Meaning of a 2 strength close-up diopter for macro

Discussion in 'Digital SLR' started by paul, Jan 4, 2005.

  1. paul

    paul Guest

    I asked this in another thread but didn't get an answer. Specifically,
    I'm wondering what effect this would have on a 400mm zoom lense which is
    only capable of getting as close as 7.5 ft. I don't understand what they
    mean when they say "changes the closest focusing distance from infinity
    to 9.9" My current 200mm zoom will get up to 1.3 ft away so I assume
    this would end up having me a couple inches away. I don't know what
    exact impact to expect.
     
    paul, Jan 4, 2005
    #1
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  2. paul

    Bob Guest

    I just got a Hoya close up filter set, and it has +1, +2, and +4 element, and
    they can be added up... for anything up to +7.

    I use them on my Sigma zoom, since it can only focus from about 3 feet.

    I just did a test using the +2+4 combination, and if I zoom to 300mm on a
    subject 3 feet away with out the filters, I can then take the same picture from
    about 7" away zoomed back to 28mm, with the +6 filters. This gives some
    advantages...

    I posted the example on alt.binaries.photos.original under "Glasses for my
    camera".

    The focus range with the +6 is very small, just a few inches back and forth. You
    use the zoom to adjust the frame... but the camera MUST be about 7" from the
    subject.

    These filters are ONLY for macro work - you must take them off to take any other
    type picture. But they work very well indeed! ($80 is a good price!)
     
    Bob, Jan 4, 2005
    #2
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  3. paul

    mc Guest

    +2 means it has a focal length of 1/2 meter. This is a diopter unit
    (reciprocal focal length) as used in eyeglass prescriptions. It is very
    handy for calculations, as this example will show...

    You said your 400-mm lens focuses down to 7.5 feet without a close-up lens
    attached to it. (This is something you have to know from the lens
    specifications, of course; it has nothing to do with the 400-mm focal
    length.)

    7.5 feet is about 2 meters. We'll call it 2 meters from here on.

    The reciprocal of 2 meters is 0.5. The reciprocal of infinity is
    effectively 0.

    So your lens currently covers a distance range (measured in diopter units)
    of 0 to 0.5.

    The close-up attachment will add 2 to this, making its range (in the same
    units) be 2.0 to 2.5.

    It will then focus from 1/2 meter down to 1/2.5 meter (i.e., from 20 inches
    down to 16 inches).

    That's how close you can get.
     
    mc, Jan 13, 2005
    #3
  4. paul

    Bob Guest

    That's useful info - thanks. Do most lenses have the diopter info for them
    available? Or do you have to figure it yourself?
     
    Bob, Jan 13, 2005
    #4
  5. paul

    mc Guest

    You have to take the minimum focusing distance (given in the lens
    specifications) and convert it to diopters by convering it to meters and
    taking the reciprocal.

    I need to make a web page about this. This system of measurement is
    familiar to every eye doctor (which I am not) but photographers rarely
    encounter it.

    --
    Clear skies,

    Michael A. Covington
    Author, Astrophotography for the Amateur
    www.covingtoninnovations.com/astromenu.html
     
    mc, Jan 13, 2005
    #5
  6. paul

    Bob Guest

    I think I'll make a spread sheet of all my lenses and my +1 - +7 diopter kit
    for all the distances... right now I'm kind of guessing on which lens to pick
    out!

    Now, if I can just figure out the field of view... !
     
    Bob, Jan 13, 2005
    #6
  7. paul

    mc Guest

    Not too difficult, but more than I can express in a short newsgroup posting.
     
    mc, Jan 13, 2005
    #7
  8. paul

    Bob Guest

    Is there a spec for distance for field of view with macro? I guess I should take
    pictures of a yardstick in focus and get an idea that way...

    I really am only concerned with taking product shots in focus for my site.
     
    Bob, Jan 14, 2005
    #8
  9. paul

    mc Guest

    A direct experiment is the quickest way to find out.

    Field of view is basic trigonometry. Just pretend your lens (of focal
    length X) is a pinhole X distance from the film, and solve the triangles.

    Adding a +2 close-up lens (or whatever) keeps your lens the same distance
    away from the film, but lets you get an in-focus image of a nearer subject.
    That's the simplest way to approach the calculations.

    I've placed on my "to-do list" the making of a web page that explains all
    this in more detail...
     
    mc, Jan 14, 2005
    #9
  10. paul

    paul Guest

    So an out of focus shot at the distance calculated is going to be what
    you get, that's simple.
     
    paul, Jan 14, 2005
    #10
  11. paul

    mc Guest

     
    mc, Jan 15, 2005
    #11
  12. paul

    paul Guest

    Hmm, so not that simple? I thought I could figure the dioper strength as
    with your example for the lense that normally focuses to 7.5 feet, the
    +2 diopter lets it get to 16 inches so one could just hold that lens 16
    inches away & see the field of view (even though it's badly out of focus
    without the diopter. Is that too simple?

    I actually don't have that lens yet, I have a smaller slower 200mm that
    goes to about 16 inches without the diopter so the diopter on the fast
    7.5 foot lens would be the same as my current lens (I think).
     
    paul, Jan 16, 2005
    #12
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