Rolling shutter in Sony Z7U. NOT from strobe.

[dj2thew]

I'm using the Z7U to shoot a feature in Russia. I did several camera tests in the States and decided on the Z7U with an assortment of 16mm lenses with a converter. I was aware of the flash/strobe issue but decided it was the right choice anyhow. All was well in the States, but here in Russia I have found that when shooting at night using public light (Streetlights mainly) there is sometimes a roll that looks just like the rolls from tube TVs as they refresh.

I adjusted the shutter speed up and down, which minimized and maximized the problem, but never eliminated it. Has anyone encountered this? Thoughts on it being from the different power cycle the lights may be on?

I need a work-around besides just shooting daylight or with our own minimal lights. Help!

 

[Barry_Green]

This is one of the more pernicious rolling-shutter artifacts, and it can happen from ballasted light sources such as the street lights you're encountering. If you're shooting at an exact multiple of the frequency of the light source you might be able to minimize or eliminate it, but you'd probably have to use 1/50th or 1/25th.

 

[BobDiaz]

Barry answered the question, but I'll add just a bit more...

Here in the USA, our power is 60 Hz. Because every cycle has a positive and negative swing, there are two pluses to each cycle, thus 60 Hz has 120 Hz (120 times per second) pluses.

With normal tungsten lights, this is not seen, because it takes time for the filament to cool down and heat up, but other lights, like fluorescent lights, do show the pluses of light. The flashing is so fast, that our eyes see this as continuous light, but a video camera can catch this flashing.

When shooting with a video camera from the USA, the 59.94 Hz frame rate is very close to the 60 Hz power (120 Hz flash rate), so it's hard to catch this with the camera.

However, when you take this camera to a 50 Hz power country, the flash rate there is 100 Hz. This will create a beat pattern of the differences in frequencies.

A CCD camera will show the effect evenly across the whole frame, but a CMOS camera will show a difference across the frame. That's because it's scanning different parts of the image sensor at different times.

The solution as Barry pointed out is to set the shutter speed to the frequency of the country you are in. So a 50 Hz country you could use: 1/25, 1/50, or 1/100. These are all even multiples of the flash rate of the lighting. My guess is that the longer times, 1/25 and 1/50 are the best choice.


Bob Diaz