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#1 Ken Minehan

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Posted 24 July 2008 - 02:22 AM

How many stops does a full silk take out?
thanks
ken minehan
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#2 J. Lamar King

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Posted 24 July 2008 - 02:28 AM

China silk is one stop and an artificial (poly) is one and a half.
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#3 Chris Keth

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Posted 24 July 2008 - 02:32 AM

I'll add that Mr. King's answers are correct for clean silks. If they are a little dirty as often happens, they will cut more light and be a bit warm.
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#4 Ken Minehan

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Posted 25 July 2008 - 12:20 AM

ok, thanks guys. I have another question. I always struggled with this.

I read the photometrics of the arri 5k fresnel and it says that at 10metres we will get 3775 Lux. So then what will be my f stop if i'm shooting on 250asa?

I have read on this forum before that:
100ASA @ f2.8 = 100fc

and knowing that 1fc = 10.764 lux.
therefore 3775 lux =350.7 fc

would i be right in saying that i will get just over f8


regards
ken minehan
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#5 Ken Minehan

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Posted 25 July 2008 - 12:28 AM

and i forgot to add, with a silk in front of the light that takes 2 stops off, i should be getting some where around f4?
How does that sound. havei got my calculations right?

regards
ken minehan
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#6 Christopher Wedding

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Posted 29 July 2008 - 11:45 PM

Hey Ken,

You're right that an F4 is two stops less than F8.

at 250 asa - an F4 needs 80 fc
an F5.6 needs 160 fc
an F8 needs 320 fc
an F16 needs 640 fc

So a little over an F8 would be about right if your reading was 350 fc.

However, alot of variables come into play. If you put any gel on the light,
you'll be losing some of those 350 fc's. Also, you can always through
a scrim in the light to reduce the output. Plus, is the light at full spot or full
flood for that reading? By controlling the spill, you can spot in or flood
out to reduce or increase your reading.

What's most important is the stop you WANT to be shooting. Usually it's
good to try to stick to an F-stop and then adjust your lighting to that stop.
It's not always possible, but it's the right approach. Obviously if you want
stuff to be more out of focus (shallow Depth of Field) you'll try to shoot
with a lower F-stop like a 2.8. To pull that off, you would need to light
accordingly. IF the 5K was your key light, then you would need around 40
fc's for a 2.8. So throw some scrims in your light (a double takes away one stop of
light), or back your light off, or put an ND filter in front of the camera lens.
Of course, at some point getting a smaller light might be the best option.
I'm only talking about reducing the INTENSITY of the light.

Obviously a silk will reduce your stop as well, but it effects you QUALITY of
light by softening and spreading the light. If this is the look you want, then
great. A normal poly silk will eat up about 1.5 stops of light. That puts
you at about a 4/5.6 split if the 5K was your key at 10 meters with a poly
silk in front of it. So use any of the above options to reduce the intensity
to what ever stop you want.

Check out the Set Lighting Technicians Handbook. It has all the photometrics
for most lights in foot candles. ;)

Hope that helps

Best,
Chris
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