ND Filters

[David Gascon]

I have a simple question, how ND filter affect light. Are you loosing any of the light property or it's only simply cutting light. Also, from your experience, would it be better to use ND on camera directly or on light? Thank you. I know it is basic but... need to know.

[David Mullen ASC]

ND filters evenly cut the amount of light. They don't affect color or contrast.

 

While you can use ND gel on a lamp, it is more common to use wire scrims to cut their intensity.

 

You can't use ND lighting gel on a camera because it is not optically good enough. You would use ND camera glass filters or ND behind-the-lens Wratten gels IF your camera could take behind-the-lens gels.

[Laurent Andrieux]

ND means neutral density. That means it only cut light, doesn't affect the color.

 

The most used grades are ND 3, that cuts 1 stop, ND 6, two stops, ND 9 three stops etc...

 

Gelling a source will only affect the source and will cause a loss of electrical power (since a ND 3 cuts one stop, it means it devides the light intensity by two, then a 1 kw projector acts like a 500 w one, it's a waste of electrical power) but it's sometimes so quicker to put a gel than changing the projector... when you made a mistake and find the intensity is too strong... (a dp shouldn't make the mistake of setting a source he then has to be powered down, but should set the proper one that gives the amount of needed light).

 

We sometimes put ND Gel on windows. You also have CTO (85) + ND gels for both powering down the incoming light and convert from daylight to tungsten)

 

Putting a ND filter on the camera will affect the whole image. It is somehow a way to cut down the stock sensitivity. We also have combined filters : 85 + N3, 85 + N6...

 

It's mostly done with exteriors where light is strong and you don't want to work at too close Iris F stop., and not changing the stock to a lower sensitivity.

 

Definition : D = log (opacity), where opacity is incident flux (lumen)/ transmitted flux (lumen). You see that a filter that devides the light by 2 has a Density of Log (2) = 0.3 that explains the figures. Since you can add Densities, it becomes 0.6 for the loss of 2 stops (light devided by 4 =2.2 remember log (aXb) = log a + log b ; log (a^ b = b.loga) etc...

[David Gascon]

Hey thank guys, that make sens and you made it clear for me. It's what I was thinking, but wasn't to sure if there were a twist that I couldn't tell. Thank again.

 

David.

[Michael Nash]

but it's sometimes so quicker to put a gel than changing the projector... when you made a mistake and find the intensity is too strong... (a dp shouldn't make the mistake of setting a source he then has to be powered down, but should set the proper one that gives the amount of needed light).

<{POST_SNAPBACK}>

 

 

I don't agree with that. There are many different reasons for choosing a particular unit; the light intensity is only one. For one thing, it's much easier to set a light that's slightly brighter than what you need, because you can then lower or raise the intensity by adding or removing scrims. If you use a lamp that just gives you the right stop, you need to set another lamp if you should need more light for some reason (like shooting at a higher frame rate). Things change quickly on a film set, so you're best off with a little flexibility. The AD loves it when a lighting tweak is just a scrim-pull away, instead of waiting for another light to be hung.

 

Another reason you might pick a too-bright unit is for the size and spread the fresnel lens gives you. For example, a 10K fresnel does a nice job of filling up a 12'x12' frame in a moderate amount of space, even if it's too bright for the scene. If you were to use a smaller unit like a 5K fresnel, you'd have to back it off almost twice the distance to get the same effect as a scrimmed 10K.

 

Sometimes you're at the mercy of what units are left on the cart, because everything else is working.

 

But mainly it's easier to use a big light and knock it down, rather than have to replace a light if it's not bright enough. Believe me, a gaffer ends up looking far worse when he has to swap out a light because he chose the wrong unit, rather than simply dropping in a scrim! ;)

[Felipe Perez-Burchard]

Hi,

 

In my student experience, for whatever that's worth, ND filters DO shift the color slightly depending on the brand. I did a test a while back, if I remember correctly, Tiffen were just slightly green and I think the other set we had was Schneider and that was slightly warmer.

The effect was just slight enough that you could easily time it out.

Do you guys think this was due to old equipment or is there any validity to my observations?

 

-felipe.

[J. Lamar King IMPOSTOR]

I agree with Michael, I go for the biggest appropriate instrument.

