On a ‘lighter’ note from my previous post, this week I wanted to bring up one of the daily scourges of the working photographer; that of LED lighting.
Of course I’m a huge fan of energy-saving devices, and I appreciate the part LED lighting plays in this (though I do have some questions around that – perhaps best saved for another day), but they can cause issues for photographers.
What most people don’t realise is that LEDs are not constant lights in the way halogen lamps are/were. LEDs have a frequency, that is to say they pulse on and off. For the most part the human eye cannot detect this, but cameras, because of the way they work, can.
Each frame is 1/800th of a second, but slowed down here so you can see the changes more easily.
The Invisible Flicker
If you look at the video above, it shows a sequence of images taken at high speed over the course of a few seconds. This is not a light being dimmed up and down, it’s the LED effect. The camera settings haven’t changed from one frame to the next, but you can see how the light dims on a cycle. You wouldn’t see this when you look at the light, but the camera’s sensor is recording the variable output of this light source.
What you’ll also notice is that the ‘bulb’ isn’t changing all at once. That’s because these LED bulbs consist of clusters of LEDs, each with their own frequency. If they’re not in sync with each other you get this wave effect of light brightening and dimming.
The issue this can cause to photographers is seen as banding in images (see image below) where horizontal strips of the image are darker than the rest of the picture. It looks horrible and can’t be corrected in post-production, at least not easily. The effect in the example here is exacerbated by my use of the electronic shutter function of the camera.*
Note how the banding affects the background wall, the seats and bench in front of the students. It’s less visible in the subjects, even so I rejected it.
LEDs Everywhere
LEDs are now ubiquitous in our everyday lives. They’re in all our workplaces, shops, homes, cars; basically, they light our lives. Better quality LED lamps will cause fewer problems than cheaper ones. However, even expensive LEDs can cause issues, especially if they’re dim-able.
When an LED light is dimmed, it isn’t actually dimmed, so much as its frequency is extended. In other words, the moments when the LED is off are extended. We’re talking microsecond differences here, which is why the human eye doesn’t see more flicker, just a dimmer light.
As of writing this article, I believe there is still only one photo-centric (not video-centric) camera with a sensor design which eliminates this issue completely, and that costs almost £6,000.00 just for the body. Other slightly cheaper cameras use faster processors to try to mitigate the effect, but it’s still an issue to be aware of.
My cameras don’t show banding when I’m using the default Mechanical Shutter mode. In some situations it would be preferable to have the camera operating completely silently (by using the Electronic Shutter setting), but if banding is ruining the photo then there’s no choice but to allow the shutter to click. It’s not exactly loud, but can be noticeable in certain situations.
LEDs Triggering Issues
The flicker of LEDs can have other detrimental effects too. If I’m setting up portable studio lighting for a photo session, I’ll often use a handheld flash meter to set the output (brightness) of my flash units. To do this, I press a button on my flash meter which is then waiting for my flash to fire so it can measure the intensity of the flash. But the flicker of an LED light is often enough to fool the meter into thinking it’s detected the flash, when all it’s detected is the pulse of an LED light.
This can make it incredibly difficult to get an accurate flash meter reading. I can try shielding the flash meter from the LED light source, or I can plug the flash meter into the flash unit and not use the non-cabled setting (apologies if this is getting technical!) Unfortunately, not all modern flash units have a flash cable socket, so this often rules that option out. It’s also not always possible or desirable to switch off all the LED lights in a space to prevent the interference.
There have been occasions where I’ve just had to set the flash output by taking a series of test shots until I’ve got the correct exposure. It’s not ideal, but it gets job done eventually.
Irritation and Mitigation
I say the human eye can’t detect the flicker of LEDs, but that isn’t always true. I used to work in an office which had LED strip lights, and they irritated my vision and fatigued my eyes. Some people are more sensitive than others and you can even buy special glasses to counteract LED lighting.
The trickiest thing about LEDs is you can’t easily predict when they’ll be a problem. I’ll take test shots or look through the electronic viewfinder of my camera, but neither of these is a 100% reliable predictor of when banding will wreck a photo. A different angle, moving from one location to another or someone making changes to the lighting can all have an impact.
Many audio visual engineers have moved to LED stage lighting and this can be brilliant or a disaster depending on their system.
So next time you’re at an event and the photographer’s camera is clicking away, even if you know they have a silent function on their camera, perhaps now you’ll understand why they might not be using it.
Sorry about the length of this article, but I do hope it’s shed some light (see what I did there?) on a mostly invisible issue.
*Electronic shutter setting means the camera is completely silent, however the imaging chip is more prone to recording banding because it exposes the sensor by scanning in lines from top to bottom. The example photo I’ve used was shot at 1/200th of a second which shows just how fast the frequency of the LED cycles is.
