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Infra Red and the RED


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#1 Michael Kubaszak

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Posted 18 April 2008 - 07:09 PM

I was on a shoot Wednesday in which the RED ONE was used entirely outdoors for a commercial. On some shots the image coming into the LCD and monitor looked really blue with ND in the front of the lens. The basic reason (as was explained to me) for this is that the Mysterium chip sees visible light and infrared light the same so when you throw ND on the visible light is vastly diminished and the infrared light pours into the lens. To compensate you need a 486 filter to compensate/block infrared. There are no manufacturers for cine-style 486 filters.

This link gives some examples and has more explanation.

ttp://www.reduser.net/forum/showthread.php?t=9412&referrerid=1799


Michael
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#2 John Sprung

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Posted 18 April 2008 - 07:55 PM

There's another recent thread on this:

http://www.cinematog...showtopic=30130




-- J.S.
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#3 Keith Walters

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Posted 18 April 2008 - 07:59 PM

I was on a shoot Wednesday in which the RED ONE was used entirely outdoors for a commercial. On some shots the image coming into the LCD and monitor looked really blue with ND in the front of the lens. The basic reason (as was explained to me) for this is that the Mysterium chip sees visible light and infrared light the same so when you throw ND on the visible light is vastly diminished and the infrared light pours into the lens. To compensate you need a 486 filter to compensate/block infrared. There are no manufacturers for cine-style 486 filters.

This link gives some examples and has more explanation.

ttp://www.reduser.net/forum/showthread.php?t=9412&referrerid=1799


Michael


This subject was briefly discussed on this forum here.

The problem appears to simply be that, because 3-Chip cameras and colour film don't react appreciably to infrared, historically the filter designers never worried overmuch about how well their NDs blocked IR.

The same problem has been reported with digital still cameras, but because in those it is easier to reduce the exposure by shortening the "shutter speed", NDs are not so often used. The only time they would be really required is where a combination of shallow DOF and motion blur are needed.

Just because a transparent material looks grey or black to you does not necessarily mean it looks like that to all wavelengths. Actually if you look at a piece of exposed and developed colour negative with an IR camera under IR light (eg from a security or door minder camera with built in IR illumination) the image on it will be either be not visible or extremely faint. In fact negative film blackened by overexposure was once widely used by night vision experimenters to convert ordinary flashlights into infrared sources.

Some high-end telecines have an IR sensor as well as the usual red, green and blue ones. If the colour film is undamaged, there should be little or no output from the IR sensor; it should "see" the film as completely transparent. However physical scratches or other irregularities not part of the original image will show up on the IR scan, and this can be used to automatically hide the defect, often by substituting the equivalent pixels from an adjacent frame.

I have actually been experimenting with old-fashioned smoked glass filters which are made by blackening ordinary glass with soot from a kerosene flame. In theory they should offer the same attenuation for all wavelengths. At the moment I don't have access to a camera that suffers from this problem so I'm not sure if they will address the issue or not. The only problem I can see with them is that they are very fragile.
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#4 Dane Brehm

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Posted 22 April 2008 - 01:03 AM

I recently had the chance on a Commercial to try both a Panchro Set from Woodland hills and a new Tiffen 1.2 NDIR. The Panchros are awesome but watch out hits! We had some inner-filter bounce that the Red sensor caught during a few 14mm steadicam sequences

For Daylight I'd use a .9IR and a Polarizer for the best stop.

I've also been recommending using a 80C/80D filter for Tungsten Interiors but Build 15 is a Major improvement over Build 14 as far as Dynamic Range and the 5000k Sensor Issue.


Dane Brehm
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