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Strange CPL-filter effect..
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PostPosted: Sat Jan 12, 2019 2:35 am    Post subject: Strange CPL-filter effect.. Reply with quote

I bought a Kenko and a Hoya cpl filter and tried to put them together as a variable nd filter, but the only thing that happened was that there was some color change when I turned on one of them. From a little added yellowish to bluish to the motif.

If I flipped one of them 180 degr, then I got the desired effect, but then the pl effect disappeared on the wrong-facing filter.

Is there something wrong with one of the filters, or is this something that sometimes occurs?


PostPosted: Sat Jan 12, 2019 8:12 am    Post subject: Re: Strange CPL-filter effect.. Reply with quote

Eriksen wrote:
I bought a Kenko and a Hoya cpl filter and tried to put them together as a variable nd filter, but the only thing that happened was that there was some color change when I turned on one of them. From a little added yellowish to bluish to the motif.

If I flipped one of them 180 degr, then I got the desired effect, but then the pl effect disappeared on the wrong-facing filter.

Is there something wrong with one of the filters, or is this something that sometimes occurs?


I've heard of "weird things" happening when using two circular polarisers in conjunction, including a "large dark cross" occurring when nearing maximum effect. When I tried, with two circular filters of different manufacturer, that effect wasn't noticeable, but neither was an extreme darkening of the viewfinder image!

Using two linear polarisers proved to be totally satisfactory (and significantly cheaper) so I've not experimented further.

YMMV - Good luck Smile


PostPosted: Sat Jan 12, 2019 8:54 am    Post subject: Re: Strange CPL-filter effect.. Reply with quote

Eriksen wrote:
I bought a Kenko and a Hoya cpl filter and tried to put them together as a variable nd filter, but the only thing that happened was that there was some color change when I turned on one of them. From a little added yellowish to bluish to the motif.

If I flipped one of them 180 degr, then I got the desired effect, but then the pl effect disappeared on the wrong-facing filter.

Is there something wrong with one of the filters, or is this something that sometimes occurs?


A circular polarizing filter is a linear filter with a quarter wave plate behind, so it depends in which sequence you mount them together. Nothing "strange" about that. A variable ND filter however consists of two linear polarizers ... I remember that from physics class in school long ago. No longer tought I guess Thank You Dog


PostPosted: Sat Jan 12, 2019 11:54 am    Post subject: Reply with quote

Using a Linear Polarizer in front of the CPL should work as a variable ND Filter.

I took Physics so long ago that they've changed the definition of the Meter a couple of times.


PostPosted: Sat Jan 12, 2019 10:07 pm    Post subject: Reply with quote

fiftyonepointsix wrote:
Using a Linear Polarizer in front of the CPL should work as a variable ND Filter.

I took Physics so long ago that they've changed the definition of the Meter a couple of times.


Laugh 1
It does, and this is how most Variable ND filters are made, if you use two linears it messes with AF & exposure on many SLRs.

Flipping the front CPL allows two CPLs to be used as a variable ND (the 1/4 plate first being fairly irrelevant, so the front one is effectively a linear polarizer.)
If you reverse the 2nd CPL, so as to use 2 CPLs back to back, you get weird effects! My physics isn't up to explaining them!

While the definition of a meter has changed it's length hasn't. They've just come up with definition that's more precise, constant & globally relevant. IIRC the original definition while based on the diameter of the earth, actually referred to the length of a particular solid object (a metal bar in France?), and it's now defined using a multiple of the wavelength of light produced by a specific reaction.
As science has improved the need for an accurate definition has been pushed several times in relatively recent years.


PostPosted: Sat Jan 12, 2019 10:21 pm    Post subject: Reply with quote

Polaroid Sunglasses marketing department taught me about that. Laugh 1

Seriously, i remember in early 1960s each pair came with a small card holding some polarization film; the piece of film was rotated in front of glasses to show effect and "prove" the glasses are indeed polaroid not fakes.