Polarizing filter for 500 f4 IS II

[madencbm]

I will be in rainforest of Peru taking bird pictures with the 500 f4 IS II handheld.  Any opinion as to need for polarizing filter for bird shots.  I have read that it is needed for landscape in rainforest.

Thanks.

[RobertTheFat]

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@TCampbell wrote:

I took [my 500mm f/4] outside and metered (using my Sekonic incident meter) with and without the filter.  You're going to lose about 1.3 stops of light with the filter.

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If you're measuring incident light, what difference does the presence or absence of a filter make?

[TCampbell]

With the dome in the "down" position I let it meter the full-sun sky with no filter.  

Then I place the filter (just the filter -- no lens) on top of the light meter's sensor so that anything making it into the light meter had to pass through the filter first.  This gives me a reading which is roughly 1 and 1/3rd stops less light.

This makes sense consider that light leaving the sun has random polarity (if it hasn't bounced off any surfaces... and it hasn't)

That means that the tolerance of the filter is accepting light with a polarity which is within roughly 15-16% +/- of the tuned polarity of the filter (since it's +/- you get to add them up and this means roughly 66% of the light is blocked and roughly 33% of the light will pass.)