No OM content at all. Just a perplexing exposure problem that I'd like
to present to the combined wisdom of the list.
Yesterday I was assisting a friend in shooting a collection of water
color paintings for a local artist. The paintings were framed and
under glass.
The paintings were held on an easel in front of the camera. The
lighting used studio flashes with large softboxes on either side of the
painting at about 45 degrees. There was subdued daylight in the room
with lots of reflections off the glass from the ambient light. To kill
these reflections the exposure was set at 1/250th at f/8 at ISO 100
which put the ambient light reflections about 7 stops down from the
flash exposure.
The camera was a Canon 20D set on a tripod in front of the easel. I
decided I would also like to shoot some of the same images with my
Minolta A1 so I could do a detailed image comparison of photos shot
under identical conditions. Since the Minolta A1's ISO 100 setting
actually has a sensitivity of ISO 160 I had to close down 2/3 of a stop
from f/8 so the A1's lens was set to f/11.
The first painting was shot with the 20D at 1/250 and f/8 and resulted
in a perfect exposure. Then the A1 was substituted on the tripod at
1/250 and f/11. The result was an image that looked like it was
overexposed by several stops or else was suffering from serious
reflections from the flash off the glass. It was nearly totally washed out.
We ultimately dismissed reflections off the glass since we couldn't see
any visually and the 20D image didn't show any evidence of it. I didn't
want to disturb the settings on the lights but couldn't close the A1's
lens any smaller since f/11 is it's minimum aperture. I thought of
trying an ND filter but the only thing I had in the camera bag was a
polarizer. So the polarizer was pressed into service as an ND filter.
The first exposure with the polarizer was also at 1/250 and f/11. I
expected to see something that would be much closer to the correct
exposure but possibly still overexposed a bit. Much to my surprise the
shot was noticeably underexposed. There followed lots of wailing and
gnashing of teeth but, to make a long story shorter, we ultimately
measured the lights through the polarizer and discovered that the
polarizer was cutting the light by 1-2/3 stops. Opening the lens by
1-2/3 stops from f/11 to f/5.6 produced a perfect exposure. That
deepened the mystery since it only further proved that the correct
exposure without the polarizer should have been f/11. A couple more
trials at f/11, however, showed the same gross overexposure. I began to
suspect a mechanical problem with the aperture. Perhaps it was slow in
closing down to f/11.
Finally we took a black/white/gray card and put it in front of the glass
covered painting and shot again at 1/250 and f/11. Glass area nearly
totally washed out as before but the gray card showing as perfectly
exposed. Another shot with my face next to the painting. Glass area
washed out but face perfectly exposed.
The conclusion would seem to be reflection from the glass. However, the
20D mounted on the same tripod in the same spot showed none of it. And
how does the polarizer fit into the equation? It was clearly correcting
the problem but, as far as I know, it shouldn't have been acting as
anything but a 1-2/3 stop ND filter. Furthermore, since the polarizer
wasn't expected to be acting as a polarizer, no steps were taken to
rotate the polarizer into any particular position. Taking the A1 off
the tripod and taking shots from different angles also didn't change
anything. The glassed area was washed out no matter where it was shot from.
I'm totally mystified. Anyone have a hypothesis about what was going on
here?
Thanks,
Chuck Norcutt
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