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doubling ND IR filters?

filter NDIR

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#1 christian mann

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Posted 16 March 2014 - 04:30 AM

Hi everyone,

a DIT told me  one shouldn't use more than one ND IR at the same time and rather use straight NDs in combination with one NDIR in order to get your stop compensated. But I was wondering why? Color shift?

And why actually ND IR only from 1.2 an up ? It is not like there isnt' infrared light out there if I use a ND9...

 

Thanks for your input,

M


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#2 Adrian Sierkowski

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Posted 16 March 2014 - 10:11 AM

It's all about ratios (and they do make lowerNDIRs I think). So as you decrease the visible light, the IR becomes more apparent, 1.2 is a good "start point" for where you actually start to see this effect.

 

I don't see how a double NDIR would matter at all as far as I can figure it-- aside from it just taking up a lot of room in your filter trays. But truthfully, why would you want to combo ND filters? It's better just to grab the one you need, or the one which is closest to that. I used to, when I didn't have enough money to buy my own 4x4s on film, to get the stop down, I'd use a Pola just for the stop compensation needed.

 

If you wanted to block all ND all the time, you can use a HotMirror filter, or just an IR filter. I find that makes things look muddled though, but that's just me.


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#3 christian mann

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Posted 16 March 2014 - 02:26 PM

ja, here in South Africa the sun can be quite intense so we easily end up with very high f stops. for example the F5 has a ISO setting of 2000....so at times you need to stop down sth. like 10stops in order to get anywhere close to f2.0. Hence using for example a NDIR 1.8 AND  a NDIR 1.2


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#4 Adrian Sierkowski

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Posted 16 March 2014 - 09:16 PM

Doesn't the F5 also have the build in NDs? Also it's only about a stop and a third over an Alexa's 800, which I normally throw a NDIR 1.5 on to get in line, then go with the internals.


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#5 christian mann

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Posted 17 March 2014 - 02:15 AM

yes, F5 has internal NDs. But just for arguments' sake, if you need to stop down +/-10stops; can you use 2 ND IRs without creating color shift or other side effects?


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#6 Adrian Sierkowski

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Posted 17 March 2014 - 08:24 AM

You would have to test, but think of it like this, once you remove all the IR with your first NDIR, the second one is just an ND filter with an added bit. But since you've already removed all the IR with the first filter, the second filter isn't going to have anything to remove but visible light.

That said, every filter has a color shift, it's just a question then of how bad of a color shift you're getting. That would depend on brand of filter. I've had Formatts straight NDs which just turned everything green (I chucked 'em quick!).


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#7 christian mann

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Posted 17 March 2014 - 09:02 AM

ja, that was my assumption that a possible color shift is rather caused by the (ND) filter than a second IR...

 

thanx,

m


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#8 Adrian Sierkowski

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Posted 17 March 2014 - 09:05 AM

My pleasure. Though also if you wanted a 10 stop you could do a NDIR2.1 and a Pola ;)


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#9 Ben Rowsell

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Posted 17 March 2014 - 04:36 PM

IR or "Near Red" is always present, its just that the camera only starts to see it at heavier levels of ND, often from 1.2 onwards but thats not to say you won't see it at .9, .6 or even 3 under the right circumstances - just not often or its very unnoticeable. Thats why general thinking is that it's not a problem until 1.2.

 

The heavier the ND, the more of a colour shift you get as most gel based ND is not made from a neutral colour, but rather very heavy shades of off colour gels (purple, green, blue etc). On top of this some of the IRNDs such as Tiffen use a colour correction element (green) to combat the IR so you see that tint as well, which can be corrected in camera by introducing a magenta shift.

 

So if you stack a IRND that has a colour shift on top of another IRND that also has a colour shift then you will see it even more which may be visually undesirable/unacceptable. In the case of the 2nd filter being less than 1.2, you may well be able to get away with using a non-IRND which will overall have less of a colour shift so will give a better result.

 

Having said this, IRNDs less than 1.2 tend to be a lot more neutral than their heavier counterparts (1.2 >) anyway. IR pollution is a lot harder to correct in post than the shifts introduced by using IRND filters.

