Speed of flash units for High Speed flash photography

(the reason camera speedlights are called speedlights)

High speed photography requires a very fast flash (very short duration) to freeze the motion up close. Camera shutters and studio lights are not fast enough for this special duty, however camera speedlights used at their lower power settings are dramatically fast enough. The next page is about why this difference, but first, this page shows evidence about the truth of it.

But first, see the 40-second bullet-stopping YouTube video from Gretel Producciones in Spain. In the video, you can catch glimpses of the regular Yongnuo speedlights used to create the included still pictures of the stopped bullets (at about 30 seconds). At lowest power levels, speedlights have incredible capability to stop action. The extreme cases often require timers to trigger the flash accurately at the right instant, and these in the video delay a few milliseconds from the sound of the shot, to trigger the flash when the bullet appears at the expected location. More common photo cases may involve milk drop splashes (a falling drop sensor and a timer makes it easy and repeatable) or stopping hummingbird wings (which don't need any timer).

Lowest speedlight power (for speed) requires lower ambient levels, not dark, but dim, like indoor interiors, or shade outdoors, to prevent ambient exposure from being sufficient to blur the motion. The basic idea of high speed flash photography is that the ambient exposure (shutter speed) can be long and slow, so long as its result is too dim to show any blur. The speedlight flash at low power level is extremely fast and quick, occurring during that longer shutter time. Lowest speedlight power levels to be very fast, and stopped down aperture for depth of field, will require flash distances of only about arms length, but normally should not be a problem.

Then the photo series below compares a Nikon SB-800 speedlight to Alien Bees B400 (160 watt seconds maximum) and B800 (320 watt seconds maximum) studio lights, to show the comparison and difference of how well the speedlight flash stops the motion of a falling milk drop. The Alienbees are great, and relatively fast in the studio flash category, generally extremely adequate for any usual use, with some advantages such as higher power levels and can easily mount accessories like large softboxes or grids, etc, but these studio flash units are NOT speedlights.

A typical flash tube power pulse is shown at right. There is a strong fast initial peak of light, and then the intensity falls off relatively slowly (a few milliseconds). That's a bright initial flash followed by less intensity as the drop falls and the trailing tail weakens slowly. This is how flash capacitors discharge. At speedlight Full power level, the standard engineering flash duration specification called t.5 is a guideline (not the actual flash duration). Flash duration is measured to when the intensity is half the peak value, called the t.5 method (shown at right at T at 50%). Another spec is t.1, measured to the 10% power point, but the standard default flash spec is t.5 (specified by ISO). See t.5 vs t.1 on next page.

Most studio monolight flash units control power level by lowering voltage level instead of terminating the flash pulse. But speedlights are unique in that they implement lower power levels by always charging to full voltage, but shutting the flash tube off sooner, simply chopping off the trailing tail (stopping the flash tube current, next page), like perhaps at the half power point marked above, and results in much faster speeds (shorter durations, also measured as t.5). That speed freezes the milk drop splash above (1/64 power, about 1/30,000 second duration). The speedlight full power level is its slowest speed (not chopped shorter), but if at 1/16 power, might be only 1/10,000 second duration, which will stop most motion (called Speedlights).

The Alienbees B400 is has t.1 specs of 1/2000 second at full power, and 1/1000 second at 1/32 power, so that's great normally, with power advantage. But camera flashes are speedlights, and at their lowest powers (at closer range) are incredibly fast (and normally are still about 1/1000 second at higher 1/4 power level, very adequate for the kids activities). And the Nikon SB-800 is rated 1/17800 second at 1/32 power, and 1/42600 second at 1/128 power. Speed vs power adjustments in most studio monolights vary in opposite directions than speedlights, and higher power studio units (like the B800) are naturally slower than smaller units (B400), see next page.

For the pictures below, the milk drop falls through a photo-sensor gate, which triggers a timer, which delays for a time corresponding to the milk drop falling 24 inches (about 61 cm), and then it triggers the flash to capture the picture. The timer is adjusted so that the drop has fallen to the exact place where the camera was waiting, 24 inches below. The front of the Nikon 60 mm macro lens was always about 4 inches from the milk drop, and was set to f/16 on all frames (even for the SB-800 at 1/128 power). Shutter speed was on Bulb, manually opened typically about 1.5 seconds, to be ready while the drop was dropped and the flash was triggered, then the shutter was closed. The continuous ambient room light was dim, so that the 1.5 second shutter speed would not register at f/16, to prevent blurring the fast action already stopped by the faster flash. Flash distance was varied in each frame as necessary, to always hold the same metered f/16 exposure, ranging from several inches to about 15 feet for the B800 at full power (ISO 200).

The ruler markings below are cm and mm. The bright streak is the reflection of the flash on the moving milk drop. The trailing tail below the drop shows where the milk drop was at the time when the flash slowly weakened to zero output.

