How is it possible for LED lights to photograph as unlit?

[DSchroader]

I recently did a photoshoot with a couple models where the lighting source was LED christmas lights. The lights were constantly on.  I had quite a few images where the christmas lights showed NO light at all. Others where some were lit and others not. I don't see how this was at all possible. 

I shoot with a Canon 7D. I shot at ISO 6400, f2.8, and a 180 shutter speed.

Can anyone help me understand what happened here and how to correct it?

This is an image where the lights showed properly.

A moment later, I took the following image:

Here are some examples of some on and others not:

[cicopo]

I'm just guessing but since LED's aren't very bright I suspect you'll need longer shutter speeds for the camera to see them. Try re shooting the lights under similar circumstances & progressivly longer (slower numerically) shutter speeds.

[ebiggs1]

You are certain they aren't "blinkers"?

[jrhoffman75 · Accepted solution]

Since LEDs are diodes I believe they actually "flicker" at 60 Hz. Since you were shooting shorter than 1/60 second you were catching the "off" cycle.

[ebiggs1]

The lights are actually blinking on and off at the same frequency of the electricity in your house!  Electricity is 120V 60Hz AC. And LED's work on DC.  This means they are only on half of the time.  It makes them "blinkers" 60 times every second.

That is why I asked if he was sure they sre not "blinkers"!

[DSchroader]

So, If I understand this correctly, I will need to drop my shutter speed to 1/60 in order to get the lights on consistenetly? There were 7 other photographers shooting at the same time as I was (at least 3 were shooting the same model at the smae time as I was). Why is it that I was the only one who had this problem? All of us tried to have the fastest possible shutter speed to try to avoid blur from movement of the model or camera shake since all were hand held.

[HPL]

I would say 1/45 to be sure that they were on (and get a tripod).