 

With a camera such as the F5, I would be inclined to use the internal filters to get you to a decent base level, then supplement with either ND or IRND filters on top of those. The Sony sensors seem to be quite resilient to IR pollution so you could probably get away with non-IRNDs to .9 or even 1.2

 

I have found that the internal NDs on the Sonys (F65/F55/F5) are all neutral and IR protected to their level of ND, but any additional external filters will introduce IR to the system - it just might not be enough to notice / be a problem.

 

The internal behind the lens filter module for the Alexa is also excellent (no colour shifts, consistent stop loss, full IR protection, etc).

 

The external filters that are the best are supposedly the TrueNDs, that use a metallic cut system but I have not yet seen them and they are very expensive to buy.

 

As pointed out you can also use Polas to being the level down but they do have an effect on the reflected light in the picture and that effect will vary as the camera moves relative to the light.

 

Ultimately it's the choice of the DOP as to what approach to take as it all effects the final look, but it pays to be informed so you can answer any questions from a technical standpoint. Also remember that if they are any good, your DIT is probably the best person to critically (technically) analyse the image as you are only seeing it through an onboard monitor or EVF, so their opinion is also valid.

 

Ben R

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#10 Brian Drysdale

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Posted 18 March 2014 - 02:29 AM

Just to add further information on the variations found in IR ND filters, here's an Art Adams article on the subject together with some tests he's done: http://www.dvinfo.ne...e-them-all.html


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#11 christian mann

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Posted 18 March 2014 - 05:44 AM

Thanks Ben for elaborating.

I just read the article written by Art Adams; on page 1 he concludes "You can mix hot mirrors with regular NDs, but you can’t mix IRNDs with regular NDs."

Didn't we just establish that using one IRND in combination with straight NDs is the way forward to avoid color shift etc?

 

And on another note: on page 3 Art says "IRND filters do not work on RED cameras." which makes sense since you need to use hot mirrors. He continues saying "These filters(hot mirrors) only work on RED cameras." So now I was wondering what effect a hot mirror would have when used with a ALEXA ? Doesn't a hot miror cut light from 680nm up to 750nm hence it should work like a IRND only that it would be a small overkill since the ALEXA doesn't have that leak over 700nm and therefore doesn't "need" a hot mirror ?


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#12 Ben Rowsell

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Posted 18 March 2014 - 03:43 PM

Re: IRNDs + NDs, I think what Art probably means is that ANY standard non-IR ND will be letting IR through, so if you use one there is a risk of pollution. You can see from his tests that this can even be on a ND3. I would be fairly confident on knowing what the brand that does so badly on the tests are as I have seen similar results. But keep in mind that he does a fairly harsh test - synthetic black fabric under tungsten light with NDs, so you are unlikely to see that say in a day ext setting (with a ND3 or ND6).

 

What I meant is that although the above is technically the case, in normal non-test scenarios you may fine that the combination of IRNDs and low level NDs (.3/.6/.9) give you a more desirable result visually as you are not ALWAYS effected by IR pollution and there may be less of a colour shift. If you were pedantic then you would say you have to use only IRNDs, but there is so many variables and only the end result matters.

 

Re the cut Hot Mirrors, I haven't had to use them recently so would be catching up on what the latest is. I know that Dragon is going through quite a few early changes on OLPF, etc, so it might be different to the standard Epics, I have heard less effected by IR.

 

I don't think it would have an adverse effect to put a hot mirror on the Alexa, apart from risking cutting near red in the 680-700nm zone, if the filter worked in that area. (i'd have to check but I though they targeted 700-750nm)  As Art says, Arri intentionally didn't cut this part of the spectrum (680-700) as it has an adverse effect on skin tones, so we have to deal with it by filtering when it becomes a IR problem.

 

Art is a pretty thorough tester and a very good source of technical knowledge.

 

Ben R

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#13 Adrian Sierkowski

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Posted 19 March 2014 - 02:10 AM

Last time I was on an Epic in day exts here we used a hotmirror for a hot second before I pulled that thing out because it just made everything look, in the words of my 1ac "blergh."


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