This is just a guess, but image sensors are sensitive to a wide range
of frequencies from infrared to ultraviolet. Some camera makers have
been less careful about filtering the extremes in some of their
models leading to lots of sales of "hot filters". It is hard to draw
the line since people who regulary do IR photography have complained
loudly when all the IR spectrum is filtered out. It could be that the
20D sensor filter is more carefully designed than the one in the A1
and the light you are seeing in the image is not visible to the eye.
Your polarizing filter may have a well designed cutoff in the non
visible spectrum, especially if it is a multicoated one.
Winsor
Long Beach, California, USA
On Oct 6, 2005, at 4:29 AM, Chuck Norcutt wrote:
> 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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