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2x IRND = green ?

IRND color shift green magenta

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#1 christian mann

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Posted 11 April 2016 - 05:49 AM

Hey,

 

so I was wondering if it's now true that when you use 2 IRND picture will turn green...

 

If one IRND blocks Infrared light shouldn't there be "no more" Infrared light left the be blocked for the 2nd IRND?

Why would the 2nd IRND filter have an effect on the image if the Infrared has already been blocked?

Does the IRND have a green layer that blocks the Infrared and hence will turn the image green when using 2 IRNDs or how excactly does an IRND work?

 

I've seen some DoPs using 2 IRND without any hesitation, others don't...

 

What's your take on it?

 

Thanx

 

 

 


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#2 Bruce Greene

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Posted 11 April 2016 - 10:33 AM

Hey,

 

so I was wondering if it's now true that when you use 2 IRND picture will turn green...

 

If one IRND blocks Infrared light shouldn't there be "no more" Infrared light left the be blocked for the 2nd IRND?

Why would the 2nd IRND filter have an effect on the image if the Infrared has already been blocked?

Does the IRND have a green layer that blocks the Infrared and hence will turn the image green when using 2 IRNDs or how excactly does an IRND work?

 

I've seen some DoPs using 2 IRND without any hesitation, others don't...

 

What's your take on it?

 

Thanx

 

 

 

One IRND will make a green/yellow image.  I can't imagine wanting to use two at once!


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#3 Satsuki Murashige

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Posted 11 April 2016 - 11:26 AM

Depends on the filter manufacturer and the particular camera. Some IRNDs are designed to counteract the magenta shift by adding progressively more green to the image. Tiffen IRNDs and Schneider Platinums work this way. Since every sensor responds to IR differently, some will see the green. For example, on a Red camera with the old MX sensor you should instead be using a Hot Mirror or a dichroic IR Cut filter instead of IRNDs. You gotta test different filters with your particular camera to make sure.
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#4 Bradley Stearn

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Posted 14 April 2016 - 12:05 PM

I noticed this for the first time today whilst prepping an Alexa classic kit for a shoot i'm on this weekend. We had some sun today in the UK, so I took the kit into my garden to set it up. I chucked in a Tiffen IRND 2.1 to get a reasonable exposure at around t2.8. I couldn't figure out why the image was so green, I checked my white balance and colour tint, and I had set to 5600k +-0. Wasn't until I took the filter out that I noticed what was going on. I can't believe I have never noticed this before, or had any DOPs request a magenta push to me whilst I've been focus pulling on other shoots. 

 

I found this thread earlier on today, someone has tested the Tiffen and Scheider IRND filters on the Alexa, and has provided a ballpark figure on what the colour tint should be in camera: http://www.arridigit...hp?topic=6795.0


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#5 Stuart Brereton

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Posted 14 April 2016 - 12:47 PM

Tiffen IRNDs do have a slight green tint. I actually don't mind it too much. Schneiders seem to be more neutral, but I've found them to be inconsistent in their densities.


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#6 Guillaume Cottin

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Posted 17 April 2016 - 05:40 PM

IRND filters should really be called IRD because they are not Neutral at all.

 

Someone correct me if I am wrong, but if you are using a "hot mirror" type of IR filtration, theoretically you could stack them, because the wavelenthgs already have been filtered at the first stage. If you are using dye-based IR filtration such as the Tiffen T1, there will be a more noticeable color shift. 

But in real life, no filter is perfect and there will be color shifts with both anyway.

 

What about having a hot mirror or T1 filter and regular NDs behind it? Wouldn't that work better than stacking IRNDs.

 

The best ND filters IMO are the Formatt Firecrest. I don't know how they react to stacking, but they are so neutral that it should probably be fine.


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#7 Satsuki Murashige

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Posted 17 April 2016 - 09:41 PM

I would avoid stacking hot mirrors, since you'll get reflections from the inner filter. The hot mirror always should be the outermost filter in the stack.

Pretty sure Tiffen IRNDs are just T1+ND or some variation on that theme. A Schneider IR750 in front of a Tiffen IRND 2.1 on the Alexa works.

Very curious about the Firecrests. Apparently they are similar in construction to the popular Mitomo TrueNDs which work very well with the Arri cameras but apparently less well on Red Dragon/Weapon with the Standard OLPF. So I'm not sure there is a 'best' set of NDs that is camera agnostic. Would love to test myself, but no rental houses in my area carry the Firecrests.
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#8 Guillaume Cottin

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Posted 17 April 2016 - 10:12 PM

Ok. So you can't stack them. What if the filters are right against each other, like you can do with double filter holders on some Arri matteboxes?
I heard of the Mitomo filters. I couldn't find any online though. There a video somewhere on Vimeo where the Firecrests are tested on a Dragon. Can't find the link... From memory it looks very impressive, no cast at all. But it wasn't the standard OLPF.
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#9 Satsuki Murashige

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Posted 17 April 2016 - 11:27 PM

That's still stacking, won't work. You have two mirrored surfaces with a gap between them so incoming light is going to be pong-ponging between them.

I've seen tests with the Firecrests online but still have yet to see them in person. All the rental houses near me have the True NDs.
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