Inconsistent exposure in sequences

[leonardturner]

I have recently noted a rather marked problem with sequences shot with LED Trond lights; multiple exposures within the stack were 1-2 stops underexposed in a random pattern. I've not noted any flickering of the lights, but have not noticed this in a smaller stack lit with a different (battery powered) LED. I'm hoping this is not a problem with the electronic shutter in my Nikon 850. Has anyone else noted this camera or lighting problem?



Thanks,

Leonard

[physicsmajor]

Most LED lights attached to mains use switching power supplies and dim via pulse-width modulation (PWM), where instead of controlling the continuous current to the LED it is instead driven at 100% for a fraction of the pulse repetition cycle. This is done because it's cheap and efficient, but depending on the pulse cycle frequency chosen can manifest as flicker or inconsistent exposure. The Trond advertises "flicker-free" but this doesn't exclude a PWM driver - it means they're switching high enough they think the human eye won't notice. Constant current driving circuitry is generally much more expensive.

Pretty much every dimmable LED system has a different pulse repetition frequency so some may be fine while others create problems for you (usually when your shutter speed is on the order of half or less the pulse repetition frequency). You may be able to "blend" out these effects by exposing significantly longer per shot.

Without getting too far in the weeds though, you can usually isolate these effects simply by running the light at the highest intensity. Are you using these lights on their highest settings? For most PWM systems setting to max will keep the LEDs on 100% of the time so the underlying PWM is irrelevant.

Even with 100% I occasionally see minor variation in intensity from my panel LED lamps in stacks which I attribute to their switching power supplies, but this is usually less than half a stop and Zerene corrects it completely.

[leonardturner]

physicsmajor, thanks so much for your prompt and very helpful reply. I believe that the Trond lights were at brightest intensity, but am not absolutely sure. I repeated a stack using a pair of Janso lights, and had a similar, but not as marked variation, much as you described having experienced. I then shot another stack with a pair of battery driven units of different manufacture, and can see no variation at all, so my worries about camera malfunction are resolved. Incidentally, the Tronds were of two different generations.

I think we have proven your case and removed my worries.

Thanks again,

Leonard