EFFECT OF SHUTTER ANGLE
The actual time that exposure is allowed during the period
that the camera shutter is open, is determined by two factors. The speed
the shutter is open and the shutter angle. Nominally we refer to the motion
picture camera shutter as being a 180 degree shutter. Some cameras have
adjustable shutters. These shutters may adjust from 0 degrees (closed)
to up to 200 degrees (open).
If the shutter is running at 24 fps and is set at 180
degrees, the exposure time is (1/24)/2 or 1/48th of a second. This is the
period of time it takes a 24 frame video signal to complete the raster
scanning of a single field.
If the shutter is running at 24 fps and is set at other
than 180 degrees, the closed period will be the compliment of the open
period.
A 200 degree shutter refers to 200 degrees open and 160 degrees closed.
example: 360 (total) - 200 (open) = 160 (closed) degrees
A 160 degree shutter refers to 160 degrees open and 200 degrees closed.
example: 360 (total) - 160 (open) = 200 (closed ) in degrees
24 Frame Shutter Angle Percentage of 24 Frame TV Picture Exposed
(1 Film frame is exposed by 1 video field)
|
CAMERA |
EXPOSURE |
PERCENT OF VIDEO FIELD |
PERCENT OF VIDEO IMAGE |
|||
|
Degrees of Shutter |
Seconds |
Seconds |
Not Exposed |
Exposed |
w/a White Bar |
w/a Black Bar |
|
90 |
1/96 |
0.0104 |
150% |
50% |
na |
50% |
|
100 |
1/86 |
0.0116 |
144% |
56% |
na |
44% |
|
110 |
1/79 |
0.0127 |
139% |
61% |
na |
39% |
|
120 |
1/72 |
0.0139 |
133% |
67% |
na |
33% |
|
130 |
1/66 |
0.0150 |
128% |
72% |
na |
28% |
|
144 |
1/60 |
0.0167 |
120% |
80% |
na |
20% |
|
160 |
1/54 |
0.0185 |
111% |
89% |
na |
11% |
|
170 |
1/51 |
0.0197 |
106% |
94% |
na |
6% |
|
173 |
1/50 |
0.0200 |
104% |
96% |
na |
4% |
|
175 |
2/99 |
0.0203 |
103% |
97% |
na |
3% |
|
180 |
1/48 |
0.0208 |
100% |
100% |
0% |
0% |
|
195 |
1/44 |
0.0226 |
92% |
108% |
8% |
na |
|
200 |
1/43 |
0.0231 |
89% |
111% |
11% |
na |
1. If less than 100% of a video field is exposed, the
remaining Video Image will be a dark bar.
2. If more than 100% of a video field is exposed, that
amount of the Video Image will be a white bar (Double Exposed).
3. The position of the error bar is determined by the
exact shutter to field phase relationship.
4. Panaflex cameras have a fixed 120 degree viewing shutter.
Seeing a dark bar at the bottom or top of the Video Image through the viewing
shutter does not mean an error bar will be exposed on the film. The bar
size is determined solely by the setting of the shutter angle.
5. Arriflex BL cameras have an exact 180 degree viewing
shutter and exposure shutter.
With a little practice, you can identify the exact manner
that an improperly exposed and synchronized video image was shot when you
see the result on film. The experienced video operator should be familiar
with the following shutter errors as the appear in the filmed video image.
Wide Black Moving Bars
Wide black fast moving bars are the result of 24 frame
filming with 30 frame video at 180 to 200 degree shutter angles.
Narrow Slowly Moving Lines - 2 of them
Narrow Slowly moving lines moving through the picture is a result of filming 30 frame video at 144 degree shutter angle and there is no video sync connected between camera and video system. At times there will be 2 lines visible. Precise and careful shutter angle tweaking to 144 degrees will cause the lines to virtually disappear. At best one line will be off the picture and the other in the middle.
Narrow Black Moving Bars
Narrow black slowly moving bars is indicative of 24 frame
filming of 24 frame video with no sync and a shutter angle of less than
180 degrees.
Narrow White Slowly Moving Bars
Narrow white slowly moving bars is indicative of 24 frame filming of 24 frame video with no sync and a shutter angle of greater than 180 degrees. Probably 200 degrees. Most photographers like to film at 200 degree shutter angles and by default will often forget to adjust to 180 when filming 24 frame video.
Skinny Slowly Moving Line
One skinny slowly moving line in the picture is noticed
when you are using 24 frame video, 24 frame camera, at 180 degrees and
have no sync connection.
Two Lines - moving is probably 24 frame camera filming 30 frame monitors at 144 degree shutter with no sync.
Stationary Skinny Line
The stationary skinny line could be one of two things.
1. 24 frame Video and 24 frame camera in sync but out of phase. The line could be anywhere in the picture.
2. 30 frame video and 24 frame camera in sync and operating
at 144 degrees shutter angle. One of the skinny lines is phased out of
the picture.
Narrow White Bar At Top of Picture
24 frame video and 24 frame camera. This is the condition
where the video operator has properly phased and set up the camera but
the shutter angle has been adjusted to 200 or more that 180 degrees.
Narrow Black Bar at Top of Picture
24 frame video and 24 frame camera. This is the condition
where the video operator has properly phased and set up the camera but
the shutter angle has been adjusted to 160 or less than 180 degrees
Quick Reference
If the photographed video picture has moving bars of any
size running from bottom to top or top to bottom, there is no sync connection
from video to the film camera. In most cases the bar will take several
seconds to go completely through the picture.
The size of the bar is determined by the shutter angle. 180 degree shutters will exhibit the smallest bar and is there fore easier to hide. 200 degree shutter angles will exhibit a narrow white bar that may be hard to hide on the video frame line. The same difficulty is found at 160 degree shutter angles. 160 degree is the maximum angle that can be achieved with most effects plate cameras. With 160 degrees you will have to hide a narrow black bar on the video frame line {the vertical interval}.
180 degrees is optimum for all 24 frame filming.
The position of the bar is determined by the phase of
the camera sync system. The video operator must insure the bar is phased
out of the video picture.