diopter question

Discussion in '35mm Cameras' started by Celeste G, Oct 17, 2003.

  1. Celeste G

    Celeste G Guest

    Did I understand that using a diopter on a 70-300 mm lens will reduce the
    minimum focal lenth (which is usually 5 ft)?? I often want to get in
    closer than the 5 ft. In some situations my macro will not work.

    Thanks
    Celeste
     
    Celeste G, Oct 17, 2003
    #1
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  2. Celeste G

    Tony Spadaro Guest

    It will reduce the minimum focus distance not the focal length. YOu will
    lose the ability to have infinity in focus while using a diopter. The best
    close up filters are the two element type. THe single element filters are
    essentially the same thing as a magnifying glass - with all the same
    distortions. THe two element ones are not perfect but are much better.
    You can also use extension tubes to get closer - infinity focus will
    still be lost but the picture will be as good as your lens can do as
    extension tubes have no glass.
    Another method is to use a teleconverter, which does not reduce minimum
    focus distance but increases focal length - giving the appearance of closer
    focus. At 600mm 5 feet is a pretty tight close up. THere is a quality loss
    with this as the TC has elements. I recommend only using top quality TCs and
    be prepared to accept some drop in quality.
     
    Tony Spadaro, Oct 18, 2003
    #2
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  3. ......And also a loss in speed.......
     
    William Graham, Oct 18, 2003
    #3
  4. Reducing focal length is exactly what supplementary close-up lenses do.
    (The exact formula IIRC depends on the distance between the
    supplementary and the front nodal plane of the prime lens.)

    Keeping the image distance constant and reducing overall focal length
    forces a reduction in object distance and an increase in magnification.
    Just because they have no glass does not mean there is no effect on
    quality. Lenses are optimised to work best at a particular set of
    object/image distances - usually with object distance somewhere around
    10-20x focal length - and anything away from this will be sub-optimum.
    How much the quality falls off is highly dependent on the design of the
    lens - in very general terms zooms deteriorate faster than fixed focal
    length lenses.

    If there were no quality fall off on moving "normal" lenses further from
    the film, there would be no demand for purpose-designed macro lenses -
    we would all just use extension tubes.
     
    David Littlewood, Oct 18, 2003
    #4
  5. Celeste G

    Don Stauffer Guest

    I take it you mean a closeup lens attachment. Diopter is merely the
    reciprocal of the focal length in meters (rather than mm). One can use
    the diopter of a lens (including the objective lens to ease
    calculations.

    But, assuming you mean a closeup or supplemental lens, then yes, a
    supplemental lens allows closer focus, and also slightly alters the
    focal length of lens.

    I also find I do not want to use Macro feature of my lens a lot of the
    time. My macro lens is only macro in telephoto position, and sometimes I
    want closeup with shorter focal length, so I have a good closeup
    supplemental lens system.
     
    Don Stauffer, Oct 18, 2003
    #5
  6. Celeste G

    Rudy Garcia Guest

    A closeup diopter lens will most definitely change to focal length of
    the system (the prime lens plus the supplementary).

    It will also, to a smaller extent, change the f-stop, from the marked
    f-stop to a usually larger f-stop. This is due to the fact that the
    mechanical aperture diameter does not change when adding the closeup
    diopter lens, while at the same time the focal length of the system has
    become smaller. This has a slight, but opposite effect on the DOF
    calculations, compared to using extension tubes.
     
    Rudy Garcia, Oct 18, 2003
    #6
  7. Celeste G

    Gordon Moat Guest

    A diopter on the front of the lens is not always the best solution. While a
    fractional diopter would work, those are usually special order items, and
    not really low cost. An easier solution would be to use a short extension
    tube. This is just an extender that moves the lens mounting outwards. A
    really short one would still allow you to focus near your 5 ft mark, though
    may not help the macro ability much.

    Depending upon your brand of camera, you should be able to find a selection
    of extension tubes. There are some third party companies that make these as
    well.

    Ciao!

    Gordon Moat
    Alliance Graphique Studio
    <http://www.allgstudio.com>
     
    Gordon Moat, Oct 19, 2003
    #7
  8. Celeste G

    C J D Guest

    Er, are you sure about that? AFAIK, the actual aperture as marked is correct for
    infinity, or when the lens is at its nominal focal length from the film plane.
    For close focussing, as in a macro lens, the effective aperture is modified by the
    amount of lens extension required to achieve focus, e.g. at 1:1 object-to-image
    size, the lens is at twice the nominal focal length from the film plane, and the
    effective aperture is two stops smaller than marked, e.g. f:8 becomes f:16.

    When you add a positive lens in front of camera lens, the focal length of the
    combination is shortened so the f-stop becomes larger than marked, but the lens is
    unusable at shorter than its nominal focal length since it cannot focus until the
    object is close enough for the lens at nominal infinity to render a sharp image -
    at which point its effective focal length is the same as its nominal focal length,
    so the stop is unchanged at that point. From there, if the lens is moved away
    from the film plane to achieve yet closer focus, the preceding paragraph's remarks
    apply.

    I hope that's not too anal,

    Colin D.
     
    C J D, Oct 19, 2003
    #8
  9. Celeste G

    Rudy Garcia Guest

    Yes, the aperture diameter remains the same, with or without the
    supplementary. Since the supplementary changes the overall focal
    length, the f-stop --which is the ratio of focal length to aperture
    diameter changes.
    That is correct, when using lens extensions, the effective aperture
    becomes smaller by a factor of (M+1)*fmarked.
     
    Rudy Garcia, Oct 19, 2003
    #9
  10. Celeste G

    Don Stauffer Guest

    I am sure you mean larger aperture, not larger f/# Larger aperture is
    lower f/#.
     
    Don Stauffer, Oct 19, 2003
    #10
  11. Celeste G

    Rudy Garcia Guest

    You are right. Larger aperture -smaller f/#.
     
    Rudy Garcia, Oct 20, 2003
    #11
  12. Celeste G

    Don Stauffer Guest

    Actually, this is one area where a closeup (supplemental) lens
    attachment may be preferrable to a macro lens. A macro lens does
    increase effective f/# when focusing on close things. A supplemental
    lens largely avoids this.

    Now, if you have enough light, and an autoexposure camera, you don't
    need to worry about this. But if you are doing manual exposure, you
    need to take it into account when using a macro lens. Also, if light is
    too dim, the greater effective aperture size with the supplemental lens
    can be a plus (didn't mean a pun, but it did come out that way :) )
     
    Don Stauffer, Oct 20, 2003
    #12
  13. If you don't care about depth of field, that may be correct. But
    in macro photography, you're almost always stopped down quite a bit
    for depth of field reasons. The effective aperture for DoF purposes
    is the same as the effective aperture for exposure purposes. So
    if you need a given amount of DoF at a given reproduction ratio,
    you'll need to stop down to a given effective aperture, and it
    doesn't matter how you achieve that reproduction ratio. Neither
    extension nor auxilliary close-up diopter lenses have a real
    advantage. Close-up supplemental lenses WILL typically give a
    brighter viewfinder and narrower DoF on the viewing screen, but
    that's about the extent of their advantage. Extension will
    typically allow you to stop down to smaller minimum effective
    apertures, to allow greater DoF (of course, you must pay the
    exposure penalty for this).

    --Rich
     
    Richard Cochran, Oct 20, 2003
    #13
  14. Celeste G

    Don Stauffer Guest

    Good point. Actually, when I do macro work I normally go to manual
    exposure and go to very high f/#. So it becomes a moot point.
     
    Don Stauffer, Oct 21, 2003
    #14
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