Cool! Not only do I get some good tips on testing shutter accuracy, but a quick lesson in the physics of motion as well! Thanks to everyone for your ideas! Phil Penne -----Original Message----- From: Steve Hodges [mailto:shodges@wantree.com.au] Sent: Tuesday, June 11, 2002 7:53 AM To: List for Photo/Imaging Educators - Professionals - Students Subject: Re: Testing shutter speeds Roger Eichhorn wrote: > > In a time t in seconds, a heavy object will fall a distance x (in > meters) = gt^2/2, where the t^2 is time squared and g is the > acceleration of gravity, 9.8 m/s^2. Thus, at 1/400 second, the > object will fall 0.0306 mm, hardly a simple blur to measure on film. ummm, no s = 1/2 ge(2t + e) where s is the distance, g is acceleration due to gravity e is the exposure time in seconds and t is the delay between the object being dropped and the shuter opening. so the blur distance actually depends on both the shutter speed and how long you wait between droping the object and firing the shutter. And, of course, the actual blur on the film depends on the magnification factor. the 0.0306mm figure would be rather significant if the object were being recorded at 10 times actual size, and the 1.2 cm you mention later would be all but immesurable if the magnification was 1/10000. > _______________________________________ > R. Eichhorn > Professor of Mechanical Engineering Clearly I shouldn't expect you to be unaware of these simple facts :-) A major problem would be releasing the object and firing the shutter simultaneously. If we assume that this is done electrically, then all we need to consider is the latency the camera exhibits. (remember that a meter reading may be taken, mirror may need to be lifted and an aperture stopped down, all before the shutter can be opened). In any case I've heard of a camera with a claimed 6ms latency (I expect that this is without the need to lift the mirror, but bear with me) Let's assume your 1/400th of a second with various latencies: latency object travel 0 0.0306 mm 6ms 0.1776 mm 10ms 0.2756 mm 100ms 4.961 mm So the error is certainly larger than what it is we're trying to measure. If deliberately delay the fring of the shutter for 1 secons (and obviously drop the object from around 10 metres above the camera) the error due to the latency in the camera is less, but errors resulting from air resistance affecting the velocity of the object may start to become significant. We also can't ignore the fact that a focal plane shutter will conspire to add even more variables. If the curtain runs in a direction parallel to the falling object, the actual path recorded on the film will be longer or shorter depening on whether the curtain moves in the same direction as the image or in the opposite direction. Even with the curtain running in a direction 90 degrees from the direction of the falling object, the curtain will only open and close incrementally, with the time taken to open being some time a little shorter than the flash sync speed. so it can take between 2 and 16 ms for the centre portion of the film to be exposed after the shutter starts to open. (for sync speeds between 1/250 and 1/60 respectively). *however* if you measure the start and finish of the blur against a calibrated background, you could determine the shutter speed bt calculating the time at which thestart of the exposure of that particular piece of film started, and when it stopped. Incidentally this might tell you something about the latency too :-) Steve p.s. for the curious... s = 1/2 ge(2t + e) = 1/2g (e)(2t + e) = 1/2g (t - t + e)(t + t + e) = 1/2g ((t+e) - t)((t+e) + t) = 1/2g ((t+e)^2 - t^2) = 1/2g(t+e)^2 - 1/2gt^2 = distance object falls in t+e seconds less distance object falls in t seconds = distance object falls in the e seconds after t seconds n.b. assumes starting from rest, ignoring air resistance, and local garavitational anomalies :-)
