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Minus green as a lens filter?


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#1 Ville Pakarinen

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Posted 28 January 2015 - 03:19 PM

When shooting in an office where changing a lot of tubes is both impractical and too expensive, is it possible to correct green with a minus green filter in front of the lens, then add some green to any additional high CRI light's you use?


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#2 Brian Drysdale

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Posted 28 January 2015 - 04:51 PM

Yes, I've done this when shooting film in shops. You have to use the correct colour compensating filter on the camera and plus green on the other lights.


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#3 Adrian Sierkowski

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Posted 28 January 2015 - 04:59 PM

Yes, they make minus green filters, like an FLB for a mattebox. I have used them before many a time and yes you'd +green any lighting you are putting up then in order to balance it all.


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#4 David Mullen ASC

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Posted 28 January 2015 - 05:22 PM

Technically you'd use CC Magenta filters. You could use FLB or FLD filters instead, I think the FLB was designed to correct Cool White for tungsten Type-B film, so it's magenta + orange.
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#5 Ville Pakarinen

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Posted 28 January 2015 - 05:50 PM

Man... Always love to have my intuitions confirmed. Thanks!


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#6 Dennis Couzin

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Posted 28 January 2015 - 08:02 PM

Your question boils down to: if you have two illuminators, and you like the pictures you get by shooting with filter X on the first illuminator and no filter on the second illuminator, then will you also like the pictures you get by shooting with no filter on the first illuminator, filter X on the camera lens and filter anti-X on the second illuminator?  

 

Both ways, the image receives the exact same light from the first illuminator.  But the image receives much less light from the second illuminator when it and the camera lens are filtered.  So you must increase your second illuminator to keep your shot from changing.

 

How much increase depends on what X and anti-X are.  It's not enough to speak of green, magenta, anti-green, etc. filters.  Filters block light wavelength-by-wavelength, and how a filter looks doesn't show what the filter does, especially with funny light and a somewhat funny sensing camera.  I found spectral data for the FLD (not FLB) and this is no ordinary slightly pinkish filter.  It has a deep drop centered at around 545 nm whose purpose is to tame the fluorescent lamp's spike at around 545 nm. (Its strong shape belies its weak color -- exactly what you'd expect from a filter able to "correct" a white light for a camera.)

 

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It's unlikely you can find a compensatory greenish filter.  (The X and anti-X filters stacked on top of each other must become a spectrally flat ND filter.)  With the wrong greenish gel on it the second light will contribute wrong colors to the picture taken through the peculiar pinkish filter.


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#7 Ville Pakarinen

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Posted 28 January 2015 - 08:27 PM

Didn't understand any of that :)


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#8 Mark Dunn

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Posted 29 January 2015 - 08:01 AM

It'srather academic and much more than you, I or most DPs need to know practically.


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#9 Brian Drysdale

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Posted 29 January 2015 - 12:07 PM

If you add combine the two together they cancel each other out, so you end up with a straight line, although, reality tends not to be quite so convenient.


Edited by Brian Drysdale, 29 January 2015 - 12:07 PM.

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#10 Stuart Allman

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Posted 29 January 2015 - 01:10 PM

Ville,

 

Typically cameras allow you to do a custom white balance.  Can you do a custom white balance to the fluorescents and then add plus green (or whatever compensation needed) to the non-fluorescent lights.  That seems easier than filtering both your lens and the non-fluorescent lights and taking the double light loss.

 

I've never seen a filter that can actually remove the green spike from a fluorescent light.  The filter takes down the entire green-ish range of wavelengths, which includes the green spike.

 

S.


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#11 Ville Pakarinen

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Posted 29 January 2015 - 03:07 PM

Typically cameras allow you to do a custom white balance.  Can you do a custom white balance to the fluorescents and then add plus green (or whatever compensation needed) to the non-fluorescent lights.  That seems easier than filtering both your lens and the non-fluorescent lights and taking the double light loss.

Hmmm... yes. Doing it in camera would be better. I'm using my BMCC and as far as I know, there's no option for adjusting color on the green-magenta axis. I know this is possible with my Canon 60D DSLR.

But since it's raw, how would a custom white balance help since it's not "baked in"? Wouldn't there have to be some kind of digital filter before the raw was even recorded?


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#12 Tom Morrow

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Posted 30 January 2015 - 04:07 AM

Using digital color balancing (in camera or in post) is becoming a common way of dealing with flourescent lights. 

In the early days most overhead flos had horrible green spikes and they were fairly consistently horrible in the same way.  So a minusgreen filter was a good "one size fits all" solution.

 

Today there are many different types of flos in use in commercial environments all with different color balances and amounts of green spikes.  More and more often the green spike is becoming smaller as manufacturing techniques improve and more R&D is going into making flos look like incandescent spectrum. 

 

Correcting digitally allows you to dial in just the right amount of correction for how your particular camera's sensor spectral sensitivity reacts to the particular flos you are shooting under.  And if you are shooting RAW you can wait for post.

 

The downside to doing it fully digitally is that a green spike (or imbalance in Red or Blue) could exceed or approach the sensor's dynamic range limits and so not be fully correctible.   And if you have mixed lighting you will have to do something to get them to match if that is your goal, no matter if the correction is in the digits or a filter in front of the camera.

