You learn something new every day

It is one of the universal truths of photography that you never stop learning. Almost every time you pick a camera up something happens that you squirrel away in your memory banks that will make a difference to how you shoot something else at some time in the future. Normally these are small things but this week I was shooting the wonderful Talvin Singh performing at an arts festival and something happened that made me scratch my head because I’d never experienced anything quite like it.

©Neil Turner, October 2104

©Neil Turner, October 2104

The two pictures above were taken within a fraction of a second of one another under ‘exactly’ the same lighting without flash and with identical settings (manual everything apart from focus) on the same camera with the same lens and have been processed through Adobe Camera RAW identically. So why are they different? The answer seems to be LED stage lighting. You might conclude that both pictures are ‘quite nice’ and move on but that’s not really an answer when you are shooting something that has a moment that you absolutely have to capture. I’ve had issues with un-balasted HMI lighting and of course strip lights but this was in another ‘issues league’ entirely.

It appears that these brand new lights installed in a state-of-the-art theatre are an absolute nightmare for stills photography. Now that I’ve experienced this, I have been reading up on it and it seems to be a known phenomenon where the lights cycle between the red, green and blue LEDs in the light at a speed that the human eye chooses not to detect but that a camera shooting at shutter speeds of 1/125th of a second or higher has a real problem with. The higher you go, the worse it gets. Of course you could shoot at 1/60th of a second and all would be reasonably well – apart from any movement being a little/lot blurred.

Being a complete anorak I decided to shoot a series of tests at an even higher shutter speed (having first racked the ISO up a way) to see what happened:

©Neil Turner, October 2014

©Neil Turner, October 2014

That makes shooting under these lights at the kind of shutter seeds you need to freeze action nigh on impossible. Once I had realised that there was an issue I dropped down to 1/100th of a second and then to 1/80th and shot lots of frames. The first five frames above were shot at 1/400th and the last one was at 1/100th – what a difference.

Most of what I have read about LED stage lights concerns white balancing – well that was the least of my worries here. It was almost as if shooting with shutter speeds in excess of the cameras maximum flash synch speed (the highest speed at which the entire chip is exposed at the same moment) was part of the problem. The LED stage lights in this theatre were effectively pulsing or flashing and the only way to get a consistent image was to work with that pulsing and use shutter speeds below the maximum flash synch. I have read something on what appeared to be a well-infomed website which implies that this only happens when the lights are dimmed – which makes some sense. I haven’t got enough experience with this to work out whether the speed thing is a coincidence or whether it is directly related but I now know how to shoot in this one venue with these lights.

Shooting with shutter speeds that are a long way below “ideal” some of my pictures were sharp and many weren’t – but the job got done. Constant reference to the screen on the back of the camera is frowned on by a lot of people but this case proves that there are times when it is exactly the right thing to do. Imagine having had to shoot this on film with no LCD…

Presumably more and more theatres and venues will use these lights and the problem will grow. Maybe there’s a solution out there already?

©Neil Turner, October 2014.

©Neil Turner, October 2014.

Techie stuff: Canon EOS5D MkIII with a 70-200 f2.8L IS Canon lens. 1/100th of a second at f3.2 on 2000 ISO. WB set to daylight but adjusted in Adobe Camera RAW removing quite a bit of magenta and adding a small amount of yellow.

6 comments

  1. It’s interesting that when shooting at slower shutter speeds the sensor reads the light as a composite of the different coloured pulses.

    I may be wrong, but do Bayer sensors have three separate micro-lenses for each colour?

    I wonder how x-trans sensors or Foveon sensors handle LED lighting?

    Would you do a post detailing how you add and subtract colour in Camera Raw?

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    1. Interesting thoughts and questions David – to which I have no answer apart from the one about adding colour.

      There I simply meant moving the colour sliders up and down… the top one (blue to yellow) towards the yellow and the lower (green to magenta) towards the green. Nothing clever, just a colour balance tweak.

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  2. The concept that the dimming causes the problem makes sense. I suspect LEDs are dimmed by changing their frequency rather than their brightness. Rather like changing the output on flash by lengthening or shortening the burst rather than the brightness of it. Does that make sense?

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  3. UPDATE… two evenings later I had to shoot another music event at the festival. It was also lit with LEDs but these ones behaved very differently and there were no banding or flickering issues. I shot most of the evening at 1/320th of a second and the lighting behaved exactly as you’d expect it to and want it to. I’m guessing that there is more than one type of LED being used for stage lighting and the advice has to be check your LCD screen to see which type you are working with. So that’s yet another day where I learned something new!

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    1. LEDs are generally dimmed by a technique called PWM (pulse-width modulation). This is usually done by picking a frequency (this can be from as low as 100Hz up) and setting the on-time of the LED power to a percentage. Because the better dimmers apply a CIE-Lab formula to correct for perceived brightness, the lights are actually off for more than half the time or less from about 80% brightness level down. This means at 100 Hz PWM frequency and 50% brightness 3/4 of the image could be black with a 1/100 shutter speed. Luckily most LEDS are modulated at higher speeds, but the units containing slower microcontrollers will have lower modulation speeds. Its obvious these lights are simply not designed with photographers in mind.

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