>Talking with a friend of mine, he had an strange problem a few days ago with a HD from Sony.
>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...
>It seems like a camera with an auto iris on, but with a prime lens, so there was no auto iris. The detail was off, and it wasn't a flicker problem. When he lowered the shutter, the problem gone.
>Any thoughts about this?
DoP Spain, Barcelona
class="style9">>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...
>The question here is what was the light source : tungsten or HMI/fluorescent???
>Camera speed 24p, 23.98p, 30p ???
>Daniel Villeneuve, c.s.c.
Directeur-Photo/Director of Photography
>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.
class="style9">>The question here is what was the light source : tungsten or >HMI/fluorescent???
>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 =Shutter° Angle Stop Gain or Loss
1/24th =360° Wide Open +1
1/30th =315° Qtr Stop +3/4
1/36th =270° Half Stop +1/2
1/42nd =225° Qtr Stop +1/4
1/48th =180° One Stop BASE
1/72th =135° Half Stop -1/2
1/96th =90° One Stop -1
1/144th =67.5° Half Stop -1.5
1/192th =45° One Stop -2
1/288th =33.75° Half Stop -2.5
1/384th =22.5° One Stop -3
1/576th =17° Half Stop -3.5
1/768th =11° One Stop -4
1/1152th =8.5° Half Stop -4.5
1/1536th =5.5° One Stop -5
1/2200th =4.25° 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.