Andy,
Did this calculation on the back of an envelope last night.
At 68 mph a car travels at 100 feet/sec.
According to Wikipedia, typical flash durations 1/1000 to 1/200 sec, (Much
longer than Chris suggests, but these figures in line with my recollections
20 years ago), so would get movement
1/1000 sec 1.2 inches movement
1/200 sec 6 inches.
Of course driving a car that fast close to the photographer could constitute
a traffic hazard.
Jim Thyer
----- Original Message -----
From: "ADavidhazy" <andpph@xxxxxxx>
To: "List for Photo/Imaging Educators - Professionals - Students"
<photoforum@xxxxxxxxxxxxxxxxxx>
Sent: Saturday, January 29, 2011 3:16 AM
Subject: Re: flash duration
I think that what Alberto suggested re: CD player or such running at 7200
RPM just might work if its cover is black (paint it?) and put a white
stripe along radius. It will be running at 120 RPS or close to it (wow
that seems real fast!) or about 4300 degrees per second so a 1/1000 second
exposure time should leave an arc of about 43 degrees. But I suspect the
flash will last longer than that and also that the output of the flash is
not constant. It should show a fast rise time and then a longer decay
time. Duration is typically expressed as time for 1/2 peak to 1/2 peak so
it will be less than the total length of the arc ... you should see a
tapering off of the light output along the blur.
It is surprising how long flash durations really are when you give a more
realistic estimate of this at 1/10 to 1/10 or so of peak output.
Now what else might make a good measuring stick for this .... falling
objects as he suggested also. Chris's suggestion is a bit extreme but
interesting. A car going past you at 60 mph carrying the flash and firing
it as it passes by a camera with open shutter? Hmmm got to think more
about this. ;)
FWIW,
Andy
