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Focus in a Dark Room Scene-Red Light


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#1 Allen Achterberg

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Posted 10 September 2007 - 11:21 PM

I'm pulling on a feature. Something came up today that I'd never really thought about or really heard about. It was a Dark Room Scene, flooded with red light. The DP Asked if I knew that there was a Difference in focus distances due to us exposing mostly only the red channel. I said yes, but I did not. So I called around to learn the scoop on this before I just ignored it.

What I was told was to Eye Focus. That's what I did while shooting the scene. I'll hear about the dailies tomorrow I'm sure.

Anyone have experience with this want to help out a bit?

Thanks in advanced!
Allen
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#2 Jamie Metzger

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Posted 11 September 2007 - 12:28 AM

Eye focus :huh: ??

If you were shooting film on lenses that have foot/meter markings you should have gone off of them. There is nothing changing about the focal distance and the distance to your subject just because of a red color. I'm pretty sure that digital see's red differently than film, and on both formats red is not as contrasty and gives the effect of never being crisp. Let us know what the dailes look like.
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#3 Jonathan Bowerbank

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Posted 11 September 2007 - 04:18 AM

Focal distance is always the same no matter what color light you're using...don't know why that would even be assumed.

Shooting video though, red does appear softer because of how much it bleeds. So focusing and getting it as sharp as possible is always preferred...as with any other color of light.
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#4 David Auner aac

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Posted 11 September 2007 - 06:09 AM

I'm pulling on a feature. Something came up today that I'd never really thought about or really heard about. It was a Dark Room Scene, flooded with red light. The DP Asked if I knew that there was a Difference in focus distances due to us exposing mostly only the red channel. I said yes, but I did not. So I called around to learn the scoop on this before I just ignored it.


Hi Allen,

I have never heard of focus marks being off with VISIBLE red light. You can forget your marks when using INFRARED light and infrared film, but I doubt you'd do that on your shoot. Maybe your DP was confusing red with infrared or she/he wanted to tease you...

Cheers, Dave
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#5 Mike Panczenko

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Posted 11 September 2007 - 08:23 AM

[quote name='David Auner' date='Sep 11 2007, 03:09 AM' post='192811']
Hi Allen,

I have never heard of focus marks being off with VISIBLE red light. You can forget your marks when using INFRARED light and infrared film, but I doubt you'd do that on your shoot. Maybe your DP was confusing red with infrared or she/he wanted to tease you...

The focus marks will be off and you should eye-focus. Because it is so far to one side of the spectrum, pure red light, as in a club scene with super saturated red, will look soft. I would have eye focused, because that seems like the best way. You couldn't even really relabel your lenses in prep unless you knew the specifics of exactly how red the scene would be, etc. I think eye focus was the way to go.
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#6 Sam Wells

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Posted 11 September 2007 - 12:18 PM

You'd be surprised how fast some lenses misfocus at the deep red end of the spectrum.

-Sam
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#7 Allen Achterberg

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Posted 11 September 2007 - 12:24 PM

thanks for the responses. the lady confused the hell out of me on that one. I thought I'd ask around because she was pretty strong about making sure I knew what she was talking about.

Thanks!
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#8 David Auner aac

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Posted 11 September 2007 - 12:27 PM

You'd be surprised how fast some lenses misfocus at the deep red end of the spectrum.


Really? Anyone can explain that & it's cause in optics/physics terms?

Cheers, Dave
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#9 Allen Achterberg

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Posted 11 September 2007 - 12:29 PM

You'd be surprised how fast some lenses misfocus at the deep red end of the spectrum.

-Sam



So Some Lenses will do this? Is this something that should have been tested before we shot the scene?
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#10 Chris Keth

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Posted 11 September 2007 - 02:53 PM

So Some Lenses will do this? Is this something that should have been tested before we shot the scene?


Yeah, some will. It maybe should have been tested. Most modern lenses, though, should be well enough corrected that you'll be fine. Another problem with shooting in very red light is that even if it is in focus, it can look soft to your eyes since we're really not all that good at seeing in red light. Without some green light coming from something, our detail vision kind of goes out the window.

Hope everything comes out fine.
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#11 Miguel Bunster

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Posted 11 September 2007 - 09:20 PM

When shooting purely red the is a lack of definition in that layer in film (as i udnerstand and red and di a test ssome time ago). What some dps do (talked about it) when shooting dark rooms with jsut red light, they will actually shoot for example with magenta which will expose more of the three layers and not just the red layer of the film stock and in post color time it to red.
But if you shoot pure red (one wavelength in theory) it will or shoud look softer even if you hit the focus right....considering your lens works fine.
BEst
M
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#12 David Auner aac

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Posted 12 September 2007 - 01:35 AM

After thinking about this for a while I think I know what causes this. It's the same phenomenon that causes chromatic aberration. Every color has it's own focus point behind the lens. To compensated for using red only you'd have to adjust focus. But what I am surprised at is that that's enough to really make your images soft. What range of adjustments are we talking here?

Cheers, Dave
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#13 Jonathan Bowerbank

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Posted 12 September 2007 - 03:07 AM

Perhaps that was Rosco's thinking with their CalColor line of Red. It actually leans towards orange just a bit, and as CalColor's were tested and formulated especially for film emulsion and for replicating what we see onto the film, it now makes perfect sense.
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#14 Clément Brewer

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Posted 12 September 2007 - 01:59 PM

Hi,
I think as far as film is concerned, red footage will always be soft because of the very structure of film stock. Red is the last layer, so when it's exposed the light has already been diffused twice (by the blue and green layers). Kodak and others improve this by tweaking the thickness of each layer and its content. (Check out the MTF curves)
I don't know how it goes for video.
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