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JUST A FEW QUESTIONS


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#1 Ckulakov

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Posted 17 July 2005 - 09:51 PM

I have heard some filmmakers say they use a ND filter to "soften the video look". But doesnt a ND filter just decrease the amount of light hitting the lens?

If I use a 500w photoflood, and put a full CTB on the barndoors how many watts will be the light worth, or to what wattage will the light be compared to? I know it tells me the fstop loss but I need to know it by the wattage?

Also I have a tiffen, 1/2 black promist filter. Can I use a normal 'lens wipe' to clean it or will it ruin it?

thanks

Edited by Ckulakov, 17 July 2005 - 09:52 PM.

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#2 Laurent Andrieux

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Posted 17 July 2005 - 10:02 PM

Yes, a ND is supposed to only cut light, not to alter the image in any other way. If the filter is of bad quality, or old, it might alter it, occuring a loss of detail, accuracy and contrast. But ND is not supposed to do so.

A full CTB cuts about 2 stops, cause it devise the intensity by 4. You then can consider it's equivalent to deviding the lamp's power by 4 as well, leaving you the amount of light a 125 w would give you, when you put it on a 500 W.

Using the same materials for cleaning filter and lenses is a good thought. That's what everybody does.

Regards

EDIT : Just thinking : using a ND to soften the video look can refer to the fact that using a ND reuires to open the iris, therefore giving you less depth of field. May be this is what was refered to...

Edited by laurent.a, 17 July 2005 - 10:04 PM.

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#3 timHealy

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Posted 17 July 2005 - 10:21 PM

Video tends to have a larger depth of field than its film cousins so that if one uses an ND filter, you can open up the lens and decrease the depth of field, and defocus the background.

Many believe that this helps to make something shot in video look like film. I don't agree with this idea in that if you look at a film like Citizen Kane, which Orsen Wells is well known to have had a huge depth of field, it still looks like film. It doesn't look like video. So personally, I believe depth of field has nothing to do with making a video look like film or vice versa.

On to your second question, what are you trying to achieve with trying to figure out how CTB affects wattage? They really have nothing to do with each other. But if I were to hazard a guess, if full blue cuts down light by two stops, then it might be comparable to a 125 watt size bulb.

And yes you can use a lens wipe on a filter.

Best

Tim

PS Soory for the some duplication. I started writing this before laurants was up.

Edited by heel_e, 17 July 2005 - 10:23 PM.

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#4 David Mullen ASC

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Posted 17 July 2005 - 10:41 PM

It's true that film can be shot with a lot of depth of field, so shallow focus isn't inherently an attribute of film.

But consumer DV typically has a very deep-focus look due to the small target area behind the lens, so one way of disguising some of its origins is to create a shallow-focus look.

The main problem with deep focus is that it is a huge increase in the amount of information that the viewer has to see, which is distracting if that information is not contributing anything of dramatic value. Good deep focus movies tend to be very well-designed; low-budget indie movies shot in DV tend to not have the money to control every element in the frame like that, nor do they often have to directorial skills to take advantage of the deep focus. So the deep focus of 1/3" CCD consumer DV becomes a liability often because it becomes very hard to direct the eye to what's important and eliminate ugly distractions in the frame.
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#5 drew_town

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Posted 17 July 2005 - 11:03 PM

The main problem with deep focus is that it is a huge increase in the amount of information that the viewer has to see, which is distracting if that information is not contributing anything of dramatic value. Good deep focus movies tend to be very well-designed; low-budget indie movies shot in DV tend to not have the money to control every element in the frame like that, nor do they often have to directorial skills to take advantage of the deep focus.  So the deep focus of 1/3" CCD consumer DV becomes a liability often because it becomes very hard to direct the eye to what's important and eliminate ugly distractions in the frame.

<{POST_SNAPBACK}>

Some tools you can use to make a deep focus shot interesting are silhouette (my personal favorite), variations in texture and/or color (I'll often try to do this with lighting- make the background dominantly one color and contrast the foreground elements by using gels/color temps), shoot handheld (focus on the action more than the composition), create bold or unique compositions.

My biggest peeve about the wide DoF is the inability to rack focus.

And just a side note: It really annoys me that this idea that using ND filters will magically give you the depth that you want. I've heard a good number of people say this and they aren't able to tell you why that works. I actually had someone come up to me while I was shooting and ask why I didn't have an ND filter on the lens.
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#6 David Mullen ASC

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Posted 17 July 2005 - 11:49 PM

Well, in higher light levels, an ND filter will allow you to shoot at a wider lens aperture, which DOES reduce depth of field. But obviously you don't need an ND filter if you're already shooting wide-open because the light levels are low.

Plus how does someone know just looking at your camera that you aren't using an internal ND filter?
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#7 Gordon Highland

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Posted 18 July 2005 - 12:38 AM

That's what I was gonna say. If you're already shooting wide open, the ND isn't going to affect your ability to further limit DoF. You're better off shooting on a longer lens with the camera further away then zoomed in, if that's your goal.

However, an ND grad filter could give you more of a "film look" on outdoor scenes with skies, given video's crappy lattitude. It can help mange the highlights in one part of the frame and give you more to work with.
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#8 drew_town

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Posted 18 July 2005 - 09:18 AM

Well, in higher light levels, an ND filter will allow you to shoot at a wider lens aperture, which DOES reduce depth of field.  But obviously you don't need an ND filter if you're already shooting wide-open because the light levels are low.

Plus how does someone know just looking at your camera that you aren't using an internal ND filter?

<{POST_SNAPBACK}>

What annoys me are the people that don't know WHY that works. They think the ND filter is the magic trick. "I need more light so I can use an ND filter!" You'd be suprised how many times I've heard that.

>Plus how does someone know just looking at your camera that you aren't using an internal ND filter?

They obviously don't know what they're talking about, do they?
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#9 Laurent Andrieux

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Posted 18 July 2005 - 05:47 PM

On a video cam, just looking at the wheel on the left side allows you to know what filter is in use...
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#10 Brian Wells

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Posted 18 July 2005 - 10:35 PM

What annoys me are the people that don't know WHY that works. They think the ND filter is the magic trick. "I need more light so I can use an ND filter!" You'd be suprised how many times I've heard that.


Ahh, that's funny. I tend to only use the ND built in the video camera as to keep my filter trays open for other options. I heard some cameramen prefer 4x4 ND's to the built-in ones, but have no idea why.

Brian
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#11 Brian Wells

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Posted 18 July 2005 - 10:42 PM

However, an ND grad filter could give you more of a "film look" on outdoor scenes with skies, given video's crappy lattitude.  It can help mange the highlights in one part of the frame and give you more to work with.

Combined with a polarizer it can work wonders, sometimes. It also appears to create a distracting posterization problem. I have found underexposing a stop or so helps. This mandates a bit of gamma correction on the post side to bring up the overall levels, but ensures a smooth gradient in the skyline.

Brian
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