 

More and more the easiest thing to do is just shoot without doing any color correction and see if it needs it in post. 


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#13 Stuart Allman

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Posted 30 January 2015 - 11:28 AM

Ville,

 

My experience in shooting raw is limited to the Red cameras, so I don't know the caveats of doing it on the BM cameras.  What I've had on set in the past is a DSC Chroma Du Monde chart.  Since the footage is shot in raw it should be just a matter of having a *good* white reference (not a white piece of paper) and using the color grading software to balance white to that color as a first pass color correction.  So unless there is a BM specific limit on correcting green in camera, as Tom suggests might happen, you'll probably be as good as it gets just balancing it out in post.  That doesn't mean all the primaries and secondaries will look right, but at least the neutrals should look neutral.

 

I was playing around with my friend's A7S last night and noticed that when you do a custom white balance it shows you not only the color temperature it chose, but also the amber-blue and green-magenta balances it chose.  This could be a nice little tool to have to balance out your film lights so you can check that they mostly match the fluorescents. 


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#14 Ville Pakarinen

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Posted 01 February 2015 - 03:57 PM

What about shifting hue in post? I got this idea of using the green to our advantage: I could correct only the lights above the talent with minus green gel and leave the background lights uncorrected. Then shift the hue of the high CRI lights a little bit so that when I shift hue in post, I'd get slightly bluish background and correctly balanced light on the talent.


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#15 Tom Morrow

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Posted 01 February 2015 - 05:07 PM

What about shifting hue in post? I got this idea of using the green to our advantage: I could correct only the lights above the talent with minus green gel and leave the background lights uncorrected. Then shift the hue of the high CRI lights a little bit so that when I shift hue in post, I'd get slightly bluish background and correctly balanced light on the talent.

 

Sounds like a good plan.  If your scene has both green-spike flos and incandescent light, you will need to correct the green at the flo source not at the camera or post.  Otherwise you will accentuate the magenta of the incandescent if correcting with minusgreen in camera or post. 


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#16 David Mullen ASC

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Posted 01 February 2015 - 05:19 PM

What about shifting hue in post? I got this idea of using the green to our advantage: I could correct only the lights above the talent with minus green gel and leave the background lights uncorrected. Then shift the hue of the high CRI lights a little bit so that when I shift hue in post, I'd get slightly bluish background and correctly balanced light on the talent.

 

But if you've corrected the light hitting the talent's face with minus-green gel, then why would you shift the hue (magenta-green axis) in post?  If you wanted a colder background but no green overall, then you'd put a warming gel on the Cool White tubes (or switch them to Warm White tubes) over the talent but leave the green, then once you take out the overall green in post, your background Cool White tubes would be blueish compared to the foreground but not green as well.

 

But if you took out the green already on the lights over the talent, then they would be normal but the background would still have green, so if you tried to take the green out in post, your talent would shift from normal to magenta.


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#17 Tom Morrow

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Posted 02 February 2015 - 12:57 AM

As I understood it, Ville was talking about shifting the talent's face with a CTB type blue shift in post.  It sounds like he was using the word hue in the colloquial sense of changing the color, not the more precise way that hue means moving along the magenta-green axis to camera folks. 

 

Shifts along the Warm/Cool (Orange/Blue) axis are called changing the Color Balance, or Kelvin

Shifts long the Magenta/Green axis are called changing the hue.

 

But in art classes and computer graphics (including photoshop), Hue changes mean going around a color wheel in any direction to any destination (as distinguished from saturation and brightness axes).

 

Sometimes terminology gets in the way of communication!


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#18 Ville Pakarinen

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Posted 02 February 2015 - 01:07 AM

Oh yes! Didn't realize the magenta-green axis was referred to as hue. I meant exactly that: moving the color wheel in Resolve (although it's not exactly a wheel in Resolve) so that greens shift towards cyan/blue. This, of course, shifts skin tones towards yellow, so the lights on the talent should be shifted towards red - probably no more than 1/4 stop loss?


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#19 David Mullen ASC

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Posted 02 February 2015 - 09:51 AM

You shift cyanish green to blue by adding magenta (because Cool White is already cyan), not yellow, so the faces are going to get very pink if they started out corrected which is why I said if you just wanted a cooler background and a warmer foreground, then don't correct just the foreground lighting with minus green, leave it green so that you can then take out the green overall in both the foreground and background with post or with a magenta filter. Then you just have to make the lights over the talent warmer with gel like CTO, or switch those tubes to Warm White, leave the background Cool White.

Though the truth is that if your foreground is lit to 3200K with no green in it, then the color of Cool White tubes in the background is an interesting cyan. It's only in daylight balance that Cool Whites look sort of a sickly yellow-green.
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#20 Tom Morrow

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Posted 02 February 2015 - 12:15 PM

As I read ville's post he was thinking of shooting in daylight camera balance for the flos then correcting the 3.2k incandescent (high cri means no green spike to me) on faces to white with ctb in post. I think where David was going was that just shooting with camera at incandescent balance would achieve the same effect more simply.
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