What is actually happening
- The tube pulses at twice the frequency of the supply feeding it, putting a full cycle somewhere around 1/100 to 1/120 of a second.
- Within each cycle the output varies in intensity and in colour temperature, not just in brightness.
- A shutter faster than that captures a slice of the cycle rather than an average of it, so consecutive frames land on different parts and come out differently.
- The effect appears as inconsistency between frames, and on cameras that expose the frame progressively it can appear as banding within a single frame.
The other problem with fluorescent light
One answer pointed out that flicker is only one of the reasons these lamps are difficult. Their output is missing a substantial part of the red end of the spectrum, which is what makes skin look greenish and unwell, and tubes of nominally the same type frequently do not match each other. Those are colour problems that a white balance setting cannot fully undo, because the light itself has gaps in it rather than a simple cast.
What to do about it
- Use a shutter speed at or slower than the flicker cycle, so each exposure averages a whole cycle instead of sampling part of one.
- Many cameras now include a flicker-detection mode that watches the light and times the shutter to fire at the same point in each cycle. One answer noted these arriving some years after the question was asked.
- Where possible, replace or overpower the light rather than working around it.
- Test before it matters. The inconsistency is obvious across a burst and easy to miss in a single frame.
Why this matters beyond stills
The same physics produces the flicker and rolling bands familiar from video shot under these lamps, and the same fix applies: match the shutter to the cycle of the supply rather than choosing it freely. It is one of the few technical problems where the stills and motion answers are identical.