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Working consistently with natural light


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#1 Ville Pakarinen

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Posted 28 August 2014 - 11:32 AM

The problem with natural light is that it does not decrease or increase in full stops or half stops or third stops. It changes gradually.

I'm shooting a scene in a room with natural light from the windows working as a general ambient light. I have decided on a fixed aperture (let's say F2.8) that gives me consistent DOF in all shots. For all setups, I first meter the ambient light always at the same location - let's say the back wall - then underexpose two stops to make the room murkier and the windows less over exposed. Let's say I get F1.4 at the back wall. Perfect! F2.8 is exactly two stops smaller aperture. Then I light the talent to match F2.8. Perfect! The talent is two stops brighter than the back wall.

Now, I shoot a wide master, then move on to coverage. I turn off the lights and meter the back wall again, this time getting F2.2. The light from the windows has increased 1 1/3 stops!

Now, what do I do when changing the aperture isn't an option? I want it to be F2.8. There is no 1 1/3 stops ND, I would really like to refrain from using any gradual ND filters. What are the options?

Also, how often do you recomment I check the exposure at the back wall?


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#2 Mark Kenfield

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Posted 28 August 2014 - 11:59 AM

Just move your iris up or down 1/3-1/2 a stop and fix the rest of the imbalance with ND.

 

No one (probably not even you) is going to notice the difference in depth of field between f/2.8 and f/3.2 (or f/2.8 and f/2.6)


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#3 Ville Pakarinen

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Posted 29 August 2014 - 08:57 AM

Sure, no one's going to notice such a small difference. But let's be perfectionists here for a while :D

In the context of the question, if I really want to be adamant about the F-stop, are there any other options:

1. Use a gradual ND
2. Cut the light from the windows by closing the blinds a little (if possible)
3. ...


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#4 John Miguel King

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Posted 29 August 2014 - 09:58 AM

Hello mate,

I am not too sure "perfectionism" is helpful on set!. The small variations in luminance you're mentioning are easily matched and corrected at any stage of the colour and data pipeline.

However, if it helps, when I work as a DIT I'm very often in charge of setting iris. When outdoors this translates as riding the iris remote during takes due to changing cloud cover.

Provided you've got good density, a healthy waveform picture and the variations are not extreme, you can get away with murder in the grade and nobody will ever notice.

Life and schedules are too short for perfection.


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#5 David Mullen ASC

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Posted 29 August 2014 - 10:15 AM

I don't see why you can't adjust the f-stop.  The perception of depth of field varies by subject size and distance to the lens anyway -- often I try to work at a deeper stop on close-ups just to give the focus puller a break.

 

But for minor adjustments, less than the 1-stop of an ND.3, you can try adjusting the shutter angle if you want to maintain the same f-stop.  Closing down from 180 degrees to 144 degrees, for example, is close to a half-stop loss, maybe a bit less.  If this is a digital camera, you can combine this with an ISO change of a 1/3-stop, no one is going to spot that adjustment especially if you are lowering the ISO by a 1/3-stop like from 800 to 640 ISO.

 

I usually have iris control at the video monitor and don't hesitate to adjust the f-stop...


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#6 Stuart Allman

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Posted 29 August 2014 - 10:30 AM

Ville,

 

First, I'm with the others in saying that minor adjustments to aperture and shutter angle are probably your best options.  However if you really want to be picky about aperture Tiffen does make a 0.1 (i.e. 1/3rd stop) ND filter.  Buy two of those and you can tweak all day long to your heart's content.

 

On a lot of these cameras you can adjust the ISO without too much of a change in highlight range as well.  I would personally go for the less expensive option that makes the production run faster.  A camera setting change takes 2 seconds.  An ND filter change always seems to take 2 minutes.

 

S.

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Edited by Stuart Allman, 29 August 2014 - 10:31 AM.

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#7 Ville Pakarinen

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Posted 29 August 2014 - 10:36 AM

Yeah, thanks guys. I guess I'm just eager to give the colorist perfectly exposed shots since this is my first pro shoot :lol: If was shooting for myself and doing the grading, I wouldn't care half as much.

The camera is my Black Magic Cinema Camera.


Edited by Ville Pakarinen, 29 August 2014 - 10:37 AM.

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#8 Bill DiPietra

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Posted 29 August 2014 - 12:19 PM

The unpredictability of natural light is half the fun of working with it for me.  But you also need to go with the flow a bit when you're dealing with nature. ;)


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#9 Satsuki Murashige

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Posted 29 August 2014 - 11:47 PM

Well, if you really wanted to go crazy and kill your schedule then you could have the grips put up large frames of single and double nets over the windows and switch them out as the light changes. The problem is that by the time you've done all the work to switch them out, the light probably will have changed again. Also, you'd still be working only in 1/2 -1 stop increments. Unless anyone makes lavender nets in large sizes like 6x6 and up...

Really, if you were being a stickler about maintaining a precise stop, you would not be using natural light. You would instead flag off the natural light with a tent and use a large HMI to give you a consistent source all day. Most productions on a low budget can't afford to do that, so something has to give.
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#10 Ville Pakarinen

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Posted 30 August 2014 - 09:00 AM

Turning the aperture ring a notch certainly does sound a lot more appealing.

Thanks!


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