Circular Polarising Filter

[Omni Images]

I recently went for a helicopter ride and found many images were ruined by reflections from inside the cab.
I was asked to go again last week. I though best get a polarising filter to try and cut those reflections out, and also to help with cutting through the haze over the mountains we were to shoot.
But while I was up there shooting today I noticed an issue with using it.
As you dialed it in to cut the haze down and get the saturation up on the scene outside, the reflections inside got worse ... as I dialed to take the reflections away the scene outside became washed out ...

I vaguely now remembering reading something about this, perhaps about how using 2 filters cancel each other out, like there is a odd even odd even odd even, sort of back and forth in the wave length or what is going on there.
Of course the solution would be to take the doors off, but he wont do that .... so I have low saturated shots with no, or little reflection and high sat shots with more reflection.
I'll be going up again to try get these shots, but maybe now not till our Autumn, winter when the sky is clear in the afternoons, as we need the afternoon sun on these cliff faces ....
Anyway, it would be nice to hear a little more about this Polarising effect and have it worked out for my next trip.
It is about getting the chopper at the right angle etc etc also ....
So please discuss !

 

[ajfotofilmagem]

Are you sure you used multicoated CPL filter? If the filter does not have multi-layer coating can cause reflections.

You left a space between the lens and the glass of the helicopter? If you have space between the lens and the glass of the helicopter will be reflections that a polarizer will not help. Shooting through the window requires enconste the lens (or lens hood) on the glass. Try a rubber lens hood.

The glass of helicopter is curved? Curved glass cause reflexes and some weird chromatic aberrations. The use of CPL filter through a window with curved glass can cause similar effect to a rainbow on the camera images.

It would be possible to open the door of the helicopter during the shoot? If security is maintained, tying his body firmly and making the pictures lying on the floor of the helicopter will avoid problems, because the glass was not designed to do photos (high quality) through it.

 

[Omni Images]

Hi ajfotofilmmagen,
Yep, paid $160 for an Advanced CPL Manfrotto filter, made in Japan, multi coated (12 layers) black rimmed glass .. so it's certainly not a cheapy.

I also bought a big rubber plumbing sorta funnel thing that I fit around the lens as a big hood, but a lot of times it was hard to press it up against the front window ...

The guy won't fly with the doors off, so ..... but I will be able to get him to position the chopper so I can shoot through the side window and have the rubber hood against the window etc ... I'll work it out.

My main query was this weird thing with the filter, dialing one out and the other coming in ... I couldn't turn it to take out the window glare and also make the scenery darker, sky deepr blue bushes more green etc ... it was one or the other, not both, so they seemed to be polar opposites.

 

[DFM]

All plastics have some innate polarization, it's a byproduct of the internal stresses that form when they're molded (and why Perspex models of components were used for stress analysis before the days of CAD/CAM software). Even planar windows in fixed-wing cockpits are polarized, so a polarizer on your lens (LP or CP) will always introduce problems.

Realistically, the only way to get pro quality aerials is to remove the door, or mount the camera outside on a remote gimbal. With a wide angle lens and a cooperative pilot you *might* be able to get away with a fixed mount on the skid (I'm talking GoPro-style gear, not a 5D) - but they may not accept 'amateur' stuff clamped to the airframe. Without vibration isolation a DSLR can be shaken to bits.

From inside the canopy, you have to accept that you're shooting through a hideous optical surface (even a brand-new bubble is covered in bumps and scratches, they're only meant to keep the rain out). You can't prevent image softening and loss of contrast, but you can reduce the internal reflections in the same way you'd shoot in an aquarium - take a black cloth and hide underneath it, making yourself a lightproof tent against the inside of the canopy. Try to keep the lens a little way back from the inside surface so any imperfections in the line of sight are comparatively small, and avoid the smallest apertures so it's never in focus. If the sun's hitting your side it'll still create flares, nothing you can do to stop that. It's a good idea to wear gloves, so when you rest your fingers on the window to steady yourself you don't make fingerprints.

 

[Valvebounce]

Hi Omni Images.
There are pilots out there using choppers with removable doors and a harness, one even offered to take the misses up to do ariel shots of the estate she works on. He was due to do this on a routine flight to collect her boss but there was a problem with the chopper with the harness (I think it was in use for paid photography!) so it didn't happen, well it did she had to shoot through the Perspex but it was only a jolly anyway.
My advice find a pilot that will fly sans doors and go with them, compromising the images a third time will cost more than doing it the right way this time!

Cheers, Graham.

Edit, sorry no idea on the polariser question.

Of course the solution would be to take the doors off, but he wont do that

 

[alexanderferdinand]

As mentioned before: achiving top quality aerial pictures it is necessary to get every glass or plastic out of the optical way.
Maybe you find a plane with a window to open, an ultralight with open doors or a gyrocopter.

