Incandescent bulbs do flicker even when not dimmed, simply because of the sinusoidal mains voltage, made even worse by the U^2/R power relationship.
Thermal "inertia" of the filament smoothes the flicker, but not completely. Flicker is small enough so that practically everyone, even those sensitive to flicker, are fine in incandescent light. But if you shoot animation / stop motion / etc. (where you expect every frame to have the same exposure), or do scientific measurement expecting to get repeatable photographs, then this flicker with maybe 10-30% of amplitude in intensity, plus a color shift during the cycle, is a showstopper. The simplest solution is to use long enough exposure time. If unable to do that, rectifying the AC power and feeding the bulbs with DC is really the only option. High-frequency (maybe a kHz) PWM is acceptable. This allows you to build a feedback loop based on measured power, or even better, light intensity.