A few scanning tips

www.scantips.com



Milk drop collision - column going up, next falling milk drop coming down

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. Bigger studio units (like B800) are naturally slower than smaller units (B400), see next page.

For these pictures, the milk drop falls through a 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   
1/128 Power 40dsc_8293.jpg   SB-800 duration specifications

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

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.

But note that this photo is NOT an absolute speed test. 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 probably don't mean what you might assume they mean (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.

Main Next