Got a question from a reader on Twitter over some strange camera performance.
@SomeGadgetGuy 1. Could you explain why this happens? I assume its static electricity? 1/2 pic.twitter.com/t8xUPzZO6n
— Vincent Soon (@Vsmh96) September 16, 2015
The first time I ran into something similar on my DSLR I had a minor panic attack that my sensor was damaged. While it can be frustrating to deal with, it’s a fairly common issue we face with consumer fluorescent lights.
The light that comes from a bulb is not constant. The electricity we use is AC, or alternating current, and cycles between 50 or 60 times a second. This refresh rate is fast enough that the human eye perceives it as constant. You can also think about film and video, where 24fps and faster frame rates produces fluid movement, even though each individual frame is a still photo.
Inside incandescent bulbs, light is created by heating a filament. As the AC cycles, there might be a subtle pulse, but the filament retains heat in between cycles, so the lighting appears more constant.
Fluorescent bulbs work by exciting mercury vapor, and they have a more distinct flicker. On consumer grade bulbs, the quality of each cycle can be very inconsistent, and some people can be very sensitive to that strobe effect.
That brings us to your camera. Digital sensors don’t work quite like film cameras. On a film camera, when you hit the shutter, the entire frame is exposed all at the same time. On a digital sensor, the scene is scanned row by row of pixels from top to bottom. The “shutter speed” is tied to how quickly the sensor will scan. The longer the shutter, the longer it will take to get to the bottom of your shot.
This is why we sometimes see strange distortion in video. A fast moving object can move quite a bit during the time it takes to scan from the top of the frame to the bottom.
Getting back to your original question, as the light from fluorescent bulbs is flickering, and the output is inconsistent, AND the camera is exposing parts of the frame at a time, this means that we’ll run into situations where part of the frame is darker AS the sensor scans it, and part of the frame is brighter.
It’s really difficult to compensate for this issue on our cameras. Often the best we can hope to do is match the shutter speed to the AC cycle rate, and hope that we catch the photo at the right time to sync with the lighting. While that’s kind of a bummer, at least you don’t have to worry about static discharge.
For more info on what’s happening here, search for the terms “rolling shutter effect” and “focal plane shutter artifacts”.
[Incandescent bulb image courtesy of KMJ, fluorescent bulb image courtesy of Oimax]
