class="style10" Polariser Advice For Air Show
>Publisher : 8th Feb. 2006
>I am going on a shoot for a documentary and will be shooting portions of an air show. It will be 16mm and 7245 50D is the planned stock. I will be panning the sky tracking the planes mostly from the ground. A polariser has been requested to enhance the sky.
>Now the question : I have heard various pro and con reasons to use a circular polarizer and a linear polarizer. I understand about the possible 'black out" a linear polarizer might cause in the viewfinder but don't think that will be an issue.
>I have heard concern that a circular polarizer might degrade the image slightly more than a linear one. On the other hand I have heard a circular polarizer will maintain its effect over a wider range pan.
>Anyway any advice from aerial shoot experience with polarisers would be helpful.
>Lance Griffing wrote :
class="style11">>"...shooting portions of an air show...A polarizer has been requested to >enhance the sky...the possible 'black out" a linear polarizer might >cause in the viewfinder...a circular polarizer might degrade the image >slightly more than a linear one...a circular polarizer will maintain its >effect over a wider range pan."
>Circular and linear polarisers for imaging applications are virtually the same in how they function in polarization effectiveness and image quality.
>After all, the circular polariser is at its heart a linear polarizer to which has been added a clear retarder layer that makes it into a circular polarizer. The retarder does not degrade the image in any noticeable way.
>General imaging rule of thumb...a linear polarizer has many applications but may not work best in all...a circular polarizer can be used in any application. When in doubt, go circular.
>If your viewfinder has a partially silvered mirror or prism in it, it may be susceptible to being hard to see through with a linear polariser. Check your manual for this equipment, as it may be an issue.
>The key issue you face is in panning with a polarizer in general, realizing that it doesn't matter whether it's circular or linear. The degree of polarisation effect changes as you change your angular direction relative to the sun in the sky. So if you are panning to follow a plane as the angle changes the sky will go from lighter to darker and back again. This may (or may not) be visually objectionable. The way to minimize this is to try to set up your shots, as much as possible, so that the camera's viewing angle stays perpendicular to the line-of-sight of the sun in the sky. This will maximize the sky-darkening effect. If you find that your pan causes too much change in the sky, then turn the polarizer so that it isn't at maximum effectiveness so that the image will be improved but not be so dramatically different from where it isn't affected at all.
>Note that either type of polarizer has no effect when aimed directly toward or away from the sun. And that you need a blue sky to start with...and that you want to be sure that the change in sky darkness is not causing your camera to change exposure as you pan...you will want to use the recommended filter factor for exposure compensation with a fixed iris if that is otherwise acceptable for your situation. Typical polariser use a 1-2/3 stop
>>The key issue you face is in panning...The degree of polarisation effect >changes as you change your angular direction relative to the sun in the >sky.
>I often use a pola on sky shots but fell victim to this problem last year. Luckily it was on something I was shooting for myself with no great consequences, but it was very noticeable afterward and quite distracting. I was shooting a travelling landscape from a car with a wide lens on video. The effect looked like an unnatural dark area in the blue sky which moved as I panned the camera or the car turned direction on the winding road. I should have made adjustments on the pola.
>The pola works best on lock-offs. Setups facing in the same general direction may be able to withstand a bit of panning before the pola changes density noticeably. Or for quick action such as planes you may be able may pan from a good polarized position into an unusable section only to use the better polarized section. It can get tricky. get yourself a polarizer and play with it looking at the sky at various angles and directions. You will get a sense of how much latitude you will have before getting into trouble.
>I have a 4x4 polarizer which has a focusing type of ring on the holder to dial it in. It is essentially a round filter in a 4x4 holder. This helps in making quick adjustments on the fly without taking off the filter. finding the sweet spot, marking it, and resetting it in the filter holder.
>On a similar note, a well tuned-in polarizer works very well in sunlit scenes producing less glare on objects and renders more saturated colours with the glare eliminated.
>Hope this helps.
>Bear in mind that if you pan a significant angle across the sky, the brightness of the sky is likely to change considerably during the shot as the angle between the sun and the moisture in the air that you are "polarising out" changes. This could be really distracting. It all depends how much of a pan angle change there is within a given shot...the further back from the flight line you are, the less likely this is to be a problem if you are shooting what is happening over "air show central" but if you are panning planes in on their approaches or out as they recede you may get yourself into trouble...depending of course on time of day and where the sun is and etc
>Your mileage may vary.
>I believe the viewfinder issue has to do with whether the viewing system uses a plastic fibre optic block in the viewing system or not.
>Well, I managed to avoid answering any of our specific questions while bringing up others
>If you shoot from the end of the runway and if there are jets at the show, bring earplugs.
>Thank for the information.
>From what I understand I will be panning as much as 270 to 360 degrees as the planes circle the field. This is going to be a mess. I was thinking if I stay with a pan of 90 to 180 degrees I might not have too much change in the blue sky. Fortunately these will be bi-planes and fairly slow, well under 150mph.
Perhaps on a 180 to 270 pan with the polarizer turned at half strength it wouldn't be too noticeable. I am not use to shooting exclusively "sky scenes" so any more hints and advice are welcomed!
>Lance Griffing wrote :
>"Perhaps on a 180 to 270 pan with the polariser turned at half strength it >wouldn't be too noticeable."
The change in the polarization effectiveness in relation to the camera's angle to the sun in the sky is such that it goes from minimum to maximum in just a ninety-degree quarter-turn pan.
>In a similar manner, the effectiveness of the polarizer also goes from minimum to maximum as you rotate it a quarter-turn.
>Best bet is to do as Jim suggested and look through the filter while turning both yourself and the polarizer to see what the range of polarization effectiveness is and what it looks like, prior to shooting.
>Nice thing about polariser is that the effect is easy to judge just by looking at it.
>Lance Griffing wrote:
class="style11">>Perhaps on a 180 to 270 pan with the polarizer turned at half strength it >wouldn't be too noticeable. I am not use to shooting exclusively "sky >scenes" so any more hints and advice are welcomed!
>Personally, I think the polariser idea is trouble. Test it, throw one on a camera and go out and shoot some birds or something. Even with the polariser at half strength It is still going to produce density changes in the sky as you follow these planes. Net result is that it will produce something that will be more of an artefact than an enhancement, the way I see it.
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