help with Nikon 50mm 1.8 af

Discussion in 'Nikon' started by Steve, Feb 23, 2008.

  1. Steve

    Steve Guest

    I recently purchased two used lenses from different sources, and just
    received them in the mail (yea!). Both are nikon: 85mm and 50mm 1.8
    lenses. However, when comparing images from the two, I'm noticing that
    the 50 is consistenly 1 exp position underexposed. I can compensate in
    the auto modes by bumping up the exposure, so I first thought this
    might just be a meteringt thing. However, I just tried a comparison
    shooting outdoor shots in full manual. For the same shutter speed
    (1/60), f/8 ap on the 85 gave the same as f/5 for the 50mm. f/8 on the
    50 was a LOT darker...

    Is this normal, and if not, what can I do (short of getting rid of the
    lens) .

    Thanks.
    -Steve
     
    Steve, Feb 23, 2008
    #1
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  2. Steve

    Joel Guest

    In general you don't care 2 different things that way, and it depends on
    several other thing's beside the brightness.

    - 85mm vs 50mm have 2 different set of leneses (requirement)

    - If you capture the same subject then the 85mm should be 35mm closer, the
    subject should be around 1/3 closer (larger), so the subject should be
    brighter (this many people don't pay much attention to)

    Same with the very same ZOOM lens, you can get the same subject larger,
    brighter, sharper by zoom in closer. Or this is one of the tricks or
    advance of long zoom lens when shooting in low-light situation.

    - Also, you can look at the very end of the lens (the part attachs to the
    body) and you may see some difference there (or I don't have yours to
    confirm but I know some do have the size difference, and when talking about
    size I also mean LIGHT).
     
    Joel, Feb 23, 2008
    #2
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  3. I doubt that you actually have a problem, if for no
    other reason than a full fstop difference is a very
    significant amount of light reduction (from scratches,
    fungus, misadjustment, etc), and you would almost
    certainly be able to see *anything* that would cause
    such a huge variation.

    Regardless of that, I have a 50mm f/1.8 and also an 85mm
    f/1.8, so it was rather easy for me to check mine for a
    variation. I'd expect though, that you will be
    interested in just how I checked, because you will need
    to do something similar to determine if your lenses are
    actually different.

    I set up a test scene. I pinned an "Eskimo mask", that
    is about 10 inches in diameter to an off-white colored
    cardboard box, and used a single light from about 5 to 6
    feet away directly behind and above the camera (a D2x on
    a tripod). Neither the box nor the mask showed any
    reflections. The area of the mask is perhaps 15% black
    and it has no white areas. (The face is a tan color,
    which with a spot meter measured at the same reading as
    the entire scene did when the 85mm lense was used.)

    The camera to object distance was adjusted with the 85mm
    lense in place for a full frame top to bottom image,
    which left some white off to the sides. When the 85mm
    lense was replaced with the 50mm lense the angle of view
    changed, and a larger area of white background was
    framed than had been true with the 85mm lense.

    The camera initially was set for full frame metering,
    and shutter priority was selected. The shutter speed
    was at 1/10th of a second for all images. With the 50mm
    lense it metered at f/11, and with the 85mm lense it
    metered at f/10. (The camera was set for 1/3 fstop
    steps, so this is less than the difference you reported,
    but is not insignificant.)

    I then set the camera to manual mode, and used the
    blink-on-overexposure LCD display as an aid in setting
    the aperture. With either lense there was no blinking
    at f/6.3, and there was at least some blinking at f/5.6.

    There are several conclusions which can be reached from the
    above:

    1) The *correct* exposure was at f/6.3.

    2) The light tranmission of these two lenses differs
    by less than 2/3 of an fstop.

    3) Full screen metering is very easy to fool.

    A) The overall brightness of the scene was
    greater than 18% gray.

    B) The 85mm lense's field of view allowed
    enough of the white background to be part
    of the scene to skew the measurement by
    1-1/3 fstop.

    C) The 50mm lense's field of view allowed
    enough of te white background to be part
    of the scen to skew the measurement by
    1-2/3 fstop.
     
    Floyd L. Davidson, Feb 24, 2008
    #3
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