NO rolling shutter in Super Slow Mo!

[Alister Chapman]

Anyone else noticed that there is no rolling shutter when shooting super slow mo? I was shooting planes with propellors at the weekend and there are NO rolling shutter effects when shooting Super Slo Mo and I do mean none at all. Does this camera have a global shutter in Super Slow Mo? Other modes exhibit some skew and rolling shutter.

[Chris Medico]

I noticed the lack of skew on mine when playing around with the slo-mo modes. I planned to do a test to better document the difference.

Wouldn't it be great if they could enable that readout mode when shooting normal video frame rates??

[Walter Brokx]

Maybe it has something to do with the faster shutterspeed?
1/50 or 1/60 vs 1/240 or even 1/480 (at 180 degree) is quite a difference: maybe something to try and compare in normal mode?

Global shutter would be nice to have all the time :-)

[Jim Michael]

Typical prop speed is about 2500 rpm.

[Alister Chapman]

Normally skew and rolling shutter effects become worse as the shutter speed is increased (shorter). At high shutter speeds with a conventional CMOS sensor a narrower band of the sensor is scanned at any one time and this accentuates the skew effects. However on the FS700 this does not appear to be the case. I shot a fan at 25P 1/100th and at Super Slow Mo 100fps with 1/100th shutter, so the shutter speeds are the same. The rolling shutter effects on the slow mo footage are reduced compared to the normal speed, so something different is going on. It's probably just a much faster scan speed.

NEX-FS700 Significantly reduced shutter when in Super Slow Mo! | XDCAM-USER.COM

[Rob Lind]

Same with heli, perfect straight with no rolling shutter.

/Rob

[Chris Medico]

One thing I've noticed in SSM is decreased battery life. I've not quantified how much less but it looks like the camera draws a good bit more power when in this mode. For sure the electronics are revved up in this mode.

[Jim Michael]

Doesn't the number of lines scanned change with the much higher frame rates on the FS-700?

[Chris Medico]

Originally Posted by Jim Michael

Doesn't the number of lines scanned change with the much higher frame rates on the FS-700?

Not till you get over 240fps.

[Walter Brokx]

Originally Posted by Chris Medico

One thing I've noticed in SSM is decreased battery life. I've not quantified how much less but it looks like the camera draws a good bit more power when in this mode. For sure the electronics are revved up in this mode.

Faster scanning draws more power.

So now we just have to wait for a hacker to add global shutter to normal mode? ;-)

[Dave Sperling]

Has anyone tested it at 240fps with strobe flashes to see if only part of the frame is illuminated by the strobe?

[Alister Chapman]

I've tested with a strobe light and the frame grabs are on my blog. It suggests the scan rate is twice as fast in SSM compared to normal modes as the flash band is twice as wide (almost full frame) in SSM as normal modes.
If you have an external recorder you can always record the Sdi or HDMI output at normal speed while the camera is set to SSM. Does mean the minimum effective shutter speed is 1/100th and audio would need to be recorded separately but it may be useful for shots where Jello might make it impossible to do with a conventional CMOS camera. Maybe if your shooting from a Helicopter for example.

[Alister Chapman]

Here's some footage from my shoot last weekend. Note the almost total lack of rolling shutter effects on the propellors and rotor blades.

Royal International Air Tattoo 2012 in Super SlowMo! - YouTube

[Emmanuel Plakiotis]

In normal speed the sensor must be scanned @1/25th of a second, which is pretty slow for fast motion thus the rolling shutter effect. In slow motion the scanning time decreases accordingly, therefore the rolling shutter goes unnoticed, because the object has far less time to move during the line by line scan and all lines represent the object in the same position. The shutter speed doesn't have an effect on rolling shutter, only the scan speed does. If you use 1/100 second at 25fps the scan speed doesn't increase, only the chopping of the shutter, which is irrelevant of the rolling shutter effect. But when you push the sensor to scan, in let's say 1/240th of a second, the sensor scans 10X, essentially working almost like a global shutter. I have read that Red and Alexa scan faster than 1/25 in order to minimize the rolling shutter effect.

[Alister Chapman]

The scan speed in normal modes must be at least 1/50th or 1/60th. It must be at least this fast to allow the camera to shoot 50p/60p and 25i/30i. 1/25th would be unacceptable in a pro video camera and the FS700's scan speed is definitely faster than 1/25th, it's most likely 1/60th in all normal modes and S&Q. 1/60th is the speed used by most pro video camera as it allows 30i shooting. This is confirmed by my strobe test results that show a near 2x scan rate increase when you go from 25fps to 100fps. If the scan rate was only 1/25th the flash band would have been a quarter of the width at 100fps.

Shutter speed does affect flash banding which is a rolling shutter effect as the reduced scan window reduces the width of the flash band. Slow shutter speeds can also mask skew through motion blur, faster shutter speeds tend to make skew more obvious as there is less motion blur to mask the issue.

Of course as I said the lack of skew is simply down to a faster scan rate, the scan rate must be faster to accommodate the higher frame rates. Interesting to have a sensor that can scan as fast as this one. I suspect there is some line skipping in SSM up to 240fps (every other line) which explains the aliasing increase, but as the sensor is quad HD, that doesn't result in a resolution drop. The need to line skip (if that is what is happening) may explain why the faster scan rate isn't available in normal modes.