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Does far-red exist in HMI's?

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Dan Coplan
Does far-red exist in HMI's?
on Jun 22, 2011 at 8:03:36 pm

Currently working with an Alexa which benefits from using an IR filter when stacking ND's.

Question: Is this only relevant when shooting outdoors or is it also an issue when shooting indoors with HMI's as the source? Far-red falls between 700-800nm on the visible spectrum. Looking at a graph I found for HMI's, it looks like there's a little bit of red at 700 which trails off as it approaches 800 (traces of far-red?). This is in contrast to the daylight spectrum which appears to have significantly more red in that area.


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Todd Terry
Re: Does far-red exist in HMI's?
on Jun 22, 2011 at 9:08:21 pm

It's probably going to depend on the particular HMI, and might vary from instrument to instrument.

I'm just basing that anecdotally not on IR, but rather UV. Most HMI fixtures are shielded pretty well from UV, although some definitely better than others (as evidenced by the occasional sunburn that some people get with the occasional instrument).

I'm sure betting that similar to the variance you get with UV from different heads, you'd probably get a similar IR variance. Probably the easiest thing is just to test it... throw a hot mirror in the matte box and see if it helps, hurts, or does nothing. I'd be interested to know the results.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Dan Coplan
Re: Does far-red exist in HMI's?
on Jun 23, 2011 at 12:47:57 am

Thanks for the feedback. I wish I had time to test! As it is I'm barely given enough time to swap cards and reformat them, but that's a very good suggestion.

My understanding is that IR issues relate to certain black or dark fabrics. Is that correct or can it show up in other materials as well?

Cinematographer / Steadicam / DIT
http://www.dancoplan.com


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Todd Terry
Re: Does far-red exist in HMI's?
on Jun 23, 2011 at 2:11:57 am

[Dan Coplan] "IR issues relate to certain black or dark fabrics... can it show up in other materials as well?"

I think it's a bit of an optical illusion... that if you have IR noise in a frame, it's there throughout the frame, it's just more visible over dark backgrounds... just like most any other noise. But then again, if you can't see the problem, it's not a problem.

Obviously, the lower quantity of filters stacked in the matte box the better... I don't have any of the combos, but they do make filters that are combination ND and hot mirror in the same filter, which will keep your filter count in the box down. Slightly strangely enough, while they make such combos as .9 ND + hot mirrors (which makes sense), they also make much lower levels such as .3ND + hot mirror filters. That's a little surprising to me because you normally only have to worry about IR noise when using higher levels of ND. If you're only needing .3 of ND (or maybe even .6), you probably wouldn't need a hot mirror under HMI lighting, unless the instrument is leaking IR like a sieve.

Curious, in the "none of my business" department... if you're using HMI lighting, why do you need ND filters?... just don't light the scene as brightly. Unless, I'm guessing you're shooting daytime exteriors with supplemental HMI.

But to answer your question... I don't think the IR is going to be enough of a problem to have to deal with it except, as you say, over dark fabrics, etc. If you do, then the Alexa is a heckuva lot more IR sensitive that I thought (I haven't had the pleasure of shooting with one yet).

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Dan Coplan
Re: Does far-red exist in HMI's?
on Jun 23, 2011 at 7:38:42 pm

The call for ND filters is not up to me. I'm the DIT on this particular gig, not the DP. We shot part of the scene outdoors in full sunlight and then moved inside to shoot matching scenes. I can only guess that the DP attempted to match the intensity of light. In theory, however, it shouldn't matter, right? So long as the stop is the same? Is there any quality difference between shooting wide open outside with a stack of filters and wide open inside with no filters? Unless, of course, the goal is to match IR infiltration!

Here's an interesting article/test done by Art Adams with the Alexa:

http://provideocoalition.com/index.php/aadams/story/arri_alexa_and_far_red_...

Cinematographer / Steadicam / DIT
http://www.dancoplan.com


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Todd Terry
Re: Does far-red exist in HMI's?
on Jun 23, 2011 at 8:10:19 pm

Ahhh... ok. I was assuming you were the DP. Second time I've been wrong today.

I'm still a little puzzled by the need for the ND for interiors. I'm guessing that the DP had piled on the ND for the full-sun exteriors because 1) he simply needed them because it was way too bright, or 2) he was trying to open up the lens more to decrease the depth of field.

When moving to interiors, as long as the f-stop is the same... you're right, you certainly don't need that stack of ND in the box if you can get similar exposure without it. There should be no quality difference in shooting wide open outdoors with lots of ND, and shooting wide open indoors with no ND. If anything, the interiors should be better, especially if enough ND was required outdoors to necessitate multiple filters in the matte box.

I think though, it's a fairly safe bet that if you didn't have an IR problem with the ND and exteriors, you probably won't have a IR problem with the same filtration under HMIs. If so, it should show up immediately (if the monitors will let you "zoom in" to see actual pixel-for-pixel resolution). If so, scream at the DP to add the hot mirror. But if he didn't need it for exteriors, I'd sure try to get away with not needing a hot mirror for the interiors, either.

Thanks for the article link... I wanna read it soon.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Dan Coplan
Re: Does far-red exist in HMI's?
on Jun 23, 2011 at 9:02:50 pm

I should add that we're shooting at Alexa's "base ASA" of 800, hence the need for ND's to shoot wide open in lieu of less lighting. I assume it could also be a matter of it being easier to drop an ND in front of the lens rather than throwing a ton of wire in front of the numerous sources.

Is there a downside to using a hot mirror? In other words, why not use one if you have it?

Cinematographer / Steadicam / DIT
http://www.dancoplan.com


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Todd Terry
Re: Does far-red exist in HMI's?
on Jun 23, 2011 at 9:45:22 pm

[Dan Coplan] "Is there a downside to using a hot mirror?"

Not really... other than the usual the-fewer-individual-different-filters-in-the-box-the-better theory. That's why they make hot mirrors that are combined with ND, to cut down on the numbers of pieces of glass in the matte box, and reduce the chances of internal reflections within the "sandwich." But I don't think there's really any downside to adding a hot mirror. I honestly haven't had the need to throw one on in... gosh, I can't even remember the last time.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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