Flying Squirrel

Two images on a single frame  

The Subject::

   The Black Oak forests around our California studio in the Sierra Nevada support a thriving population of Flying Squirrels. These attractive little animals are regular visitors to bird feeders, but are rarely seen as they are wholly nocturnal.     A flap of skin between the front and hind legs extends to form an aerofoil surface when they jump, enabling them to glide 150 feet (46 meters) or more from tree to tree.   The Northern Flying Squirrel (Glaucomys sabrinus) is widespread across Canada and through the forests of the Rockies, Sierras and Appalachians in the United States.     A second, more familiar species occurs in the southern States.

The Technique:

   To establish a regular flight path, along which we could photograph the squirrels, we placed offerings of peanut butter on a 3ft section of tree trunk suspended vertically on a rope from above.  This feeding station with its peanut butter bribe could only be approached easily along one branch.   Gradually, over several nights, the bait was moved back until a jump of six and half feet (2 metres) was necessary to reach the landing site.    Observing the squirrels over several weeks,  we found that they followed a predictable flight path,  taking less than half a second to cover the distance.  We also confirmed  that they were not disturbed by our equipment or observation lights.  

The Photography:

   1)  Background. Multiple flash images on a single frame are best shot against a dark background to avoid ghosting.  As the squirrels are only active at night, such a background was completely appropriate.

   2) Camera.  We chose a medium format Rollei 6006 with 150mm lens, as this gave a field of view some 20 inches (½ m) square when focused at 5 feet (1½ m).  This would accommodate variations in flight path and give us the freedom to crop without loss of detail.  It also allowed us the ability to monitor timing and exposure on Polaroid instant film frame by frame as we made small changes.     We minimized focus problems by keeping the film plane parallel to the flight path.     Because the woods were in darkness this was virtually an open-shutter situation.  A half-second shutter speed would allow us to capture all the action without admitting significant ambient light.

   2) Lighting.  Applying the velocity formula (Tx c/M  or  Flash duration= velocity x circle of confusion / magnification) we found that a flash duration of 1/10,000 sec. would provide blur-free images.  We actually used two flash units with a fixed 25-microsecond pulse, which were positioned just below the camera.   These were arranged to fire independently through a delay timer. The power of each was adjusted to allow an aperture of f/11,  taking into account color balance and reciprocity law corrections.

   4) Detection.  A single, vertical light beam initially appeared sufficient, but we found moths attracted by the observation light caused false triggering, so we ultimately used a pair of crossed retroreflective infrared sensors.  These were mounted on a sturdy frame and positioned a few inches beyond the take-off point at the end of the branch.

   5) Synchronization.  As the squirrels took almost half a second to complete the jump, the latent response time of the camera was not a problem and could be included in the delay to be introduced between detection and firing.  In our Rollei 6006 this is 60 mSec. with the mirror raised.    To create a double image, two delays are needed.  We therefore used a twin channel delay timer, which was tripped when the crossed beams were broken simultaneously.

   Having initially estimated that it would take about 400 mSec. for the squirrels to enter frame after breaking the beams, we then noted their position by eye as the flashes fired and adjusted the timings empirically.    To get the images about 10 inches (0.25m) apart, the second delay  needed to be just 100 mSec.   Polaroids were used to confirm exposure and fine-tune the delays before slipping on a magazine loaded with ISO 100 film.

System Summary:  

Once the crossed beams had been broken together, the camera was fired electrically at a shutter speed of 1 second.  As the shutter opened, the flash contacts tripped the delay timers, which then fired the flashes.  The shutter contacts tripped the delay timer, which then fired the strobes at the selected intervals before the shutter closed again.

General Photographic Setup

 

 

 

 

 

 Timing Diagram Caption

A timing diagram provides a valuable way to understand the sequence of events in setting up an action photograph. The more complicated the timing requirements, the more helpful the diagram becomes. 

The lower (green) section of the diagram summarizes the movement of the squirrel as it jumps from a branch to its target five feet away.  The curve of the graph approximates the squirrel's velocity as it accelerates at take-off and gradually slows down in preparation for landing.  As the images show, just before landing the squirrel "flares out", increasing drag by adopting a more vertical attitude.  

The upper (blue) section shows the equipment timings used.  The sequence begins as the squirrel breaks the crossed detection beams on take off.  Following a latent response time of 60 milliseconds the camera shutter opens.   At the same time the sync contacts trip the first delay timer.  After an interval of 220 milliseconds the first flash is fired and the second delay timer started.  At the selected time, 100 milliseconds later, the second flash fires.   

 It can be seen that the action is completed in 0.4 second, which allowed the camera shutter to be conveniently set at half a second.