7RM3 silent shooting and high-frequency banding

[thefsb]

I took pictures at a solo viola performance yesterday. I used silent shooting. The banding in the images is pretty bad but what surprises me is that there's about 50 bands.

 

https://blog.kasson.com/a7riii/how-fast-silent-shutter/ says that with AF-C and compressed RAW, which I used, the shutter traverse takes about 1/30 second.

 

So that means the frequency of the lights was about 1.5 kHz. Is that plausible for stage lights?

 

Otherwise I don't understand the cause of the banding.

 

2018-05-19-Viola-Talus-Boston-Conservatory-_DSC3518 by The Fsb, on Flickr

 

 

[Chrissie]

Otherwise I don't understand the cause of the banding.

 

 

I would expect the sensor readout to be massively parallelized, as there is no dependency between individual pixels which would mandate a serialized readout.

The notion of a "scanline" type of readout like the way cathode ray tubes used to produce TV images would otherwise not allow for blackout-free continuous-mode shooting.

 

Btw, Sony claims shutter speeds of up to 1/8000 for the a7r3 and up to 1/32000 for the a9, respectively.

[thefsb]

Rather than speculate, take a look at Kasson's measurements.

1. "The electronic shutter travels in the same direction as the mechanical one, from the top of the image to the bottom, or the bottom of the camera to the top."
2. This linear scan takes about 1/30 second, and AF-C and compressed RAW, and double that for AF-S and uncompressed.
3. The mechanical shutter traverses at about 1/250 second.
4. For shutter "speeds" smaller than that, the second curtain starts moving before the first reaches the end.

That being the case, how to account for the banding in this image?

• Do some stage LED lights flicker at 1500 Hz?
• Is this abnormal? e.g. a fault in the lighting power supply?
• Is something else going on?

 

[thefsb]

Jim Kassen thew some lige on this (huhuh)

 

Stage lighting often modulates the LEDs at frequencies unrelated to line frequency. The frequencies can even change during a performance. The modulation frequencies can be much higher than line frequencies.

 

[Chrissie]

If you have your explanation already, why bother this forum?

 

I prefer common sense, as being an engineer myself.

[thefsb]

I didn't have the explanation until I posted #4.That stage lights may be modulated at 1.5 kHz was a complete surprise to me. My reply (#3) was to correct your statements about massively parallelized sensor readout.

 

Moreover, the knowledge could be of interest to others. One may be able to mitigate such banding by finding a shutter speed that's a multiple of the flicker period, if you're lucky and the flicker frequency is stable. Or you have to use the mechanical second curtain. Or an A9.