LED Hybrid Flash
There are a handful of "LED Flash" products on the market which will fulfill this requirement. One example is the Rotolight NEO 2 but please do shop around. This method is ideal if you cannot bring a tripod on site. The key will be to choose a flash unit which offers enough light for you to use short, "handhold" exposure times while offering a long enough flash duration to induce the desired motion blur. There are three functional modes for these LED systems and a given flash may offer any number of them.
- Continuous PWM mode: this is a constant LED output much like a flashlight. The light will flicker in order to control the amount of light output. More expensive units will feature very rapid PWM, up to 64,000 hertz. Note that there are many cheap units out there which claim to be "LED flashes" but offer no synchronization to the camera. These are not really flashes but may be suitable for your application if the frequency is high enough. If your adjustable power PWM light flickers too much you may be able to improve the situation by setting it to full power and using an ND filter.
- Continuous Direct Driven mode: this mode actually powers the LED constantly. Output power is controlled either by varying the bulb wattage or engaging less than the entire array of LEDs. The circuitry for direct drive is more complicated and these units are commensurately more costly. Nevertheless, motion will not be a concern with such a light. Direct driven LED flashes are more likely than PWM flashes to offer camera sync and some offer the choice of always on or camera triggered. Check the duration (and adjustability) of a camera triggered flash carefully before buying.
- Overdriven Flash mode: In this mode of operation, the LEDs are driven beyond their rated wattage for a short period of time. This allows a several-fold increase in brightness. Overdriving is possible for short periods of time because the p-N junction in efficient LEDs will reach overrated light output momentarily before reaching a damaging temperature. While some overdriven flash units only illuminate via a sync signal, it is more common for this mode to be a feature on a continuously illuminated unit.
The best LED flashes will allow use as flashlight, modeling light, adjustable-duration flash, and stop motion flash. The NEO2 mentioned features continuous, video compatible illumination with adjustable frequency and includes an overdriven flash mode which allows duration between 1/50s and 1/1000s. At $400, it may be more than needed for this application. Expect to pay $200-$500 for a powerful, full featured unit and assume, with exceptions, that anything for less than $50 is a flashlight with a hotshoe mount (not that there's anything wrong with that.)
NOTE: if you are using an LED flash for primary exposure you could still utilize a "normal" spark-gap flash unit either remotely triggered or manually triggered with the camera in bulb. A low power strobe from the normal flash unit will add "sparkle" to the otherwise continuous motion. If you have the equipment, a remotely synced rear-curtain fill flash would be ideal.
To capture motion or selectively illuminate in nighttime shots, I've achieved decent results with "painting" in the light using a flashlight. This can be time consuming but allows an unparalleled level of control over the scene lighting. I use a rather powerful LED light (a converted Mag Lite which is extraordinarily durable) In particular this LED has a very blue color temperature, close to that of flash. I will often add fill flash to introduce the "sparkle" you are eluding to. The flash works best at 2-4 stops under exposure so that is just adds "pop" to the scene and maybe lights the background slightly, without washing everything out. The two main disadvantages are that (A) you must be very spatially aware so that you can illuminate the correct objects in darkness when you may not be able to see them and (B) this is not quick. I've been practicing this technique for a decade and can occasionally complete a shot in 3 minutes but 10-15 minutes per shot is more realistic.
Here is a basic procedure for photographing moving water in dark environments
- Use a Tripod (obviously)
- Set the camera to bulb or use a locking remote
- Use the flashlight to help yourself focus
- Shut the flashlight off or cover it up then open the shutter
- Slowly play the light over the portions of the scene you want illuminated, avoiding overlapping as much as possible.
- Add the flash for background light or "pop"
- You will need to experiment to determine how many seconds of flashlight equals a full exposure
- To estimate exposure, perform a "test exposure" with the light stationary then estimate the size of an objects in multiples of your flashlight beam. During the actual exposure, count off seconds until you reach a total of multiples x exposure time.
- When using the flashlight, Just like dodging or burning, never stop moving.
- The aperture you use will be determined by the amount of ambient light (and how bright you want it to be in the final image) ISO will likely need to be low in order to reduce noise
- The smaller the aperture, the slower you can move the flashlight. The only limit is your patience.
- Because the shutter will be open for a very long time, you can move around to light the subject from the side or even behind (provided you move quick enough not to "ghost" the frame
- Very long exposure times also mean that you may be able to "peak" by quickly playing your flashlight over subjects if you lose your bearings.
- For precision you may wish to use a snoot over your flash or flashlight. A pringles can with the bottom cut out works surprisingly well.
- The long exposure time also means that your flash can be any time you like. I recommend rear curtain sync since the flash will ruin your scotopic vision temporarily
- The most common issue with this method is that the ground very near the camera is over-illuminated. Aiming upward, using a long lens, or carefully shaping your light will mitigate this.