The speedlight is called a speedlight because to achieve lower power levels, its output is "chopped off" short to become dimmer low power, which is also an extremely short duration. You can see that speedlights become very fast at low power, but Full power is not so fast.

Most studio flash monolights set a lower voltage to achieve less power, which then gets slower at low power. Also seen here is that a smaller maximum power studio unit (has a smaller flash capacitor) is faster discharge than a higher maximum power unit, but is still slower at their lower powers. A very few studio flash monolights do also have a speedlight mode (Paul Buff Einstein is a popular example).

You can click any picture for a larger photo, which will show the faster differences more clearly. Enlarged, you can see the 1/128 power speedlight milk drop does have sharper edges.

The Nikon flash duration spec numbers are printed in the specifications section of most of the manuals, and are shown in this table. Many recent models have 1/128 power, one goes down to 1/256 power, and the older SB-600 stops at 1/64 power. Speed varies with flash design, so instead of guessing numbers, it needs specifications. But yes, in speedlights, lower power is much faster (shorter duration). But speedlight full power level is substantially slower than lower power.

My guess is that these flash durations are reasonably typical of any other speedlights, and suspect most any speedlight is around 1/1000 second at 1/2 power and 1/2500 second at 1/4 power. That's a powerful feature, and is what a speedlight is. But they are slower at full power level, Not in speedlight mode if at full power. Speedlight mode truncates (terminates) the flash before it would finish, to reduce power level, which also makes the duration greatly shorter (much faster). But Full power is full power, so it does Not truncate the flash pulse, it decays fully and naturally.

Speedlight design varies, in the choices of capacitor size and voltage and flash tube diameter and discharge current. These parameters affect speed and ionization and color temperature, so flash color varies with power level. The more powerful flashes must have a larger capacitor to store power, which is a slower discharge than smaller capacitors. I imagine the designers chose parameters to best average out the color over much of that range, but color has to vary with power level. Speedlights are a little more blue at low power, the low power tail is cut off (see an example). Almost all studio lights run the opposite direction of speedlights, at lower power they are slower and more red in the color (next page).

Note that speedlight Full Power level is much slower (longer duration, not truncated at all, NOT a speedlight then), and its Full Power numbers are an exception regarding duration. Full power speedlight duration is measured with the t.5 method (measured at the half power points, which is standard engineering practice for vague things, like flash duration and beam width, radio signal width and strength at distance, etc.). It's difficult to decide when the decaying tail stops at zero, but the half power points are more definite. More on next page.

So the speedlight duration Numbers may be shown as similar values for Full and Half power, but the t.1 rating (10% power points instead of 50%) of the actual full power duration is routinely considered to be 3x longer then the t.5 rating (so the SB-800 1/1050 second at Full power is closer to a slower 1/350 second (which is not so slow, it is still faster than the shutter speed), but this difference is only for Full Power Level and only different on speedlights). It will be related to the Maximum Flash Sync speed, like 1/200 or 1/250 second duration (and the Nikon D-800 camera can sync at 1/320 second). And full power is plenty fast enough for portraits, but has exceptional speeds at lower power levels. Half power is truncated shorter to be around 1/1000 second duration. All the lower levels are chopped off short to be low power, which then makes them even faster, very fast, and since no decaying tail, and these numbers would be actual flash durations. In contrast, most studio flash monolight units are voltage controlled instead of speedlight mode, and they are fastest at full power, and maybe twice slower at minimum power.

Specifically, most flashes (studio lights and speedlight Full power) have a long NON-truncated tail, slowly decaying in the standard way (see the small diagram above). Full power uses the engineering standard t.5 time duration which is measured at half power points (measured like studio lights, with lots of light continuing past half power). Which is why speedlight half power rating is approximately same duration as Full power (Full power is measured at half power points, but half power is chopped off (terminated) at half power with no decaying tail). Speedlight truncation is not exactly same as a 10% t.1 level, but level is zero below the speedlight power level set. So Half power is much faster than full power, and gets much faster yet at lower power levels (speaking of speedlights). And for Full power, t.1 time at 10% points would be about 3x longer than t.5 numbers (next page). I have taken the liberty to provide divided-by-three t.1 times in the chart. But speedlights are not in speedlight mode if at full power level.

Note that if the timer stops every falling drop above at the same place at the same time, the velocity must be the same. Falling 24 inches computes to take 0.35 second and reaches 11.4 feet per second velocity at the 24 inch point. Think of these next distances as a camera sensor size, as the time the drop rapidly crosses the sensor. If falling 24 inches, the speed is 138 inches per second, or 1 inch in 1/138 second, or 1 cm in 1/350 second, or 1 mm travel in 1/3505 second, or 7.7 miles per hour, which is slow for a distant moving car, but as seen here at only 4 inches distance is quite fast across our field of view, and gone very fast. At such close distances, the movement angular distance causes blur in photos.

