Common with cheating about wide and tele conversion?

Discussion in 'Digital Photography' started by Olav Naess, Nov 27, 2004.

  1. Olav Naess

    Olav Naess Guest

    I just bought a set of Merkury wide angle and telephoto converters through eBay.
    They were advertised and marked x.45 and x2.
    I immediately checked them out on my new Minolta Dimage Z3.
    With the zoom lens set at wide (35 mm equiv.), I got 16 fence posts horizontally
    in a picture, and 22 1/2 when the WA converter was on. Its ratio was not .45,
    but .71.
    With the zoom lens set to tele (420 mm equiv.), I got 7 roof tiles horizontally
    in a picture, and about 5.3 when the WA converter was on. Its ratio was not 2,
    but only 1.32!
    I also checked out the Kenko .42x converter (KCW-042) I have for my old camera
    (a conventional little 3x zoom Medion MD 6126), but my balcony hadn't fence
    posts enough for it. The angle of this one (as well as a .5x converter I have
    tried) was so much stronger that the Merkury models were simply turkeys.
    Well, a 25 mm wide angle without vignetting and with decent linearity has some
    usefulness - the next one I order can now be a real fisheye. A Phoenix .25x
    advertised on eBay looks interesting. Anybody who knows this?
    The stated Merkury magnification factors become reasonable if areas are compared
    (i.e. the actual values are squared), but do any honest people define
    magnification factors by dividing areas?

    -- -- Olav Næss (Naess)
    -- -- Bergen, Norway
     
    Olav Naess, Nov 27, 2004
    #1
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  2. Olav Naess

    dj_nme Guest

    I use a Dimage 7i and bought a Phoenix 0.25x converter a few month ago
    (it was cheap on eBay) and found that it vignettes badly at the wide end
    of the zoom range.
    The vignette did not disappear until zoomed (for a "full frame" image)
    to about 75mm (35mm equiv), so I suppose that the equivalent shortest
    focal length would be about 18 or 19mm (35mm equiv).
    So if you desperately want a slightly wider focal length, it might be
    considered okay.
     
    dj_nme, Nov 27, 2004
    #2
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  3. No. Magnification is always a ratio of linear dimensions, not area.
    The ratings on the Merkury adapters were simply lies.

    Dave
     
    Dave Martindale, Nov 27, 2004
    #3
  4. Olav Naess

    Matt Ion Guest

    Teleconverter ratings are expressed as a factor of the focal length, not
    width of view. Your equations don't apply; trigonometry comes into
    play, and unfortunately, my trig is far too rusty to work out the exact
    numbers right now. All you can derive from simple multiplication is
    effective focal length: a 35mm-equivalent length with a .45X converter
    will give you the approximate equivalent to a 16mm lens; likewise,
    420mm-equiv. with a 2X converter will give you the same field of view as
    an 840mm lens.
     
    Matt Ion, Nov 28, 2004
    #4
  5. But those should be almost proportional.
    To avoid trigonometric effects, he can test this way: Zoom your lens all
    the way to tele (for both adapters). Instead of counting fenceposts across
    the picture, measure the size of one object near the center.

    I don't think trigonometry is throwing it off *that* much! I think he got
    cheated.

    In particular, note that .71 is close to the square root of .45, and 1.32 is
    a little below the square root of 2.

    I think the vendor "cheated" by giving the magnification in terms of area
    rather than in terms of linear size. This used to be a common scam with
    binoculars. "Makes objects appear 9x as big" would be claimed of something
    that actually magnifies 3x.

    I also seem to recall there are U.S. FTC regulations being violated if that
    is the case, if they were sold in the United States. I didn't see where the
    original message came from.

    --
    Clear skies,

    Michael A. Covington
    Author, Astrophotography for the Amateur
    www.covingtoninnovations.com/astromenu.html
     
    Michael A. Covington, Nov 28, 2004
    #5
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