What Fluorescent Light Can Do To Your Photos
Discoloring in bands
By PF Staff in Articles and Tips on Oct 9, 2012
Forum member isb.deep recently posted a series of images taken indoors under (seemingly) constant lighting, but nevertheless the images came out with varying color casts; in some cases these color casts took the form of horizontal bands.
The image above shows an example with a yellow band at the bottom of the image.
We find that this issue, its explanation and the remedy deserves a wider audience hence this blog post.
The explanation of this unwanted effect was provided by users SpecialK and Aegon and is quite interesting.
The images were shot with a fast shutter speed (1/320s) under fluorescent light. Two factors are at play here:
1) Fluorescent lamps/bulbs flicker with the cycle of the AC current (1/60s or 1/50s) and the color of the light varies over this cycle.
2) At high shutter speeds (above 1/180s for current Pentax DSLRs) the focal plane shutter of a DSLR is effectively a horizontal slit that moves vertically across the sensor and thus only a part of the sensor is exposed at a given point in time, not the entire sensor.
So what we see in the above image is that when the shutter was open over the lower part of the image the light was yellowish and we get the yellow color cast. As the shutter moved up over the rest of the image the light turned white and the colorcast disappeared.
The only way to avoid this effect is to use a shutter speed slow enough that the light goes through its entire cycle of color while the shutter is open over the entire sensor. This means 1/60s or 1/50s (or slower) depending on the frequency of the AC current in your area.
You find isb.deep's original thread with more examples of inconsistent color and the related discussion here.