>He was shooting a green screen,
with the shutter at 1/2000, and every >20 or 30 seconds the camera
went almost 1 stop under...any thoughts >about this?"
I'm thinking frequency.
I wonder what the green screen light source was because if it came
back to normal at normal shutter, perhaps there is a bit of a differential
with the light source...not unlike what you could experience with
a projection source.
Make sense... or nonsense!
Allen S. Facemire
I think you all are on the right track.
When I was first testing my camera in the field. I had it set up
in a hotel room selecting setting in the menu from a color chart
and noticed a very strange "breathing" effect. I became
instantly frustrated and had no idea what could be causing it.
I first thought that it was a software issue as I was using it in
a housing underwater and , since the camera / housing system software
was a near proto -type assumed I had fried the camera. After about
an hour of scratching my head, running through the system and a
few tears I realized the obvious...I was in a mixed lighting room
with ambient and florescent lighting. Adjusting the proper shutter
as well as for the prime light source solved the problem. I was
able to recreate the strange "breathing" once I got back
to basics. Sometimes it is the obvious and little things that cause
us the greatest grief.
Look at shutter speed relation and light source settings. Test it
and then try to recreate it. Live and learn.
I hope this helps.
>The question here is what was
the light source : tungsten or >HMI/fluorescent???
Pol Turrents adds :
> a F750
> 1/2000 shutter
Yep, I've seen this before.
It is the slight difference in application frequency and phase between
the camera and the light. At an extreme shutter speed (or narrow
angle, if you prefer) such as that, it will cause an occasional
and repetitive drop in effective exposure as they pass through opposite
sync with each other.
One easy solution is to decrease the shutter speed.
This is the chart I built a while ago (3 years) for 23.98 shutter
equivalents to film Shutter angles.
I have not built one for Interlace but it is easy to do the Math
then it should be tested to insure that all the calculations with
Stop Loss Are Good.
Then check in your ASC Manuel to determine Shutter Angle and Speed
(Framerate) to determine of your flickering HMI rate is in conflict.
Dropping 1 stop is one of the obvious conflicts. Most are mush more
subtle and may include slow Pulsing, Fast Pulsing, it is very apparent
on the WFM.
Just like a Computer screen using ECS and dialling it back a bit
1/1875.5 may keep the narrow shutter and eliminate the problem.
Shutter Speed = Shutter Angle
When using a 23.98 or 24Psf Base
ECS =ShutterÂ° Angle Stop Gain or Loss
1/24th =360Â° Wide Open +1
1/30th =315Â° Qtr Stop +3/4
1/36th =270Â° Half Stop +1/2
1/42nd =225Â° Qtr Stop +1/4
1/48th =180Â° One Stop BASE
1/72th =135Â° Half Stop -1/2
1/96th =90Â° One Stop -1
1/144th =67.5Â° Half Stop -1.5
1/192th =45Â° One Stop -2
1/288th =33.75Â° Half Stop -2.5
1/384th =22.5Â° One Stop -3
1/576th =17Â° Half Stop -3.5
1/768th =11Â° One Stop -4
1/1152th =8.5Â° Half Stop -4.5
1/1536th =5.5Â° One Stop -5
1/2200th =4.25Â° Half Stop -5.5
Extended Clear Scan or ECS must be set and frequency dialled in.
Be aware of stop loss.
Again you should build your own chart for 29.97P and 50i, and 59.94i
Hope this helps
Test it for yourself.
B. Sean Fairburn
HD DP LA
HMI source not set to square frequency (flicker-free) setting.