What was that CPL filter test....

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Steve
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I remember a little while ago to test how good a CPL filter is to put it over a 20p or 50p coin - but thats all I can remember as cant for the life of me remember what to look for...

:thinking: Was it for the colour, dullness/brightness or what - does anyone know what the heck I'm going on about or do they know of the same test :shrug:
 
You could try putting it on a lens and taking a photo.
 
:thinking: I never thought of that..... :p

Seriously thou - there was a test involving a silver coin - but I'm dammed if I can remember what it was & was just hoping that someone could've helped me out to remember so I don't go mad....
 
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Its a shame the Hoya HD C-PL wasnt icluded in the tests above....it may not have been available at the time.....but anyway, another very good piece of kit but like the other two Puddleduck mentions they ain't cheap...I guess you get what you pay for though.

Neil
 
The CPL will reduce the glare off the coin, so place a coin with a light bouncing off it and into the camera

In crappy ASCII art:


Code: 
(lamp)      (camera)
    \           /
      \       /
        \   /
       -coin-

then you spin the CPL and you should see the glare off the coin change in brightness (as reflected light is largely polarised).

This works on water (ie: fising or sailing glasses), or any surface with a reflection.

For a fun time, look what happens in front of a flatscreen monitor, or better, a digital watch.

With Polarisation, it's all about the angles - think of light as a large letter and the polariser as a letterbox, when the two line up, the letter fits into the box (light is bright), when the letter is put at 90º to the box, it won't fit through (light is dark).
 
Mikebert4 said:
The CPL will reduce the glare off the coin, so place a coin with a light bouncing off it and into the camera

In crappy ASCII art:


Code: 
(lamp)      (camera)
    \           /
      \       /
        \   /
       -coin-

then you spin the CPL and you should see the glare off the coin change in brightness (as reflected light is largely polarised).

This works on water (ie: fising or sailing glasses), or any surface with a reflection.

For a fun time, look what happens in front of a flatscreen monitor, or better, a digital watch.

With Polarisation, it's all about the angles - think of light as a large letter and the polariser as a letterbox, when the two line up, the letter fits into the box (light is bright), when the letter is put at 90º to the box, it won't fit through (light is dark).
Click to expand...

Polarisers are indeed all about angles but the angle you need to know about for reflections is 37 degrees, not 90 (that's for skies) but it won't work at all on a coin.

Reflections off metal are not polarised. It's an electromagnetic thing. By the same token, the reflections you get off salt water are very slightly different to those off fresh water - not that you can really tell, the difference is tiny, but it's there.

To the OP, I've never heard of such a test, but I can't imagine what it would tell you. As far as the polarising ability of different filters is concerned, I've never been able to tell any difference - circular vs linear, cheap vs expensive, Japanese vs German, no noticeable difference. I guess there probably is one if you did careful side by side tests, but it's not significant whatever it is.

There are differences however, and cheap polarising grids wreak havoc with sharpness on long lenses because they only look through a small section of the filter and any flaws are magnified, and for the same reason wide angles are much less affected. And the other big thing is flare and ghosting of course, that you get with any filter, so coated or preferably multicoated is much better.

I use a Hoya HD because it has the best of all these things and is unquie in only reducing the overall light by 1.2 stops, compared to the 1.7-2 stops of most others. It's in almost permanent residence on my 17-40L superwide :)
 
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