KM6VV Loc: Central Coast, CA

OK, I'm familiar with the old PL filters on film cameras. I understand that DSLR cameras need a circular polarization, as a linear PL won't allow the camera to focus properly.

A circular polarization of light (or RF) is created when a wave and a 90 degree copy of the wave are added together. This is all fine.

I tried a new CPL filter a couple of weeks ago on my D3300, and because the weather wasn't that great (light clouds, diffuse light) I couldn't see any effect. I held it up to the light and rotated the filter, no effect. 90 degree to sun, etc. No problem. I put it away to investigate later.

I was just reading an old thread (Feb 18, 15 10:39:48 "using the circular polarizer" ) and one of the posters suggested pointing the camera at the monitor (tv) and rotating the filter. Wow! very dramatic! Total blocking of the screen!

So why is that? What is polarized about the light from the screen? No effect if I just hold the CPL up to the screen and rotate the quarter wave plate. My old PL filters would show the effect if held up by them selves (to the sky, at least, no LCD's!).

EDIT:
OK, I did just find on Wikipedia that LCD screens polarize the light.

But why is the camera apparently needed to see this effect?

My Polaroid sun glasses don't show any effect when held up to the screen either (Not that I expected any).

All very curious! Anyone have any answers?

Thanks,

Alan

Macronaut Loc: Redondo Beach,Ca.

I can not get technical but, I can share my first experience with CPL. The first time was in Maui. I put on and played with it pointing it at various scenes and turning the ring some. I saw very little or no change in much of anything. Then, I was down by the beautiful clear water with sun high and to my left. I pointed it at the water, started turning the ring, the view darkened some, then, poof! the glare disappeared from the surface and I could see all of the detail on the bottom on the ocean :shock: The effect was dramatic.

So, when you use it in the situation it's designed for, such as removing glare from non metallic surfaces for example, then it will hit you :wink:

Someone will come along shortly and explain it better than my story does :)

KM6VV Loc: Central Coast, CA

I've seen my old PL filters work that way with water, haven't had the situation come up with the new CPL filter yet.

Maybe I'm just expecting too much? It seams the old PL filters were quite dramatic. Maybe new CPL's are not as obvious.

Macronaut Loc: Redondo Beach,Ca.

Different tools, different job, different results :wink:

I think you just might be surprised at the results once you use it for what it's designed for.

lamiaceae Loc: San Luis Obispo County, CA

"My Polaroid sun glasses don't show any effect when held up to the screen either (Not that I expected any)."

You sure? Perhaps not a TV or computer monitor screen, but I clearly remember wearing "polaroid" glasses in my automobile and not being able to read some of the LCD displays on the dash board or stereo. Many "things" just "black out" to polarizing filters.

Also, my understanding is that some but not all camera focusing systems need CPL filters, some can use the old "film camera" linear PL type. Though I have no idea which or how to know with out trying.

KM6VV Loc: Central Coast, CA

I haven't noticed anything in my car with my sun prescription glasses. Could be they aren't Polaroid! I know I have a pair around here somewhere.

Pretty sure the D3300 needs CPL. New filters "work" against the TV through the camera.

I most want to be able to effect the sky colors. Shooting through glass is a plus.

The biggest question I have is how the camera must be used to see the effect of the CPL filter. I'm thinking now that circular polarized light is no longer polarized in a linear fashion, and thus needs a "second" polarizer in the camera somehow.

In RF (radio) waves can be polarized either left-hand or right-hand. I've used it in UHF antennas for satellite work. I assume light must work the same way. What has been done with that? Maybe cross-circular polarization is used in variable ND filters?

John N Loc: HP14 3QF Stokenchurch, UK

2 linear polarisers at 90° to each other will almost totally block out light. Same doesn't happen with circular ones.

cmc4214 Loc: S.W. Pennsylvania

I once bought a cheap CPL, the salesman said it was almost as good as the "name brand" one...Wrong ! I found out when I later bought a "good" one, the effect was much greater than the cheap one

JohnSwanda Loc: San Francisco

It's not DSLR cameras that need circular polarization, it's autofocus lenses. A film camera with an autofocus lens also needs it.

Grnway Loc: Manchester, NH

Hi Alan,

I wear polarized sunglasses when I'm working and most LCD panels become hard to read. I don't know the technical reasons, but it may have something to do with how LCD panels emit light.
I was just experimenting with a magnetic mount for my cpl last night and was rotating it while sitting in front of my computer screen and, at a certain point, the screen wen dark.
I think you'll find the effects of the filter more dramatic during sunny days, 90 degrees from the sun angle, as you already know. However, don't sell it short during cloudy days, when near water, or taking photos in shade when foliage is shiny and/or wet. You'll find that, under those conditions, you'll reduce glare and still increase color saturation using the filter. The effect is not as apparent but it still works.

f8lee Loc: New Mexico

This is incorrect.

Here's the test: hold the polarized eyeglass lens/filter in front of any LCD screen (TV, computer monitor, smart phone) and rotate it. When the orientation of the filter is 90 degrees to that of the polarization built in to EVERY flat screen monitor made today (it's essentially the reason LCDs work at all) all light will be blocked.

CPL or older linear makes no difference.

MtnMan Loc: ID

If your sunglasses are polarized and you rotate them you will get the same effect as your CP when looking at an LCD. Try it on your camera LCD. If you do not see it then your sunglasses are not polarized.

