Neutral Density Filter

[Basil5278]

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Did you actually watch the video?

When he added another linear polarizer on top of the two that would be a ND filter, it did absolutely nothing. Only if he put it in between, which of course doesn't happen when stacking filters.

Furthermore, it's a circular filter, not another linear one, so who knows?
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The filters he used are linear polarizers, just like the microwave 'filter' he showed later. The reason the 3rd filter did nothing when placed on top of the other 2, is that they already filtered out most of the light in both planes, so placing a 3rd sheet on top does nothing (if the light source was stronger and the room darker, you probably would have seen a slight effect).

Placing the 3rd sheet in the middle means you just swap the functions of sheet 2&3 - the net effect is the same.

All a circular polarizer does is to filter out some of the light through a linear filter, then a 2nd plate (not a polarizing filter) causes the light to change polarity - thinking it as putting a spin on the polarity!

If you users linear polarizer on a digital sensor, you'll get a horrible moire patterning because of the filters in the sensor itself.

 

[Helen B]

If you users linear polarizer on a digital sensor, you'll get a horrible moire patterning because of the filters in the sensor itself.

I've never observed that in practice. Can you give an example? Which filter(s) on the sensor cause the problem? I always though that the problem came when semi-silvered mirrors / beamsplitters were used in the autofocus or exposure meter lightpaths, resulting in varying amounts of reflection depending on the polarization angle.