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Old November 13th, 2007, 12:20 PM   #1
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ND filter weirdness

I was shooting a test the other day and I ran into something weird...

The band members were both wearing BLACK shirts, but they look reddish brown.

I assume it is the fault of the Cavision ND filters I was using. Would more expensive, for example, Tiffen filters avoid problems like this?
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ND filter weirdness-mvtest01b.png   ND filter weirdness-mvtest02b.png  

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Old November 13th, 2007, 01:33 PM   #2
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http://www.slmproduction.com/mainsit...es_abound.html

Quote:
The best explanation I could get was that there is no true ND material. It is only ND relative to the chip or film emulsion that is picking up the light through it. So the internal HVX filters have the proper colormetry and do not cause a color cast. But other materials colormetry will differ and therefore can introduce a color cast.
It seems that which ND filter you use depends on your camera, and even then, it can be difficult to find the ideal filter.

That said, I use a Tiffen 3x3 .9 ND filter and have no [noticeable] color shift issues with it. I use a Sony FX-1.
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Old November 13th, 2007, 03:46 PM   #3
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Damn. Well these NDs aren't as bad as what that article describes, so I'll just stick with them. And I have at least one more test shoot beforehand.
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Old November 13th, 2007, 08:14 PM   #4
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Was black stretch or its equivalent for your camera selected "on"?

Were you using "cine-gamma" or similar preset?

Were you using one of the third party published custom "film-look" presets?

Did you manually white-balance to the subject through the ND filter before backing away and setting up your shot?

Did you do any post-production brightening and contrast reduction of vision deliberately underexposed as an attempt to broaden the apparent dynamic range?

If you were white-balanced to a fixed daylight preset, was there a red-brick wall in bright sunlight behind or to your side, bouncing a red fill back onto your subject?


It is also highly likely the black tee shirt is not quite black in reality. The human eyes say we see black.

Last edited by Bob Hart; November 13th, 2007 at 08:18 PM. Reason: sudden new random thought
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Old November 13th, 2007, 10:36 PM   #5
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Zeroed out scene file, Cinelike D or V (I'll have to check it), daylight preset, sunlight shaded the concrete wall behind me (no bounce), this phenomenon has happened before with black clothing, and no post effects besides shrinking.

While it may be that the shirts aren't a true black, the bare camera and the in-camera NDs don't have a problem with it.
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Old November 14th, 2007, 01:08 AM   #6
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Hi Jon..............

The images I'm seeing say to me - those T - shirts aren't black.

As you can see from the background, the soot covered bricks have, indeed, come out black.

I do not think this is a filter issue, rather a discrepancy between what the human eye sees and what the camera sees.

It is entirely possible (indeed, even likely) that the extended RED range of the camera has picked up something the eye cannot percieve, that there is, indeed, a good quantity of red in the colours on those T - shirts.

Problem: I don't think so, unless you require the wearers of said T - shirts to be wearing Black, in which case, get a better quality Black T - shirt.

From what you posted, all the other colours appear normal (well, as far as "normal" can be applied to any video of a band).

I really don't think you have a problem (of course, if you do, that's another story).

This from someone who's never shot a band in his entire life, and has no intention of starting now.

Let us know how you get on.


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Old November 14th, 2007, 10:08 AM   #7
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Nope, the shirts were black. I wish I had brough my SLR with me... I will next time.

Check out the link Ben Winter posted... they had some weirder color shifts.
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Old November 14th, 2007, 03:06 PM   #8
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I had the same sort of problem with ND grads I used that were made by Cokin.. blue sky would turn reddish. Guess it's the nature of cheaper filters.
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Old November 14th, 2007, 04:27 PM   #9
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Seems pretty much like the filters are skewing the colour. There might be some ultra-violet flourescing thing going on as well that the filter might be aggravating.

The only cure as I see it, is another known good filter, or manual whitebalance for all setups, filter-on and filter-off where there may be use of the filter
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Old November 15th, 2007, 04:32 PM   #10
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Same problem here. Yesterday I tried to re-shoot some footage I shot on Monday and the actor was wearing a black T-shirt. The footage on Monday came out fine (not sure what filters my DP used), but yesterday I used a Schneider .9 ND filter and it turned the black t-shirt to reddish brown. Now I have to hope for similar weather to (once again) reshoot without the ND filter.

I should have noticed it in the monitor but time didn't allow me to evaluate the footage (aside from having my kids crying next to me while I tried to get the MOS shot). Oh well...
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Old November 15th, 2007, 08:14 PM   #11
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Is all this color skewing coming from daylight? If so I have a feeling it may be due to IR light getting bent into the visible spectrum by the filter (hence the typically reddish tint to things). If anyone could slap an IR filter in front of the ND and see what that does...

These prosumer cams have IR filters built-in, but they're probably only meant to handle the amount of IR that would come in through a reasonably exposed image. I bet the camera's built-in ND filters have IR filtration, but since we are using external ND's without IR filtration we are cutting down the visible spectrum but letting a full blast of IR through to the sensor.
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Last edited by Ben Winter; November 15th, 2007 at 08:53 PM.
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Old November 16th, 2007, 01:31 AM   #12
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That would make sense, since I've seen this kind of thing all the time when shooting IR film. I haven't seen any corrective IR filters... does such a filter exist?
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Old November 26th, 2007, 05:27 PM   #13
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Quote:
Originally Posted by Jon Wolding View Post
That would make sense, since I've seen this kind of thing all the time when shooting IR film. I haven't seen any corrective IR filters... does such a filter exist?
Jon,

Schneider Optics sells a filter known as an UV/IR CUT filter. We originally designed it for the Post Office to read green and red envelopes at Christmas time. It cuts off all invisible IR light and UV rays. It is marketed under the B+W brand and available in many different sizes up to 82mm.

http://www.schneideroptics.com/ecomm...D=667&IID=4389

This should solve your problem. Otherwise, I have never seen another cause for this effect you have experienced as Schneider ND filters are a truely nuetral grey with little to no color shift over time unlike other brands.
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Old November 29th, 2007, 11:27 AM   #14
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I'll check it out. Thanks!
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