Can’t get to 180 degree shutter, is it worth getting closer?

John Sheehy wrote:

John Vickers wrote:

Some folks who should know better have claimed - and continue to claim - that frame rates that divide mains frequency eliminate mains-related flicker. That would only be true if the mains electrical supply was genlocked to the camera.

Are you mixing two different issues?

Could you explicitly state what "two different issues" I am mixing?

Synchronization would be necessary only if the exposure time was much shorter than the lighting cycle period;

The word "much" is doing some heavy lifting.

Also "much shorter than" should be "much different from". "Significantly different from" would be better. We could then go on to quantify "Significantly", for a given light source.

For light sources that have bounded, continuous output, for small differences in exposure time from a period of the light output, the recorded banding/flicker (in linear light) is proportional to the error in the exposure time relative to the period. [The assumption of bounded continuous output is not necessary. The output does not even need to be a function. It is sufficient for the integral of the output over any bounded time interval to exist. We could have lighting like HSS flash.]

Assuming we have synchronisation between the frame cycle and the lighting flicker cycle, at a given point in the recorded images, there will be no flicker.

But unless we have global shutter, which is unlikely, different parts of the images will be exposed at different phases of the lighting cycle, so we can have stationary banding, even if we do not have flicker unless the exposure time is "very close" to a period of the light output. This is equally true in a film movie camera with a rotating shutter and in a CMOS sensor with row-by-row reset and readout.

In the unlikely case that we do have global shutter, the exposure in this genlocked case depends on the phase of the fundamental lighting period at which the exposure starts.

if the exposure time is exactly the lighting cycle period or light flicker period, then it does not matter at all what part of the wave the exposure begins at, because an entire wave period will be recorded, and exposure is therefore the same in all frames.

Yes.

But the exact case is uninteresting for practical purposes.

Mains frequencies vary with the load on the power grid. Exposure times are not always clear: if we ask for a nominal 1/60 s, do we get 1/60.00 s, or 1/64.00 s? Does it depend on whether the camera is capturing stills or video? Whether we are shooting on Nikon or Panasonic?

What is interesting is: How small do the errors need to be to assure acceptable results?