There is more to a BLUR than meets the EYE!
Imaging and Photographic Technology Department
School of Photographic Arts and Sciences
Rochester Institute of Technology
This is a little project that has quite significant
ramifications for encouraging the awareness of photography and other
students to technical uses of the photographic process.
It is a project that I include as an introduction to basic
concepts in technical photography in my own high speed photography
course that eventually also deals with more sophisticated applications.
I use this project as a sort of "ice breaker" to the field. You may
find it interesting as well.
Helicopters, or the whirligig seeds of several species of
trees, are so much a part of our daily experience in this part of the
country that almost everyone has noticed and played with them at some
time or another. I bring several seeds that I pick up from the street
on my way to school to my classroom and set up a problem to the
students based on their behavior in flight. Namely, I pose the question
of what kind of photographic equipment they might use and how they
might go about figuring out the rate of fall and the rate of rotation
of one of these things.... as I drop one from my hand.
Several suggestions usually surface quite naturally. Most
involve the use of video cameras, high speed motion picture cameras or
stroboscopes. The students quite often realize that these devices exist
and that they indeed would be able to provide crucial data needed for
the analysis of this seemingly very simple event.
It is usually obvious even to them that what one needs to
determine is the time that it takes the seed to fall a given distance.
What is not so obvious is how to determine rotation rate. It turns out
that the principle essentially is the same. One needs to determine the
number of revolutions that the seed makes in a given amount of time.
Anyway, after a brief discussion of basic requirements I get
back to the "tools" that are usually suggested and flatly state that
all the equipment mentioned above is too sophisticated and expensive
and that in "real world" situations sometimes they have to deal with a
very low budget yet be able to solve seemingly very difficult problems.
Such as the one I posed by dropping the seed. Botanists would be very
interested in the solution of this problem.
Eventually we agree that the most important item that we
really need is an accurate stop watch and an automatic monitor of the
seed's location and position in space at two different times. It turns
out that a simple camera neatly provides both of these capacities.
If a seed is dropped from some height it acquires a "terminal"
velocity. Once this is achieved the shutter of the camera is opened.
When this happens the seed is located at a particular location
in space. When the shutter closes it is at a different, lower, location
in space. The elapsed time is the exposure time that was set on the
shutter. By including a yardstick next to the falling seed the location
of the seed from the start of the exposure to the end can be determined
quite accurately. Dividing the distance traveled by the exposure time
set gives the average velocity of the seed.
What is more, by examining the blurred image produced by the
falling, twisting, seed it is possible to determine rotation rate as
well. Note that the image consists of a wavy line caused by the blades
of the "propeller" being at right angles to the camera at certain
times. Two consecutive edges of the "corkscrew" that the seed describes
while falling stand for one whole revolution. With a little ingenuity
is is possible to determine rotations of even less than a complete
To determine the seed's rotation rate count the number of
edges or ridges visible between the top location of the seed and the
bottom and divide by the exposure time. This will give the rotation
rate in revolutions per second. Multiply by 60 and you get RPM. Pretty
Any kind of film can be used but to make sure that this works
well make sure you use as black a background for the falling seed as
possible. Use an exposure time of 1/8 to 1/4 second and overexpose the
film by about 2, 3 or even 4 stops. The overexposure is necessary
because the seed will not be occupying the same spot in space during
the exposure. Throw plenty of light on the flight path of the seed and
keep the light off the black background. Possibly use a spotlight
pointing straight up from below the seed.
Place the camera on a tripod, keep the back vertical and place
a yardstick next to the flight path of the seed to give you the scale
needed to determine how far it has fallen during the shutter open time.
You may have to experiment a bit to make sure the camera is not too
close to the seed or you will not get the whole blur into one frame. If
either end of the blur is missing you can not make a measurement
because you can't tell where time started or ended.
SPINNING SEED IS PHOTOGRAPHED WHILE FALLING NEXT TO A
YARDSTICK WITH THE CAMERA SHUTTER SET TO 1/4 SECOND. IN THAT TIME THE
SEED FALLS 11 INCHES FOR AN AVERAGE VELOCITY OF 44 INCHES PER SECOND.
IN THE SAME TIME IT TURNS ABOUT 5.5 TIMES FOR A ROTATION RATE OF 22 RPS
OR 1320 RPM
The idea is to have some fun, learn a bit about making
measurements with a standard camera and possibly explore other
applications of blurred photographs.
If you would like to discuss this project with me I would be
happy to provide whatever assistance is needed. Contact me at firstname.lastname@example.org
Original title of this article was: SEEDS KEEP FALLING ON MY