 

[Omni Images]

Hi DFM,

So you are suggesting the fact that the windows have some sort of polarisation in them they may react in reverse to the outside scenery.
Like I said above that I read somewhere, maybe here, that two polarising filters stacked may cancel each other out ... so by turning it around to "on" let's say ... polarising the outside scenery, the windows are off .. and turning it "off" for outside scenery turns it "on" for the windows ?

Could distance perhaps have any to do with it ? ... turning the filter effecting at different distances ?
Though I have done some tests and noticed for instance the water close and the sky on horizon both change at the same time ... so I'm thinking maybe not .... but the window was extra close compared to the outside scenery

Just really interested in the physics of the issue at hand.

 

[Omni Images]

Thanks for the advise about aerial shooting .. and I do understand the issues.
The guy taking me is doing it for free .. he's just a guy I met up at our local look out one day, we got talking ...
That's all .. so he's doing it for free, it's not costing me anything ..
I did price the same helicopter with a professional mob a 1/2 hour's fly north of us, their price was $1,000 an hour, and they would take the doors off.

The issue I am wondering about is the polarising effect on the glare on the window.
I would have though when the filter was turned around ... both the outside glare and the window glare would disappear, but it was one or the other not both ... I found that both curious and frustrating.

 

[Don Haines]

You need to track down this lady and ask her to hold the camera
http://www.youtube.com/watch?v=8oAzdbd0J2A

Seriously though, I was in a plane they were filming from... it was a Twin Otter that had a special door which had a pair of "ports" in it that could be remove for filming.... VERY COOL!!!!!

 

[Valvebounce]

Hi Omni Images.
I guess if he's doing it for free my statement of cost is irrelevant! Do I understand that this is somewhat of a you both would like pictures of the subject, he has the aircraft and you have the camera?
If so it would seem that you are going to have to show him the images and the issues the canopy is causing and see if you can negotiate (contribute towards) a door less situation or if he will let you gaffer tape a black shroud to the window that you can close around the lens barrel (or you and the camera*) to minimise the reflections from your side of the glass then concentrate on the subject with the CPL.

* I'm not sure but it seems to me the less that is inside the shroud the less there is to reflect light on the canopy?

I hope you can get the images to do the subject justice and to satisfy your standards.

Cheers, Graham.

 

[DFM]

Plastics can change the polarization of light passing through them because they're very slightly birefringent (the refractive index is direction-dependent). The degree of birefringence relates to the internal stress in the polymer chains, so when you view a scene through a polymer sheet with one polarizer, the effect it has will be degraded. If you were to put another polarizer on the outside, you'd see rainbow colors. This will mess with the CP filter's ability to effect the outside scene cleanly.

In this case you're also fighting against reflections from different sources. The outside world is, in general, reflecting primary sunlight - so the reflections have an element of polarization that's perpendicular to the incident light (physics 101, the incident and reflected wavefronts interfere with one another based on the angle between them - Google for Brewster Angle). That's why a CP filter works best when your camera is at 90° to the sun. Inside the aircraft, any reflections from the inner surfaces will be from scattered light inside the cockpit, so the incident direction could be anything - and the polarization won't be the same. This is why you can't "tune out" both sides at the same time.

So you are suggesting the fact that the windows have some sort of polarisation in them they may react in reverse to the outside scenery.

 

[Coldhands]

The issue I am wondering about is the polarising effect on the glare on the window.
I would have though when the filter was turned around ... both the outside glare and the window glare would disappear, but it was one or the other not both ... I found that both curious and frustrating.

I get the impression that your frustration may come from a lack of understanding of polarisation. (No offence intended, just an observation)

Polarisation isn't quite as simple as on/off. There is unpolarised light and polarised light, the latter of which has an associated angle of polarisation. A polarising filter only allows light with a certain polarisation angle to pass through. So if two polarised light sources with, for instance, a 90 deg angle between them arrive at a polarising filter, one can be completely filtered out (absorbed) but then 100% of the other light source will be allowed through.

So in your case the first (partially) polarised light source would be the light reflecting off the landscape, and the second (partially) polarised light source would be the light reflecting off the window, each with its own angle. Nothing to do with the window material changing the polarisation (as in a liquid-crystal, for example), but just that they have different angles at the source.

I hope this doesn't come across as overly patronising or dumbed-down, just trying to explain in the most basic terms I can.

Hope it helps.

 

[Omni Images]

Hi Guys,
Thanks for the info, much appreciated.
So basically I take it as the two sources I'm trying to cut out are coming from different angles and the polarising filter can only handle one angle.
Sounds simple enough .. Thanks.