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ND filter tints image red


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#1 Tyler Purcell

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Posted 15 April 2015 - 02:35 AM

Hey guys

Got an odd one for ya.

So I've got these cheap polymer ND 4x4 filters for my matte box. They range from .3 - .9 and I use them because I do a lot of sports cinematography and they get destroyed quick. I've already gone through three of them and they're so cheap, it's no big deal to throw them in the trash when a rock or tree hits the camera (which happens often).

When you hold them up to the light, they simply lower the brightness of the light. To the naked eye, they appear to be doing the right thing.

However…

When you shoot through them with a camera, they tint the whole image red. They almost eliminate the green and blue channels, it's quite amazing. I always have to do major color correction of my material, especially in the black levels, in order to work in the missing color.

Now, I've shot with ND filters for my entire life and never seen this phenomena. Initially I thought it had something to do with the native color balance of my cameras, but after some recent testing, it's absolutely not that. So I figured out a way of making my own LUT which fixes the problem, but anything that's black has a brown/red tint to it no matter what I do.

Here is a still image right out of DaVinci, after applying a simple 3D LUT, the base RAW to Rec709 LUT I use for everything.

catalinared.jpg


Here is a video I shot recently where I fixed the problems in post. Notice in shots with bright sunlight, the blacks are more muddy brown then red. Look especially at the guy with the green kawasaki shirt. The upper part of his shirt is green, the bottom is black… but in the video it's red/brown. 

 

 

 


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#2 Mark Dunn

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Posted 15 April 2015 - 03:45 AM

The dye isn't quite neutral. Cheap polaroid filters also tend to have a cast. You get what you pay for, I expect.


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#3 Satsuki Murashige

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Posted 15 April 2015 - 04:06 AM

This happens all the time with cheap (and sometimes not so cheap) filters. I used to use Cokin acrylic ND filters and had the same issue. Among 4x5.65 filters, I've seen Tiffen NDs from a a decade ago that were green and Schneiders from a few years ago that were magenta. The Tiffen White Water Glass NDs that I use now are fine. A set in 77mm screw on is only $250.
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#4 aapo lettinen

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Posted 15 April 2015 - 05:17 AM

this is very common with cheap filters. there is a bit of variance though, some of them tint the image towards GREEN not magenta  :lol:

 

cheap glass filters can also have colour problems. I have at least 4 different kinds of glass filters and 4 or 5 kinds of plastic filters (Cokin, Chinese stuff, etc) which all have more or less unpleasing colour cast. some of them have so badly tinted image that I can't use them at all...


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#5 Lance Soltys

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Posted 15 April 2015 - 07:11 PM

Doesn't this seem like a classic case of IR pollution? Are you using IRND's? I got a cheap set of ND' s and I get this all the time. I got a cheap hot mirror but it doesn't seem to help much. Maybe this is stupid, but I'm thinking about hanging on to the cheap filters and get a good IR filter. Does this make sense? Since I mostly work just as a hobby, I can't justify $200 on one filter.
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#6 Tyler Purcell

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Posted 16 April 2015 - 01:14 AM

Interesting, I kinda suspected I wasn't alone with this quandary. 

 

So… what's the suggested course of action?

 

I need to reduce the amount of light getting into the camera by at least 4 - 5 stops, as the damn Blackmagic Pocket camera is so sensitive, my lenses are always all the way closed with the camera set at the highest shutter speed (lowest angle) and lowest ASA. Plus, I'm about to get a speed booster, which will make the camera even more sensitive. 

 

Is there any other type of filter which will work to bring the stop down that won't have this problem? Has anyone tested any matte box filters that work well that aren't a few hundred dollars? I don't know if these plastic ones are IRND's, but that's absolutely worth looking into and a very good point because it does seem like IR interference. 

 

Thanks for the help! This is the first time I've tried to use shitty/cheap filters. 


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#7 Adrian Sierkowski

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Posted 16 April 2015 - 02:40 AM

The real solution is to get some good filters. My NDs are Schneider (regulars up to .9, then the 1.2 and the 1.5 are IRNDs).

I hear good things about the firecreast as well.

Personally, I use a nice MB; but you could get by with a cheap 4x4 MB of some kind and save some cash on the filters-- if you're really just looking for a filter holder.


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#8 aapo lettinen

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Posted 16 April 2015 - 04:31 AM

if reflection is not a problem you can also get custom made cheap glass 4x4 plates which fit to the matte box and use them in front of the filter. I used polycarbonate pieces for this earlier but then I had an optical company made me some 4x4 and 4x5.65 glasses out of normal good quality window glass. it could be something like 10 - 20 usd a piece for small runs and if you order more they will definitely give you a discount :)    this was with dimensions finished with 1/10th mm accuracy, if you can do with rougher finishing I believe they would cost a lot less.

 

Would it be possible to fit a 2 to 4mm thick polycarbonate protection layer over a good quality nd filter using some kind of oil as a fitting layer between them for preventing reflections? 

This would not of course protect you from tree branches and huge rocks but flying debris would just bounce back from it I think and you could easily replace it with a new one quickly  :lol:


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#9 Lance Soltys

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Posted 16 April 2015 - 08:15 AM

There was a great test by CaptainHook on BMCuser about this and he had test shots with the BMPCC and different brands/amount of ND with different IR filters.  Sadly, he was testing screw-in type filters, though I do think I remember a square filter test somewhere too.

