Shutter speed when using LED lights

Hi,
Several people in the past DP Review forum have commented that they carry one of those little battery-powered white LED lights (about the size of a larger phone) as an extra source of light. I've bought one on sale but haven't had much chance to try it out yet. Can anyone please tell me the range of shutter speeds at which they are actually useful? The LEDs pulse - so I'm guessing that there's a potential for higher shutter speeds to limit the number of pulses that can be captured. Is that right, or am I barking up the wrong tree? And if right, at what point do the limitations begin?

Thx in advance, Rod

The answer depends on the LED lights themselves, and how they are designed. I've not found any shutter speed that doesn't work with the Westcott battery powered wands, for example.

I haven’t tested extensively but in practice I’ve had no problems using variable color FalconEyes F7s or the color temp. adjustable LumeCube desk lamp. The latter primarily serves to make me look my best for online meetings, but is also put into service for impromptu tabletops.

Edit to add: I’ve also used free with coupon Harbor Freight LED panel flashlights without issue. (Though a diffuser helps prettify the light produced.)

If they don't pulse, then they can be dealt with just like they were any other kind of continuous light source; it doesn't matter that they are LEDs. If they do pulse, then varying the shutter speed will give a large range of banding artifacts. The best fit is an exposure time that is exactly the same length as the pulse cycle, or an exact integer multiple of it). Many lights pulse like a rectified version of the power mains, making the ideal exposure time 1/100s (or 2/100 or 3/100) or 1/120s (or 2/120 or 3/100), but your device seems to be battery-powered, so if it does pulse, it may have some other pulse length than 1/100 or 1/120, since it has to create its own clock, anyway. If your light does pulse, I would guess at a higher frequency, in which case the banding should be weak around 1/250 or slower. It's easy enough to find out, though; just set a long, fast lens to the largest aperture, with manual focus at infinity, and the shutter speed to the fastest available with e-shutter mode and put the lens right up to the light, and photograph it, If it is not blown out and it is evenly colored and bright, you have nothing to worry about.

My Wescott and Aputure LED lights appear to use pulse width modulation at some high frequency, maybe around 40 KHz. I don't think you'd want to make the efficiency compromises that go with using dc power to make variable intensity LEDs.

I was thinking a little earlier, when writing my first reply, that a light panel could be designed where individual LEDs were pulsing, but the net effect of the whole panel was not pulsing, for most intents and purposes, due to phase differences. Such a light would clearly benefit from a diffuser, not just to get the individual LEDs from reflecting in glare or causing stepped shadow edges, but also that the light panel could cause pulse banding localized only to such shadow edges.

Wouldn't the net light output of such a light be lower?

You could still vary pulse width; you would just not have to suffer the deeper or slower modulation in lighting that you get when everything is in sync, with PWM.

Let's say you had 3 phases; the only time you could have any blackout is when the duty cycle is less than 33.33%, but even if you do turn the light down that much, the blackout is 3x as frequent as when everything is in sync, allowing 3x the shutter speed without blackout in the banding. And blackout or not, the pulse period would be 1/3 as long.

None of this would make much difference with slower shutter speeds, but with higher ones, it might be beneficial to have staggered PWM. 1/16000 with a slow rolling shutter could reveal banding that you would not see at 1/50 with a fast-rolling or global shutter.

I didn't realize that some commercial lights were running that fast. That seems pretty safe for non-action camera shutter speeds.

Then there’s the phosphor persistence that smooths things over.

I was thinking about that possibility, but I was wondering if it was challenging to use such materials in a light where you might want radiation to stay a bit narrow. I would expect that it would certainly be relevant if the phosphor material was on the surface of the lamp, but if it were further inside the lamp perhaps the optics in the surface of the lamp could keep it narrow.

With the current LED technology, the only practical way to achieve broad spectra is to pump phosphors with short wavelength LEDs.

Hi John, Jim, Jaberg, & Flashlight

Thanks for your responses and the discussion. It seems to me that the answer might well depend on the specific device - eg, power source, LED pulse length and pulse rates (which might vary with intensity settings), phosphor screens and so on. Some experimentation might be in order - you can't beat empirical data. I will try with my own little light (an Iwata Genius Light) when I have some time (not for several weekends), and report back.

Regards, Rod

I just threw that in as an extra when thinking about how to have variable lighting without PWM; my main idea was just about changing the number of LEDs, and changing sizes of them to vary intensity. I guess that I just have an aversion to pulsing lights, ever since my dormitory back in the '70s had a dimmer switch installed in my room and I couldn't hear distant radio stations anymore, as long as the lights were on.

Just take a camera with a known slow rolling shutter and try a sampling of varied exposure times, shooting the light directly, up close and out of focus, and that will show you what you're dealing with, on a practical level. If you start with the fastest shutter speed and there is no banding at any of the lamp's intensity levels, then you probably won't see any banding at any shutter speed, because the fastest shutter speed yields the most distinct, sharpest banding.

Hi John,

Thx. Just out of interest, why shoot the test with the light out of focus? Is that about sensor protection or another reason? (I have an XT4 and never shoot video - I have no knowledge or experience of rolling shutter).

I must admit that I wasn't expecting the technical discussion that ensued from my original question. I'd observed the LEDs pulsing in another completely different application - LED torches. If you move an object quickly in front of an LED torch you will see a stroboscopic effect. The frequency is certainly not high in that application, but the observation lead me to ask the question about what will happen with one of these photographic/video lights.... I guess I'd anticipated that even for stills, a fast enough shutter speed would catch moments with the pulse off and the light output therefore absent (or diminished if they're not all pulsing at the same time).

Regards, Rod