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 Using NDs for bulb ramping, How to minimize color shift? 
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Joined: Mon Sep 02, 2013 7:37 am
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Post Using NDs for bulb ramping, How to minimize color shift?
Hi All,
I'm hoping for some advice/tips for minimizing color shift when using NDs during bulb ramping.
Here is my setup:
I'm using a Canon 5D mkiii for a day to night. I'm controlling the camera with GB Timelapse (which is excellent!). I'm starting during the day with two 0.9 NDs on camera. Then they are removed during dusk as prompted by GB Timelapse. There is no perceptible density change when filter is removed, GB adjusts the shutter speed and ISO perfectly. There is a slight color change though. I'm using Schneider 4x4 0.9 ND which cost around $200 each and claim to have no color shift. I'm wondering if its due to IR leakage with long exposure (my exposures are around 2-3 secs at time of filter change)? Or maybe Schneider ND filters are not as good as they claim? I started out using cheap $50 Tiffen screw mount ND filters and they were even worse. I thought the Schneider would solve my problems but alas no.

Any ideas appreciated. Thanks!


Mon Sep 02, 2013 7:52 am
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Joined: Thu Feb 24, 2011 5:37 pm
Posts: 331
Location: Córdoba, Argentina
Post Re: Using NDs for bulb ramping, How to minimize color shift?
My guess is the filter has a color cast. Even the best ones out there have this problem.

You can solve this by shooting in raw, then you can precisely adjust the WB changes in post with LRTimelapse.

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Mon Sep 02, 2013 2:08 pm
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Joined: Sat Jan 22, 2011 10:16 am
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Location: Lancaster, England
Post Re: Using NDs for bulb ramping, How to minimize color shift?
leandroprz wrote:
My guess is the filter has a color cast. Even the best ones out there have this problem.

You can solve this by shooting in raw, then you can precisely adjust the WB changes in post with LRTimelapse.

It's an infra-red shift. ND filters let IR through more than visible light. DSLRs have a filter over the sensor that blocks most IR, but not all of it. As more IR gets through vs visible light, you get that magenta shift. So, it's more than just a white balance correction.

Tiffen do some IR blocking ND filters, and they do make a big difference. But you could probably correct for it just fine with regular filters using a ColorChecker Passport, although you'd need to do exposures on the CPP of the same shutter duration and in the same light to get accurate profiles.

photoewing wrote:
I'm using Schneider 4x4 0.9 ND which cost around $200 each and claim to have no color shift. I'm wondering if its due to IR leakage with long exposure (my exposures are around 2-3 secs at time of filter change)? Or maybe Schneider ND filters are not as good as they claim?

They are as good as they claim, they're just not designed for these sorts of exposures. They're designed for film & video, where your shutter speed is rarely going to be slower than 1/50th of a second, to bring super bright daylight conditions under control.

I have the 1, 2, 3 & 4 stop set of Schneider 4x5.65" filters that I use in a Z-Pro holder or a Matte Box (depending on whether I'm shooting stills or video), and they also shift on long exposures. For video, they're great, because not a lot of IR is getting through relative to the visible light. For really long exposures, there's definitely a shift.

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Tue Sep 03, 2013 12:11 am
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Joined: Thu Feb 18, 2010 4:47 am
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Location: St Gallen, Switzerland
Post Re: Using NDs for bulb ramping, How to minimize color shift?
I bet you`ll find that now that Mike at GBTimelapse has integrated aperture ramping into his software , he`ll no longer bother with nd filters, for the reason you have brought up.


Tue Sep 03, 2013 2:25 am
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Joined: Mon Sep 02, 2013 7:37 am
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Post Re: Using NDs for bulb ramping, How to minimize color shift?
Thank you Kaouthia,
Thank you, Thats super helpful, I suspected it was an IR issue. So I should probably buy IR blocking NDs? Wish I had figured this out before I bought these $chnieder filters. Is there a IR filter I can add to my stack w/o needing buying two new very expensive IR NDs?

Ian: I'm using NDs to keep my day time exposures at .8 sec for a number of reasons; 1. to stay in bulb mode during entire time-lapse for less density flicker, 2. inaccuracy of bulb exposures is more significant bellow 0.8 sec, 3. longer day time exposures, blur motion and make for smoother films I'm really liking the look of the long exposures.

Maybe I'm doing this wrong though, maybe I should try to keep my daytime exposures at around 1/50 then switch to bulb after sunset where IR would be less of an issue? I think this would result in more density flicker though. I'm trying to make the smoothest possible set of images that require the least post-production.

Timelapse is such a strategy game! Thanks all for your advice, I'm curious to here your additional ideas and comments.


Tue Sep 03, 2013 6:39 am
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Joined: Mon Sep 02, 2013 7:37 am
Posts: 3
Post Re: Using NDs for bulb ramping, How to minimize color shift?
would this solve my problem?:
http://www.bhphotovideo.com/c/product/9 ... t_680.html
or this?:
http://www.bhphotovideo.com/c/product/567980-REG/Schneider_68_121044_4x4_Tru_Cut_Ultraviolet_UV.html


Do I need to block UV light as well or just IR to avoid color shift w/ ND during long exposures?

Thanks,


Tue Sep 03, 2013 6:46 am
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Joined: Thu Feb 18, 2010 4:47 am
Posts: 388
Location: St Gallen, Switzerland
Post Re: Using NDs for bulb ramping, How to minimize color shift?
Quote:
longer day time exposures, blur motion and make for smoother films I'm really liking the look of the long exposures.


Yes, well there`s probably no way around nd`s if you want to ramp from 0.8 sec during blinding daylight to full astrolapse exposures, unless:
you split your time lapses in half and separate them with a clip of a sun on the horizon for example (you could adjust the aperture between the halves - you might get close to 0.8 sec down at f22 iso 100, depending).

Quote:
inaccuracy of bulb exposures is more significant bellow 0.8 sec,


Not if you do mx2 bulb ramping in GBT. Then you can accurately get down to 33ms or whatever the minimum accurate bulb time of your camera model is. I believe the emotimo can do this too. But of course you`d need one of these bits of gear on top.


Tue Sep 03, 2013 10:21 am
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