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Milk drops collision

Speed of flash units for high speed flash photography

(the reason speedlights are called speedights)

High speed photography of milk drops requires a very fast flash to stop the motion up close. Camera shutters and studio lights are not fast enough for this special duty, however camera speedlights used at their low power settings typically are fast enough. The next page is about why this difference, but first, this page shows evidence about the truth of it. The photo series below compares a Nikon SB-800 speedlight to Alien Bees B400 and B800 studio lights, to show this difference of how well the flash stops the motion of a falling milk drop. The Alienbees are relatively fast in the studio flash category, but Speedlights at lower powers are incredibly fast. Higher power studio units (like the B800) are naturally slower than smaller units (B400), see next page.

A typical flash tube 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 captures the milk drop as shown, a bright initial flash followed by less intensity as the drop falls and the trailing tail weakens. Most studio monolights use lower voltage levels for lower power levels, which makes low power slower. The flash duration specification is a guideline (not the actual flash duration), measured to when trailing decay is half the peak value, called the t.5 method (shown at right). Speedlights are unique in that they implement lower power levels by shutting the flash tube off sooner, simply chopping off the tail (next page), which results in much faster speeds (shorter durations). The speedlight is the slowest light at full power, but is easily the fastest light at 1/4 power level or less. It might be only 1/18,000 or 1/20,000 second duration at 1/32 power level, which will stop about any motion (called Speedlights).

For these pictures, 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. Shutter speed was on Bulb, manually opened typically about 1.5 seconds, 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 slower shutter speed would not blur the fast action already stopped by the faster flash. The front of the Nikon 60 mm macro lens was 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). Flash distance was varied in each frame to 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 are cm and mm. The bright streak is the reflection of the flash from the moving milk drop. The trailing tail below it shows where the milk drop was at the time when the flash finally weakened to zero output. You can click any one for a larger photo.

Stopping a milk drop which has fallen 24 inches

 Nikon SB-800AlienBees B400AlienBees B800
Full Power 02dsc_8385.jpg 04dsc_8300.jpg 06dsc_8349.jpg
1/2 Power 08dsc_8268.jpg 10dsc_8306.jpg 12dsc_8356.jpg
1/4 Power 14dsc_8272.jpg 16dsc_8309.jpg 18dsc_8357.jpg
1/8 Power 20dsc_8278.jpg 22dsc_8321.jpg 24dsc_8366.jpg
1/16 Power 26dsc_8281.jpg 28dsc_8327.jpg 30dsc_8369.jpg
1/32 Power 32dsc_8283.jpg 34dsc_8331.jpg 36dsc_8372.jpg
1/64 Power 38dsc_8290.jpg SB-800 duration specifications
(in the flash manual, specs in rear)

1/1050 sec.   at M1/1 Full output (t.5)
1/1100 sec.   at M1/2 output
1/2700 sec.   at M1/4 output
1/5900 sec.   at M1/8 output
1/10900 sec. at M1/16 output
1/17800 sec. at M1/32 output
1/32300 sec. at M1/64 output
1/41600 sec. at M1/128 output

Full power is a NON-truncated tail, slowly decaying in the standard way. 1/1050 is the standard t.5 time. t.1 time would be 3x longer, or 1/350 second (next page).

But speedlight 1/2 power is truncated, chopped off. The Full/half spec times are about the same number, only because Full power is t.5, measured at half power points. But actual 1/2 power is chopped off, and actually stops at the 1/1100 second. 1/2 power is about 3 times faster than full power.

1/128 Power 40dsc_8293.jpg

Note that if the timer stops every falling drop 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 11.5 feet per second velocity at this point. Or 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.
You can click the images for a larger view, and for the studio lights, it's impossible to say just when the faint tail disappears (emphasizes the practicality of the t.5 method). I'm jumping ahead (next page), but the speedlight Full power spec says 1/1050 second, which t.5 (half power points), and its t.1 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 speedight 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.

But note that this photo is NOT really an absolute speed test. In that any "measurement" of how much motion is visible is relative to the degree of closeup (how closely we examine it).

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 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 close at a few inches (1/128 power), 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 do that easily, and I see no reason to doubt the specifications (if they say what it was they measured). However the specifications 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 close, but a thyristor-type speedlight at a low power setting is exactly what you need to do it.

Continued on next page, to the Reason why studio lights are slower.

Menu of other Photo and Flash pages here.


Copyright © 2007-2010 by Wayne Fulton - All rights are reserved.

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