Discussion in 'Photography' started by Alan Browne, Mar 16, 2013.

  1. Actually, it's worse. That calculation of yours is only 100%
    true for infinite distances (and close at longer distances).

    An extension tube means you're getting really close ... as
    otherwise you can't focus. Then the effective aperture is
    smaller (depending on the magnification).

    Similar with the crop factor: it's only true for long distances;
    at close distances the crop factor fades away.

    At least that's how I understand it.

    Wolfgang Weisselberg, Apr 12, 2013
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  2. That was an interesting link. The contributors to that thread were all
    over the place. The guy that made most sense was the one who threw a
    magic ruler into shot. (The ruler from Calumet that also spat out
    exposure adjustment)

    It's pretty close to my use of tubes. Mine are simple plastic things
    with no wires or contacts (Fotodiox Macro Extension Tube Set). Cheap as
    chips (without the chips). Slap a stack of 'em between lens and body
    and set everything to manual -- a bit tricky for setting aperture with
    an EF lens if you don't want it wide open. It kinda works for some
    subjects. The folks with super macro lenses and ring flash things are
    in a different league, but at least I have money left for booze.

    The real fun starts with focus stacking in Photoshop. One day I will
    get it right without forking over untold money for a modern version
    that does it automagically.
    Elliott Roper, Apr 12, 2013
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  3. Alan Browne

    Alan Browne Guest

    I noticed that with some admiration. Reminds me of the RZ67 focus
    exposure adjustments.
    It ain't that automagically, just less completely manual - though there
    may be some plugins ($) that do it better.
    Alan Browne, Apr 13, 2013
  4. Alan Browne

    Eric Stevens Guest

    Umm - what is changed is not the focal length but the image distance.
    The focal length is the image distance when the lens is focused at
    infinity and remains constant.
    Eric Stevens, Apr 13, 2013
  5. Alan Browne

    Alan Browne Guest

    Sorted out about 6 hours before you posted.
    Alan Browne, Apr 13, 2013
  6. Alan Browne

    Eric Stevens Guest

    So I now see.
    Eric Stevens, Apr 14, 2013
  7. Alan Browne

    Eric Stevens Guest

    Focal length is defined as the distance behind the lens at which an
    image at infinite distance is focussed. This is a an invariant
    characteristic of the lens and does not change (unless its a zoom).
    This is assigned the symbol 'F'.

    Most people take photographs of objects at distances less than
    infinity. The distance from the lens to the object is known as the
    object distance. Call this 'A'.

    The distance at which the image of the object is focussed behind the
    lens is known as the image distance. Call this 'B'. This is always
    larger than 'F'.

    The image and object distances, and the focal length are related by
    the equation:

    1 1 1
    - = - + -
    F A B

    If you are adding physical space, you are adding it to 'B' i.e. you
    are moving the lens out and making 'B' larger. It follows from the
    equation that you are making 'A' smaller at the same time i.e. you
    obtain focus when the lens is closer to the object. But the point is
    'F' does not change.
    Eric Stevens, Apr 20, 2013
  8. Alan Browne

    Eric Stevens Guest


    Parallel rays are generated by an object at infinity. When you focus a
    lens at an object at infinity the rays from that object to the lens
    are parallel (until they hit the lens).
    Eric Stevens, Apr 20, 2013
  9. Alan Browne

    Eric Stevens Guest

    Eric Stevens, Apr 22, 2013
  10. Alan Browne

    Eric Stevens Guest

    I can't help you with that one. I didn't write it and I can't find
    where it was used. Can you explain further?
    Eric Stevens, Apr 22, 2013
  11. Alan Browne

    Eric Stevens Guest

    It would if the 6 feet wide object was an infinite distance away.
    Eric Stevens, Apr 22, 2013
  12. Alan Browne

    Trevor Guest

    Actually no, the light would be warped by the gravitational field of every
    star system between the object and the lens! :)

    Trevor, Apr 22, 2013
  13. Alan Browne

    Eric Stevens Guest

    If you want to complicate matters even further, you should consider
    the effects of 'dark matter'. :-(>
    Eric Stevens, Apr 22, 2013
  14. Alan Browne

    Wally Guest

    The rays are not parallel, but are nearly so... and behave effectively
    like parallel rays when they hit the lens.

    Here is a little math exercise for you: calculate the difference in
    angle for the rays hitting the top and bottom of the lens. (Make
    assumptions as necessary.)

    Wally, Apr 23, 2013
  15. Alan Browne

    PeterN Guest

    If you want to get theoretical, the gravitational influence of randomly
    distributed objects might very well equalize each other. Therefore the
    rays would remain parallel.
    But, I think the notion of absolutely parallel lines wound run counter
    the the theory of relativity.
    PeterN, Apr 24, 2013
  16. Wolfgang Weisselberg, Apr 29, 2013
  17. Actually, nope. It's the same as adding multiple random noise to
    a signal or a random walk from a point.

    Wolfgang Weisselberg, Apr 29, 2013
  18. Of course it can. Since the view angle of the lens is greater
    than zero, the object will record only at a single location.

    Hint: Stars are much, much larger than our planet Earth, yet
    they come as single dots. And they're not even infinitely
    far away. But their rays are *very* *very* *very* close to
    perfectly parallel when recorded by your lens.
    You only need an object-space telecentric lens. Which camera
    lenses aren't.

    Wolfgang Weisselberg, Apr 29, 2013
  19. Alan Browne

    PeterN Guest

    vielleicht etwas zu tun <> etwas tun
    PeterN, Apr 30, 2013
  20. True but irrelevant.

    Getting theoretical: The theory how random influences add is
    well established. That they equalize each other is about as
    likely as that they completely add to each other, i.e. both
    bowing the light in the identical direction. That's both not
    "might very well", unless you "might very well" win a million
    dollars in the lottery ten times in a row. So I bow to your
    immense luck and admit I was wrong.

    Wolfgang Weisselberg, May 2, 2013
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