Posted 24 July 2008 - 02:22 AM
Posted 24 July 2008 - 02:28 AM
Posted 24 July 2008 - 02:32 AM
Posted 25 July 2008 - 12:20 AM
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
Posted 25 July 2008 - 12:28 AM
How does that sound. havei got my calculations right?
Posted 29 July 2008 - 11:45 PM
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