50khz question

[Guest 89e2bdf5797fbbdc17c2cc6da1413fa0]

[Of course I meant 50Hz, not 50khz]

 

Here in PAL UK, if I want to shoot 24p or 60p where there are lights, I use a 1/50th or 1/100th shutter to avoid flicker. 

 

I had assumed that setting my shutter to ANY multiple of 50 would do the same, but with my G6 and (I think) my D5300, I'm getting flicker when I set shutter speed to any multiple of 50 above 100 (e.g. 200).

 

Can anyone enlighten me as to why this happens?

 

TIA

[etidona]

nothing to do with PAL or NTSC. The flickering you see is due to the frames being too freezed because of the high shutter speed. To achieve a more natural look you need to blur a bit the image, just to smoot the transition from a frame to another. In my experience anything higher than 1/60 produces excessive flickering in moving subjects.

Of course I'm talking about normal 25 or 50 frames per second. If you shoot in high fps you can (must) also rise up the shutter speed to achieve a natural look. Unfortunately that'n not possible with your cameras.

[Guest 89e2bdf5797fbbdc17c2cc6da1413fa0]

nothing to do with PAL or NTSC. The flickering you see is due to the frames being too freezed because of the high shutter speed. To achieve a more natural look you need to blur a bit the image, just to smoot the transition from a frame to another. In my experience anything higher than 1/60 produces excessive flickering in moving subjects.

Of course I'm talking about normal 25 or 50 frames per second. If you shoot in high fps you can (must) also rise up the shutter speed to achieve a natural look. Unfortunately that'n not possible with your cameras.

No this isn't that kind of stuttering (I know what that looks like and what causes it). This is the wavy flicker you get with lights being out of synch. It was a static subject/static camera anyway. 

 

I'm shooting at fast (smaller than 180 degrees) shutter speeds because I've been playing around with Optical Flow in FCPX (like Twixtor) to create fake 120fps. OF needs frozen motion to work best, and I have RSMB to add motion blur after if needed.

 

No lights in this one, but you get the idea:

 

[etidona]

Oh ok. You sayd that you shoot @24/60 fps. Well the AC electricity in PAL regions goes at 50khz, so you need to shoot at 25 or 50 fps to avoid black frames. Or you can use DC lights such as 12v powered portables.

[Guest 89e2bdf5797fbbdc17c2cc6da1413fa0]

Oh ok. You sayd that you shoot @24/60 fps. Well the AC electricity in PAL regions goes at 50khz, so you need to shoot at 25 or 50 fps to avoid black frames. Or you can use DC lights such as 12v powered portables.

Sorry, I should have been more clear. I do shoot at 25/50fps a lot of the time for this very reason.

 

But there are times when I like to shoot at 24p (more cinematic) and times I like to shoot at 60p (to get the most slow motion out of the camera). Using a 1/25, 1/50 or 1/100 shutter means I can do this without light flicker problems. What I don't understand is why this doesn't work at multiples of 50 above 1/100th (e.g. 1/200th).

[Guest 89e2bdf5797fbbdc17c2cc6da1413fa0]

I noticed a similar thing when shooting still images with the electronic shutter with the GH3: I get horrible banding effects. The weird thing is that with my Nikon V1, which also has an electronic shutter option, I don't have this problem.

This is exactly what the problem I have looks like (though the bands are moving, in waves). Strange that you're getting that with a still! What shutter speed was this?

 

I'm sure somebody on this forum will know the answer ...

[Nikkor]

I guess it's rolling shutter. The sensor reads from top to bottom. I guess the V1 doesn't give you this because the readout is very fast. 

This happens in still cameras if you use electronic shutter -> rolling shutter.

[Guest 89e2bdf5797fbbdc17c2cc6da1413fa0]

I guess it's rolling shutter. The sensor reads from top to bottom. I guess the V1 doesn't give you this because the readout is very fast. 

This happens in still cameras if you use electronic shutter -> rolling shutter.

The thing is I'm not getting these artefacts at 1/50 and 1/100, so I don't see why rolling shutter would not affect those speeds but would affect 1/200. That is to say, my question still stands even if it is rolling shutter.

 

My best guess is that with higher numbers, any inaccuracy with timing will be amplified (i.e. it's not quite 1/200 - more like 1/204 or something). Hard to believe though. And it doesn't explain why Ivan got it in his still shot at 1/80.

[Nikkor]

The thing is I'm not getting these artefacts at 1/50 and 1/100, so I don't see why rolling shutter would not affect those speeds but would affect 1/200. That is to say, my question still stands even if it is rolling shutter.

 

My best guess is that with higher numbers, any inaccuracy with timing will be amplified (i.e. it's not quite 1/200 - more like 1/204 or something). Hard to believe though. And it doesn't explain why Ivan got it in his still shot at 1/80.

The speed might be multiple of 50 but the readout is continuous with every line offset a very small fraction of a second. At 50 cycles per second with 1/50 you will always catch one whole wavelength even if you don't catch it at the same place the sum will be the same, from that point flickering will start because you just catch fractions of the wavelength and each line advances X seconds into the harmonic function and will have a different lightvalue. At 1/100 it might not be too visible but when you start goin up you see it more and more, you actually can see how it grows and how the flicker gradiant shows perfectly the harmonic function.

 

You can see something similar at slomo soccer game replays, the readouts on these cameras are very fast and instead of variations on the same frame you get variations between frames.

[Guest 89e2bdf5797fbbdc17c2cc6da1413fa0]

The speed might be multiple of 50 but the readout is continuous with every line offset a very small fraction of a second. At 50 cycles per second with 1/50 you will always catch one whole wavelength even if you don't catch it at the same place the sum will be the same, from that point flickering will start because you just catch fractions of the wavelength and each line advances X seconds into the harmonic function and will have a different lightvalue. At 1/100 it might not be too visible but when you start goin up you see it more and more, you actually can see how it grows and how the flicker gradiant shows perfectly the harmonic function.

 

You can see something similar at slomo soccer game replays, the readouts on these cameras are very fast and instead of variations on the same frame you get variations between frames.

This makes a lot of sense. It fits with what Ivan read about keeping your shutter speed long too ...

 

I guess I'll just have to keep my shutter speed down when around lights!