Steve <srjm72499@[EMAIL PROTECTED]
> wrote:
>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) .
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 re****ted,
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 <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@[EMAIL PROTECTED]
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