 

http://www.bmcuser.c...ull=1#post97497


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#10 Peter Phillips

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Posted 16 April 2015 - 11:18 AM

I've got to admit it's an interesting look. If you know your filters are going to get damaged, it makes sense to keep the price down. My only idea would be just to shoot a color card before you use it, balance it out in post like you already have for the most part. I'd also recommend a stronger ND to maintain a 180 degree shutter angle, depending on if you're shooting high speed or not. 


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#11 Lance Soltys

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Posted 16 April 2015 - 12:00 PM

I found at least one string on square filters.  These are Skier filters and they are IRND's, but I guess now they make a plain IR cut with no ND, which is what I'm thinking of trying.  It's around $160.  They seem to get good reviews, but i have not used them myself.

 

http://www.bmcuser.c...Tiffen-vs-Skier


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#12 Tyler Purcell

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Posted 16 April 2015 - 01:21 PM

Thanks guys, 

 

Yea I have a 4x4 matte box which has two filter holders. I can't use screw on because every one of my lenses has a different front diameter. 

 

Sounds like the only real solution is to buy a piece of glass, which isn't a big deal. I'd just have to be conscious of filter care in the future and duck when tree's or rocks hit the camera! :) 

 

I've never been forced to shoot with stronger ND's, are there any other side effects I should be aware of? I'm thinking of doing something like a 2.0 or something around there because the sun is too damn bright around these parts. 


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#13 Tyler Purcell

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Posted 16 April 2015 - 01:23 PM

Would this be OK? 

 

http://www.amazon.co...4x4 IRND filter


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#14 aapo lettinen

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Posted 16 April 2015 - 01:39 PM

Thanks guys, 

 

Yea I have a 4x4 matte box which has two filter holders. I can't use screw on because every one of my lenses has a different front diameter. 

you can still try to hack it if you use only one filter at a time: http://www.ebay.com/...=STRK:MEBIDX:IT

 

"if you can't get it from China you don't need it"  :lol:


Edited by aapo lettinen, 16 April 2015 - 01:41 PM.

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#15 Adrian Sierkowski

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Posted 16 April 2015 - 01:42 PM

What you can do, Tyler, is get a cheap optical flat glass filter (white water clear) and put it infront of everything else for such situations-- it's what I do at least.

 

 

I dunno if you need a big stopper 2.1ND-- it's a nice filter, but I'd look more for like a 1.2 or a 1.5 ND as a base. I normally use an NDIR1.5 whenever I'm day ext on d-cinema cameras these days.

 

 

p.s. what lenses are you on these days and would you ever wanna try to play with some PL S16mm optics?


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#16 Tyler Purcell

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Posted 16 April 2015 - 01:47 PM

Yea, I always run a "protection" filter anyway, but believe it or not, I've had shit hit the lens and damage both filters! But thats plastic, the glass ones are a lot thicker. 

 

I wanna run my camera's more open, so I can have a more shallow depth of field. So that's why I was thinking of doing the 2.0 ND because that would make a HUGE difference. Plus, I'm gonna be buying a speed booster soon, so that will make my problem even worse. I never thought I'd have a cinema camera that required filtration! With film for me, it was always the opposite problem, always needed lights outdoors! LOL :) 

 

I'm using the Rokinon's still. They're OK for the type of stuff I shoot, though I'd love to hook up and try some PL optics someday. I don't have anything booked, but maybe on my next pay shoot I can bring you in to help! :) 


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#17 Adrian Sierkowski

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Posted 16 April 2015 - 01:55 PM

That'd be fun. But even aside from that, as I get the new apartment settled, you should stop by for a coffee and we can throw my mount on your camera and some little glass. Compare what we have "in our pocket" lol.

the 2.1 is a nice filter-- i still think it might be slightly overkill-- if you're at 800 base than a 2.1 filter will drop you 7 stops, basically around 6 ASA, which is very open-- but if that's the look you're after, go for it and I may need to borrow that 2.1 one of these days.


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#18 aapo lettinen

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Posted 16 April 2015 - 02:00 PM

this is what I got custom made for me in an optical company: 

17144059206_2d873ccf49_b.jpg

 

It's just some scratch-less 2.9mm thick window glass cut to 4x4 dimensions and the edges ground so they won't cut your fingers :)

 

If you expect your flats to get scratches this is a way more cheaper way than buying real optical flats, although you could maybe spot small differences in glaring and transmission maybe.

 

I use these for for example vaseline effects so I don't need to smear my Promist filters :)


Edited by aapo lettinen, 16 April 2015 - 02:01 PM.

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#19 Adrian Sierkowski

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Posted 16 April 2015 - 02:08 PM

I'm thinking, honestly, about making my own filters. They have glass cutting kits cheaply and I finally have some room to do it.


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#20 Freya Black

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Posted 16 April 2015 - 03:14 PM

Yup looks like IR pollution. Was a big thing on Red cameras back in the day hence all the IRND's that are around.

However some people also buy the cheapest and nastiest ND's as effect filters because they like the strange look they give.

 

Freya


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