 

As far as ND filters go, remember to make positive an ND camera filter is pure and has no color cast. Which may not be easy, see the last post in this thread for examples:

 

http://www.cinematography.com/forum2004/in...p?showtopic=416

 

It was very hard to see these color casts with the naked eye.

[Sam Wells]

"Do you guys think this was due to old equipment or is there any validity to my observations?"

 

No your observation is valid IMO.

 

There can be a subtle shift within brands.Nothing you can't fix.

 

-Sam

[Laurent Andrieux]

OK Michael.

 

This is not much the way I do it, I'll tell you why.

 

1) There is a lot increase in production cost doing this. If you double any source you set, it's cost twice the price - for what's about the energy costs. If you are used to doing so, it's means you rent the double of the average requirement, rent a generator that is the average double of your real needs, etc. Since the gaffering job is more tough as sources get bigger, you might need one or 2 more gaffers than necesseray, need a truck that is about the double size or close, etc. All that increases the productions costs.

 

I know that there is one kind of shooting you can - or may be even must - do this is commercials / adverts shooting.

 

I guess you mainly work on adverts / commercials, or rich feature lengths.

 

I must confess I don't.

 

I've been working on such shootings as an AC,I worked with Bernard Lutic on a feature, (God bless his soul, such a great man ; I think I can tell this, it wouldn't do him no harm anymore) and he was doing some adverts too. He used to laugh at production, DA etc, being impressed by the number of sources you 'd put on the stage, like if the more sources you put, the powerfull they are, the "better" you are. He used to even set light that wern't in the frame, just to justify the presence of a source and make the set more brighter, like a christmas tree! It's true that on this kinda shootings, you don't want a projector to break or cause whatever problem, so you always take more than necessary, "just in case", and if you do cover yourself, you take just the double you need.

 

2)

if you should need more light for some reason (like shooting at a higher frame rate)

 

On the kind of production I work on, you don't improvise this kind of thing. The main camera is a silent one, that go at 32 fps maximum, unless you rent an additional feature that costs without you to necessarly use it. On the other hand, this kind of thing happens, tipically, on commercial films. That makes feel more right even thinking you mainly work on commercials.

 

Any other reason for a sudden need for a total light change would make express the same advice : you don't improvise this kind of thing, one should have worked with the director (even a DA) at firts and shoot the prefigured effect, you don't always have everything for whatever of a director's sudden idea on the set or in the truck.

 

3)

Another reason you might pick a too-bright unit is for the size and spread the fresnel lens gives you. For example, a 10K fresnel does a nice job of filling up a 12'x12' frame in a moderate amount of space, even if it's too bright for the scene. If you were to use a smaller unit like a 5K fresnel, you'd have to back it off almost twice the distance to get the same effect as a scrimmed 10K.

 

I totally agree with you. Using a 5 K instead of a 10 K would only make me work one stop more open, if I'd have to suddenly lower it, I'd put a DN gel or a grid (We don't do this much in France, I like the idea)

 

4)

Sometimes you're at the mercy of what units are left on the cart, because everything else is working.

 

Then you use what is left... I do the same. It's a question of basical list. I didn't say anytime I didn't like big sources...

 

I actually like better (much better) having a 10 K thru frame at some distance than a 5 k closer ! bouh ! I like when sources are powerfull, far away, as the comedians can move whatever the Director wants, without them getting over/under exposed ! I just consider putting an ND on a source is not on my basic operations.

 

5)

But mainly it's easier to use a big light and knock it down, rather than have to replace a light if it's not bright enough

 

You also can move it a little bit futher sometimes, or enlarge it

 

6)

Believe me, a gaffer ends up looking far worse when he has to swap out a light because he chose the wrong unit, rather than simply dropping in a scrim!

 

Absolutly ! this is the kind of situation I was saying it would happen ! But I consider it's a mistake, and of course I consider you sort of never - or only if no other solution is avaiable - ask a gaffer to change a source ! I f you made the mistake at the first place, put a gel or grid ! But someone whos knows lighting can figure out you've made a mistake, say if you put a ND 6 on a 4 kw HMI !

 

 

Very interesting post, thanks for the reply !