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Old 08-06-2011, 03:48 AM   #1
pachycephalic
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Default Light Level Correction

I have noticed a shift in the light levels when shooting at our local track when using my circular polariser on the camera. It is needed due to the glare that comes off the track surface at times.

An example of this change can be seen on my clip at 2011 Nostalgia Racing NZ particularly at around the 28 second mark where it darkens significantly.

I am having problems trying to improve the clip so that the worst parts do not become too dark to see the action clearly.

Any suggestions to fix this clip or avoid this problem appreciated. Thanks in advance.
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Old 08-06-2011, 03:59 AM   #2
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Unfortunately, polarizers work best when the lens is at a 90 deg. from the light source, so what you may be experiencing is the shift in and out of its optimal angle and without manually turning the filter to compensate, there may be no elegant solution.

For your current situation you may be able to use a two-track solution...
Create a color corrected track of the original footage above and use the rubber band to fade as the camera pans. Not great, but may work!
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Old 08-07-2011, 09:01 AM   #3
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Thanks for the reply Rusty. Hope to be able to give your suggestion a try in the next couple of days.
What I was hoping is that there was some method of keyframing lightlevels.
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Old 08-08-2011, 01:12 AM   #4
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I know the you are using the polarizer to reduce unwanted glare, but would a lens mounted neutral density filter also reduce some glare( not to the extent of a polarizer) without the reduction in brightness?
Just a thought.
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Old 08-08-2011, 08:52 AM   #5
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Hi David, too me it looks like you are shooting on auto iris.
Always use a manual iris so that the iris does not adjust exposure when the frame changes.
What is happening in your shot is that when you zoom out the bright clouds are causing the iris to close. When the camera pans and the clouds are no longer in shot it opens again.

Expose for your subject and set the iris to manual. This will stop the problem you are having.
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Old 08-09-2011, 10:16 AM   #6
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Thanks for the suggestion Jerry but not sure whether it would have the effect. The glare off the windscreen and chromework can be be quite dazzling at times. The circular polariser at least enables me to tune out the worst affected areas.

Glen, I checked the original clip and the f-stop was locked on f4.8 but the shutter speed does a large drop in the area affected. I am shooting all day in very short bursts of 10 second to 3 minute clips depending on the action with light levels fluctuating quite alot at times.

I have had a chance to have another play tonight and have had some success. I agree with you Rusty as to the cause of the problem being the change in optimal angle. What seems to have worked is by keyframing the YUV and adjusting only the black levels using the Y over the affected area. It gives the area a slightly washed out look but at least the action (severe wheel shake) is a lot more visible.

I think in future I will have to be a little more careful and aware of the changing angles. At least I now have a better understanding of the YUV filter and some of it's effects. Thanks for your help.
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Old 08-09-2011, 11:16 AM   #7
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If your shutter speed drops then your exposure changes. Ensure that it is also set to a fixed value.
Make sure that you are using zebras to keep an eye on what is happening to your highlights.

I do not see that it is the change of angle that is doing this. See the clip before the one you are having the problem with. It is fine. This is because it was not zoomed out enough to get the sky in.
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Old 08-10-2011, 07:55 AM   #8
Vaughan Wood
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David,


There's nothing wrong with your clips.

The black and white chequered pants on the grid girl were perfectly exposed!

(LOL).

Cheers,

Vaughan
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Old 08-10-2011, 08:40 AM   #9
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Quote:
Originally Posted by Vaughan Wood View Post
David,


There's nothing wrong with your clips.

The black and white chequered pants on the grid girl were perfectly exposed!

(LOL).

Cheers,

Vaughan
I have to concur!!! :)
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Old 08-11-2011, 07:25 AM   #10
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What can I say? I managed to get some things right. Actually Vaughan, she is actually from Australia and was a great sport. What entertained me was that when she begins to guide the car back, the crowd response drowned out the engines ;-)
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