Minimum LED pulse-time for paper before failing reciprocity

[radiant]

Here's an easy experiment that @koraks and @vedostuu (and anyone with PWM'd LEDs) can try. You will need one or more neutral density filters, and a light-meter that can measure PWM'd light (any analog meter will work).
BTW, the spec for controller-chip in the WS2812 says its PWM frequency is 1.2 kHz, so it would be good for testing reciprocity.

I was looking for the WS2812 PWM, but I couldn't find it. Thanks for that information!

However I'm using time to control exposure so PWM reciprocity doesn't affect me. Or if it does, it has been compensated. I try to calibrate the green/blue times once I get stouffers in my hands. I don't have control over dimming currently in my fstop timer and probably not going to implement one since the output power is within good exposure times already.

BTW: you can control the LED current with quite simple MOSFET circuit too - and that doesn't cause any PWM issues if you use pots / switches to control it.