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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

Red and the infraRED

I don't know specifically what the Panchro ND is but if it's simply a partially reflecting mirror (visible as well as IR), I don't think that will do good things for your image quality- mirrors work both ways, so I'd expect a lot of ghosting and flare due to reflections and scattering from every optical surface in the light path. Just my assumption- I have not seen or tested this kind of filter on a camera (I used to use that type of ND in an optics lab to adjust laser beam strength, though).
 
I've just remembered a conversation with a lens manufacturer (Abakus) and they gave me the impression that some of the high quality video lenses had IR blocking characteristics as part of the glass.

This, probably isn't, (may not be) the case with 35mm glass as IR sensitivity isn't really a serious day to day issue. (I'm guessing based on the fact that still 35mm glass often have IR focusing marks)

Anyone know better? If the above is true it might soon be necessary to include the same IR blocking technologies in 35mm lenses as the film world moves toward more electronic acquisition.

JohnF
 
It's always best to use the minimum filters possible, so if you need to cut 5 stops it's best to use a 1.5 ND instead of stacking a 0.9 and 0.6 for example.
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I apologize for muddying the waters with anecdotal evidence... But there are a couple of comments in various threads here & elsewhere that suggest the opposite is true in the case of heavy ND's - something about the difficulty of manufacturing the darker ones with equivalent optical properties? Perhaps a filter scientist would care to chime in here.

On a recent Red shoot, we had a color cast issue (may have been IR-related) with a heavier ND, that went away when we replaced it with a pair of lighter ones. Again, this is anecdotal (and not even intuitive) - and in the general case your comment makes sense.
 
Except for the pancro's heavier filters do have more of a problem with color balance.
 
Deanan said:
Except for the pancro's heavier filters do have more of a problem with color balance.
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I wonder if this is a combination of IR contamination plus color casts in visible light? If it's just a visible light color cast, it seems to me this might be fixed close to 100% by post color correction; maybe just a white balance. If it's also an IR issue, it cannot be fixed that way (without adding an IR filter).

My comment about 1 vs 2 filters was just in general, more optical surfaces leads to more flare & loss of contrast. Admittedly, that may be unimportant if the color cast problems are serious.
 
jbeale said:
I wonder if this is a combination of IR contamination plus color casts in visible light? If it's just a visible light color cast, it seems to me this might be fixed close to 100% by post color correction; maybe just a white balance. If it's also an IR issue, it cannot be fixed that way (without adding an IR filter).

My comment about 1 vs 2 filters was just in general, more optical surfaces leads to more flare & loss of contrast. Admittedly, that may be unimportant if the color cast problems are serious.
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It's both color and IR on the heavier NDs but I've seen the color casts also be quite strong on some NDs (minus the IR). It's one of those tradeoffs one has to make between more reflections, etc. and color balance. Neither is wrong or right but both have their advantages/disadvantages. Same with pancro's, while they have great spectral response, they're also very reflective.
 
How do Polarizers work with IR - do they reflect when on?
I assume when on, they also like NDs too.

Also would a CTB - daylight blue filter - help cut down on IR, that then one could color correct for in post?
 
It's my impression that all types of filters (except hot-mirror) tend to let the IR through, including most ND, pola, and color-correction filters. Many optical color dyes are pretty much transparent in IR.
 
jbeale said:
It's my impression that all types of filters (except hot-mirror) tend to let the IR through, including most ND, pola, and color-correction filters. Many optical color dyes are pretty much transparent in IR.
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Agreed though it's important to remember that not only can they let IR through there is a chance that some filters might actually diffuse IR wavelengths. Some here have already noted that some of their shots went "soft" whilst using ND's. This could very possibly be the IR channel getting "diffused".

It may also be that the this softness might well be due to the IR wavelength being out-of-focus at the film plane by the very fact of their longer wavelength. Has anyone tried shooting a test focusing the lens more for IR than visible? That said has anyone stuck an IR pass (Vis cut-off) filter on a RED yet?

I'd be interested to hear whether or not the position of an IR cut off filter within a stack of filters effects performance in any particular way? Reflections have been mentioned...

JohnF
 
As someone who hopes to be working with the RED camera soon, I'm envisioning a quick and dirty test to see if the recorded image is being affected by infrared under various lighting set ups and in exterior locations.

Would a two part strip of black on the production slate do the trick? Half being black cotton the other half being black nylon (or any other combination of materials that reflect infrared at different wavelengths). The strip would appear to be solid to the eye and one would hope that it would photograph as a continuous strip. However when you see separation in the strip on the monitor then one would bring in the IR Cut filters and the Hot-mirrors.

Would this work, or am I missing something?

G.
 
Roberts said:
As someone who hopes to be working with the RED camera soon, I'm envisioning a quick and dirty test to see if the recorded image is being affected by infrared under various lighting set ups and in exterior locations.

