Silks

[Ken Minehan]

How many stops does a full silk take out?

thanks

ken minehan

[J. Lamar King IMPOSTOR]

China silk is one stop and an artificial (poly) is one and a half.

[Chris Keth]

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.

[Ken Minehan]

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

[Ken Minehan]

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

[Guest Christopher Wedding]

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