So we need a lot of speed to stop fast motion, especially so when magnified by being seen up close. Shutters do not have much capability for this extreme motion. The easy way fast motion photography is done is with speedlight flashes (about any camera hotshoe flash model), which at low power levels, can be much faster than any possible shutter speed. Then in a dim setting, any slow shutter speed can be used (1.5 seconds above), so long as the ambient light is not too bright. I was using f/16 which shuts out dim light. So at f/16, the room can have enough light to see, but not too bright. Taking the same picture as a test without flash should be just a black frame, to not capture any blur from the ambient light. To add to the point, I take portraits with slow ISO 100, stopped down aperture like maybe f/8 (helps depth of field too), and maximum shutter sync speed like 1/200 second, and a test picture without flash, but with the studio lights modeling lights on, is a black frame (the not fully bright normal room lighting does not show up). That works for high speed photography too. In reduced lighting (dim but not dark), the above pictures were f/16 and ISO 100, and opened the shutter for around 1.5 seconds while the drop was dropped and the flash triggered. A dim ambient cannot compete with the close flash.

You can click the images for a larger view, and for the studio lights, it's impossible to say just when the faint milk drop blurred tail disappears (emphasizes the practicality of the t.5 method, it can be better defined and measured). I'm jumping ahead (next page), but the speedlight Full power spec says 1/1050 second, which is t.5 (half power points), and its t.1 (1/10th power points) would be 3x or 1/350 second to the 10% point. We can see the tail is at least 1 cm, which computes 1/350 second (hard to identify the 10% point). But I would judge the speedlight 1/2 power motion (above) to be 3 mm, which would be 3x 3505, or 1/1168 second. The spec says 1/1100 second, so our calculation is pretty close, but it involves approximations, like velocity. The spec would instead directly measure the actual curve of the light output on an oscilloscope.

I am certainly NOT knocking the Alienbees in any way. I have four of them, I use them, and they're great flashes, normal studio design, and wonderful for typical studio portrait work, their features are a Best Buy IMO. They are surprisingly fast (and specs specify both t.5 and t.1 specs), but they are not speedlights. It is just that attempts to "chop off" the output of a high power flash speedlight-style becomes more difficult to implement at high power levels. But recent advances, the Einstein flash model is 640 watt seconds power, but it also offers a speedlight mode. But of course, 1/16 of 640 watts is still quite a bit of power, and high power is slow, not fast, however speedlight mode makes a huge difference. And since voltage controlled studio lights become more red at low power, and speedlights become more blue, the Einstein model has a mode to use both modes simultaneously (voltage and duration), programmed to maintain constant color with varying power.

The photos above are NOT really an absolute speed test. In that any "measurement" of how much motion is visible is relative to the degree of closeup (how much magnification we use to closely examine it). These pictures are at near macro distances, lens was at about four inches.

This closeup size magnification greatly magnifies the fast movement and blur. Which is hardly typical of most other uses for studio lighting. We might not even notice the tiny milk drop if the framed area at 8 feet were a normal portrait view. Your standards then of what is blurred or not would be vastly different from this extreme case. The 11.5 feet/second above is under 8 MPH, but it is a really tough job up so close at a few inches, even if trivial at several feet farther out. Stopping a moving object depends entirely on how closely you want to examine that object. However there is a very obvious difference in the relative capabilities of the lights. The technique is, low power on a speedlight (fast), in dim ambient so shutter speed does not blur it.

The flash unit's specifications instead measure the actual duration of the flash directly. Oscilloscopes might do that easily, and I see no reason to doubt the specifications (if they say what it was they measured). However the normal t.5 specifications for flash may not mean what you might assume they mean (t.5, next page).

My only point is that high speed flash photography of milk drops is fun, but tough. It is hard to stop them up so close, but a thyristor-type speedlight at a low power setting is exactly what you need to do it.

Note that HSS flash is the slowest possible flash, because HSS cannot stop motion at all. HSS is NOT High Speed Flash, it is the full opposite simply meaning High Speed Sync. Meaning, it does allow faster shutter speeds (faster than Maximum Sync Speed of conventional flash). To do that, HSS becomes a continuous light (is "on" for the duration of the open shutter). Continuous light cannot stop motion. So regular speedlight mode can be a higher power level than HSS, and can also freeze action much faster than any possible camera shutter speed. I think there is absolutely no possible reason to consider HSS mode indoors, it would be counterproductive in every way, with no advantage. You might be able to use HSS in sunlight outdoors for the purpose to allow a faster shutter speed with flash, perhaps to stop ambient action or maybe for the resulting wider aperture to obtain less depth of field, but HSS flash range will be limited compared to speedlight mode.

More detail continued on next page, to More about speed of flashes.

The third page is an easy setup for photographing splashes, like milk drop splashes.

But photos of two drops colliding requires a way to control dropping the drops, a timed fluid valve. I offer pages about that too.

Menu of the other Photo and Flash pages here.