CPL what a PAIN!!!! aghhhhhhh Canon 70-200 f2.8 II

Started Feb 13, 2013 | Discussions thread
photonius Veteran Member • Posts: 6,769
Re: CPL what a PAIN!!!! aghhhhhhh Canon 70-200 f2.8 II

dsjtecserv wrote:

photonius wrote:

Nigel Wilkins wrote:

brightcolours wrote:

Steve Balcombe wrote:

tvstaff wrote:

When using my 70-200 f2.8 USM IS II with the sun hood it's impossible to use a CPL with ease on each shot. Is there a device or some sort of trick to be able to use your CPL with this long lens and hood?

Thank you!

And to make matters worse, a CPL can be especially useful when shooting into the light - precisely when the hood is most needed.

Buy a cheap replacement hood and cut a hole in the bottom.

When exactly can CPL's be "especially useful when shooting in light"? For the blue sky/nicer greens effect of CPL's you have to NOT shoot into the light. Similar with reflections.

Polarisers are most effective when the incident light is reflected at 50 degrees from the object reflecting the light to the viewer. So, for example, if you're taking a photo of the grass if front of you at 25 degrees & the sun is in front of you at an elevation of 25 degrees, the polarising filter will be at maximum effectiveness. Try it on a window, you'll see reflections disappear as they approach 50 degrees.

With the sky, it's not so simple, but you're right, it's most effective at 90 degrees to the sun. I expect this is partly to do with the complicated compound angles involved, but I haven't got the willpower to investigate it.

I think your calculation is off above.

It's Brewster's angle, which is key here, and the angle depends on the refractive index of the material. http://en.wikipedia.org/wiki/Brewster's_angle

For water/air, it's about 53 degrees. Incident and exit angles are equal, so the total angle is 106 degrees for a water surface. Polarization in the sky I think is mainly due to water vapor droplets, so it's 106, which seems close enough to the 90 degrees that's usually given for sky.

For grass, the angles you describe would actually be 90 - 53, i.e. sun 37 degrees above horizon, you looking down at the grass at 37 degrees. That's a bit more than the 25 you mention, but not too far off. Further, of course, you also get some polarization at not quite the optimal angle, the plants and grass are orientated in random directions, and what is the refractive index of leaf surfaces? For a soybean leaf with water, it's 1.415 (yes, there are papers for this), gives a Brewster's angle of about 55 degrees, so similar to water.

For glass it's 56.

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The darkening of blues skies at an angle 90 degrees to the sun occurs for a different reason, unrelated to Brewster's angle. The best explanation I've seen is here: http://www.polarization.com/sky/sky.html

Dave

thanks indeed. The blue sky is indeed due to Rayleigh scattering off air molecules,

see http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html and

http://en.wikipedia.org/wiki/Rayleigh_scattering

This article here is perhaps the best on the polarization of the sky due to Rayleigh scattering, nice diagrams

http://en.wikipedia.org/wiki/Rayleigh_sky_model

(so, scrap all the garbage I said above about water and 106 degrees in the sky), it is indeed 90 degrees, but http://en.wikipedia.org/wiki/Rayleigh_sky_model has more on angles and sun position.

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