Re: Which Polarizer?

In article <boggissimoron-0B9809.10444426072006@xxxxxxxxxxxxx>, Dr. Boggis <boggissimoron@xxxxxxxxx> writes
In article <44C6E7DA.36F23998@xxxxxxxxx>, ColinD <nospam@xxxxxxxxx>
wrote:

Polarizers do take about 2 stops compensation. Light from the subject
is randomly polarized, and the filter passes only the light that happens
to be in the same polarization plane as the filter, so about 75% of the
light from the subject is rejected. There is also a small further
transmission loss from the polarizing medium iself, so there's your 2
stops.

OK, this makes sense, but I still have a stupid question: doesn't the
camera meter notice this reduction in the amount of light entering the
lens and compensate accordingly? Or is the meter not operating through
the lens?

Yes, the meter is operating through the lens in your Rebel XT. The meter will notice the reduction in light levels and approximately compensate. However, it may not get it quite right. For example, if you want to increase the saturation of a blue sky, and use a polariser, you actually want the photo to be a little darker, but the meter might increase the exposure too much in order to raise the overall illumination level back to what it was.

Clearly a little trial and error may be required to get exactly the effect you want - and clearly there is a great advantage in using a DSLR that you can review the results (though only rather crudely) on the screen straight away. For most occasions the reduction given by the meter will be reasonably OK. The purists in film days advised: meter without filter, fit filter, and apply compensation of +1.5 stops (or slight variant of this according to taste, determined by trial and error). This approach will still work; it is more time-consuming than the simpler approach of sticking on the filter and letting the meter do its thing, but perhaps slightly more reliable.

David
--
David Littlewood
.

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