> Wayne wrote, of cross-polarisers and flash:
> > More bad news: Reflections from an electrically conductive
> > surface are not polarised, so using the polarisers doesn't help.
>
> I think I disagree. Surely the thing about metallic surfaces is that
> they have no polarising effect, so if the light is polarised in a
> particular plane when it hits the surface, it stays polarised
> afterwards.
>
> The converse is if you have something like water, which does polarise.
> You only need one polarising filter to stop reflections from water (if
> you're at the right angle) because the polarising effect of the water
> itself is doing the same job as the part of the cross-polariser on the
> flash.
>
> Does that make sense?
Here is the problem I as I see it. Since metal does not change light
polarization, ANY light hitting it from the flash comes back without
polarization change. If the flash cross polarizer is used, all light coming
from the flash and reflected back from the metal surface is of one
polarization, and should be 'blocked' by the lens polarizer. Now
polarizers work imperfectly, and the cross polarized light that does make
it past the second polarizer usually has a strong color cast. When I used
the T10 and cross polarizer to photograph a natural gold nugget, the gold
became very red.
My guess is that non-metallic surfaces work with flash cross polarization
because some of the light reflected back DOES undergo a polarization
change induced by the subject, and makes it past the second polarizer. 'Hot
spot' reflections on such a subject presumably don't undergo polarization
change on reflection, and should be largely blocked by the second
polarizer.
Therefore, I don't expect good results when using the cross polarized
flash technique when the subject is a metallic object.
Is this perfectly muddy?
Chip Stratton
cstrat@xxxxxxxxx
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