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[OM] [OT] A very peculiar exposure problem

Subject: [OM] [OT] A very peculiar exposure problem
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Thu, 06 Oct 2005 07:29:16 -0400
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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