Would a two part strip of black on the production slate do the trick? Half being black cotton the other half being black nylon (or any other combination of materials that reflect infrared at different wavelengths). The strip would appear to be solid to the eye and one would hope that it would photograph as a continuous strip. However when you see separation in the strip on the monitor then one would bring in the IR Cut filters and the Hot-mirrors.

Would this work, or am I missing something?

G.
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Some sort of test would be great... Or better still couldn't the camera itself tell us when it is recieveing a high proportion of IR to visable?
 
I spent four days with the RED One this week and ended up just keeping my Rosco IR filter in the matte box the whole time. Besides nipping IR issues in the bud, it served as my "clear" filter to protect the lens from dust, etc.

I met with Henry Cowen at the Rosco facility outside Austin. More later today.

Michael
 
Michael

Thank you for efforts. You guys are doing so much for RED community.

Joon
 
JohnF said:
Some here have already noted that some of their shots went "soft" whilst using ND's. This could very possibly be the IR channel getting "diffused". ...It may also be that the this softness might well be due to the IR wavelength being out-of-focus at the film plane by the very fact of their longer wavelength.
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I'd guess IR goes soft mostly because (1) different focal plane for vis & IR, (2) longer IR wavelength => more diffraction effect, and (3) IR penetrates deeper inside the sensor, and if it hits or scatters at a steep angle it might hit adjacent pixels. I don't know the relative contributions of those three.

IR diffusion at the filter is obviously possible, but my guess is that for any given filter, IR would be diffused less than visible light. In general, blue scatters most and red least (and IR even less than red).

That's why the sky is blue and the sunset is red... when you look straight up and see blue sky, that's scattered blue light that came out of the sun rays passing horizontally through the atmosphere overhead, leaving a less-blue and more-red sunset for the people who are standing east of you and looking towards the sun near their horizon. ...if you could see IR, you'd probably call the sunset more "infrared" than red.
 
Michael Morlan said:
I spent four days with the RED One this week and ended up just keeping my Rosco IR filter in the matte box the whole time. Besides nipping IR issues in the bud, it served as my "clear" filter to protect the lens from dust, etc.

I met with Henry Cowen at the Rosco facility outside Austin. More later today.

Michael
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Hi Michael

What size hot mirror did you use? It seems rosco only has odd sizes (i.e. 2x2 or 5.25 round?)

Was it a "tape job" in the matte box?

Still waiting for someone to test he Panchro mirrored ND's...

Thanks.

Dylan Macleod
Cinematographer
Toronto, Canada
www.dylanmcleod.com
 
Hi Dylan,

Not sure how this stands up (or how steep it'll be) for one-offs, but Rosco lists their hotmirror glass as available in custom sizes, so should be able to get a 4x5.65 (or whatever your poison) piece cut to order.

Will be telephoning Rosco on Tues (bank holidays all the way until then, unfortunately) to see if I can get a piece - will report back on price and time to fulfillment, etc...

Happy easter all,
Dom.

EDIT: Michael, is this what you did? What size filter are you using, out of interest? Cheers!...
 
Does the IR issue only manifest itself when NDs are in play, or are there other IR heavy environments that can have the same effect on the visible/IR ratio?
 
I ordered a custom 4x5.65" cut of their current hot mirror which cuts IR to around 925nm. The RED One is sensitive to around 1200nm so there is some room for an improved dichroic formulation.

264_1204995955.jpg


Rosco's common dichro glass is borasilicate (like picture glass) and chips a bit too easily in the matte box filter holder so something harder is needed.

I spent Friday morning with Henry Cowen at the Round Rock, Texas plant that produces all of Rosco's dichroic glass (as well as steel and glass gobo patterns) and we discussed the development of IR filters with one or more of the following characteristics:

Development priority list
 1.Block IR to 1200nm
 2.3.3mm thickness
 3.Harder Glass than Borosilicate glass
 4.Dressing of edges and corners or develop a ‘bezel’ to finish the edges of the glass
 5.Labeling of filter (possible laser etch of bezel)
 6.Flat and Parallel surface treatment
I'll be having a discussion with Joel Svendsen early next week about what market exists for Rosco in the optical glass world. My first impression is that there is no reason to compete directly with Tiffen and Schneider if they are prepared to bring IR solutions to market. There may be, however, an opportunity to bring a lower-priced filter to market that includes some, but not all, of the above features.

As I noted in my visual observation of the borasilicate glass I used this week, there was no reflection perturbation of fluorescent light when observed at twenty feet from the glass. (Of course, this is hardly scientific) When I consider the horrors imposed on the image by lens optics, I feel the over engineering of Schneider filters to be largely unnecessary (even though I have them in my matte box right now.) Would a lesser piece of glass be flat enough for most shooters?

Michael
 
Thanks Michael, interesting reading. So, you're finding (other than the strength of the glass) the Rosco useable for an intermediate solution until someone (Tiffen, Schneider) gets their IR-cut solution shipping?

You've basically done exactly what I'm planning to do, for short-term use, so I'd be interested to hear in detail your thoughts on how good a solution it is in terms of affect on IQ...
 
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