I believe it is because they polarize the screens to minimize glare.

boberic Loc: Quiet Corner, Connecticut. Ex long Islander

Don't know the technical things but I do have a question. Who made the New CPL? was it a top quality filter (expensive ) or a cheap one ? If less than top quality- that could be the cause of poor results. Just asking

amfoto1 Loc: San Jose, Calif. USA

It's easier to show you some examples, than to tell you about CPLs. But first, CPLs generally do not have as strong effect as older Liner Polarizers could give. Still, it's what we have to use on our autofocus, through-the-lens metered cameras (linear pols also can mess with metering systems in some cases, in addition to the AF system).

A CPL's effect can be pretty subtle... especially on a bright, sunny day. When there is a lot of contrast and color saturation, the CPL really isn't needed. But, it will still deepen the blue of the sky and help make any clouds "pop". This is because the light from a blue sky is particularly scattered and the CPL only allows aligned light waves to pass through.

A CPL can be quite useful on an overcast day, when reflections off shinier surfaces of foliage and such can cause loss of color saturation. For example, here are two shots of similar subject shot on overcast days... The one on the left without an filter and the one on the right using a CPL dialed up to it's fullest effect:


Notice above the better color saturation in the right hand image? Yet, there's still some reflection of the white overcast sky. The CPL couldn't completely eliminate that. But the reds and greens are a bit "richer", thanks to the CPL.

As you've noted, a CPL is most effective when it's pointed 90 degrees from the light source. Here is an example where the sun was high above my head and slightly behind me. I wanted the CPL to deepen the blue sky and greenish blue water, while making the white of the fishing boats "pop"... But I also dialed it back a bit because I didn't want to completely lose the reflections of the boats on the water (that would look unnatural, IMO):


CPL will do virtually nothing if the sun (or other light source) is directly behind you (180 degrees) or right in front of you (0 degrees, actually within or close to within the image). In fact, any filter should be avoided when shooting directly into the sun or another strong light source, since it's likely to increase flare issues. A CPL is one of the worst, because it's actually a multi-layered filter. Unlike most single layer filters, a CPL has many more glass-to-air boundaries for light rays to cross, really amping up the likelihood of flare and other possible problems. I shot this sunset without any filter:


Another reason not to use a CPL for the above image was that a lot of it is reflections off water and wet sand... which I didn't want to reduce with a polarizer.

In comparison here's a test shot where I deliberately used a CPL for a sunset image (in this case a high quality B+W Kaesemann CPL):


The above is underexposed, but also there is a lot of loss of contrast and color to veiling flare, as well as some artifacts caused by ghost flare. Actually I was pleasantly surprised it was no worse than it was, and as able to clone out the artifacts, correct some chromatic aberrations, adjust exposure and significantly increase both contrast and saturation to improve the image:


Still, it would have been much better (and a lot less work) to simply shoot without the CPL (or any other filter).

There's also potential for uneven effect with a CPL, especially with wide angle lenses when capturing a broad expanse of a featureless sky or similar. This isn't necessarily a bad thing, just something to be aware of. I deliberately used it for the image below where the sun was low and off to my left, done with a wide angle lens and CPL to make a very plain blue sky a little less boring and sort of frame the lighthouse with the darker area the filter caused:


There is a big difference in filter quality... usually reflected in the price. I use a CPL more than any other type of filter and, after more than one mistake, now will only invest in high quality. I use B+W MRC or Kaesemann, but there are some other good ones from other manufacturers... Nearly all manufacturers also make various grades, too. Including B+W, who makes cheaper single or uncoated CPL, as well as the better ones. Hoya, for example, has about five or six grades of CPL.

Something relatively new I've been seeing are "HT" or High Transmissive CPL. These appear to "cost" less light lost to the filter. A typical CPL will cost between 1 and 2 stops, depending upon it's setting. That can make for a slower shutter speed, larger aperture or higher ISO than you'd like to use in some situations. The HT apparently allow more light through... but I am not sure if they also have less polarizing effect, so haven't used them. They also appear to cost more money!

Another type of CPL that's more expensive is a "thin" type. These are designed for particularly wide angle lenses, so that the filter won't cause vignetting. But, frankly, I've rarely found them necessary, even on 10mm and 16mm ultrawides I use. (Note: Never stack filters unnecessarily, such a polarizer on top of a silly "protection" filter).

Some more recent CPL have special coatings that make them more dust resistant and easier to clean... Costing a little more, but it may be worth it.

Some people will buy a CPL in the size needed for their largest diameter lens, then use "step rings" to be able to fit the same filter to other, smaller diameter lenses. Personally, I won't do that and buy a filter in the size needed for each lens, because I always use a lens hood and that's often not possible when using an over-size filter with a step ring. I have CPL for many of my lenses, but find I more rarely need one with the longer telephotos or macro lenses (though it so happens I have sizes to fit them), so if I were buying I'd prioritize ones to fit my wide and to short telephoto lenses first, then add any for macro or long tele lenses later.

In the end, I recommend spending more for a high quality, fully coated CPL. Besides scenic shots and foliage I showed in my examples above, one also can be useful for portraiture when people have shiny skin or wear eyeglasses, and for many other situations where you need to control reflections. CPL effects are impossible or difficult to emulate in post-processing, too... While most other types of filters have become largely unnecessary with digital photography, the same or better effects can be done with in-camera settings or applied with software after the fact.

f8lee Loc: New Mexico

@Alan (Amphoto)- an excellent and complete response - I think this should be somehow bookmarked in order to make it easy to find in the future when every other question about polarizers is asked. Kudos.