bob crownfield, Mar 6, 2006
  9. Geoff

    Geoff Guest

    Hmmm....He does drive a car. See my previous post.
    Geoff, Mar 6, 2006
  10. Geoff

    wilt Guest

    Now I understand the fools who must drive at 70-80 mph only one car
    length behind a car (or worse, semi tractor-trailer) on the highway!
    They are not fools, they must be depth perception challenged!
    wilt, Mar 6, 2006
  11. has someone taught him to tell how far away the other cars are?
    bob crownfield, Mar 6, 2006
  12. Geoff

    JPS Guest

    In message <>,
    Well, they did it once before, and nothing happened, so obviously it is
    not dangerous!
    JPS, Mar 6, 2006
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