Lurker here. Why not eliminate the complexities of acceleration, etc? A clean solution would involve a dark room, a stroboscopic light that is known to be reasonably accurate, a meter stick, a large ball bearing, and a very smooth, very level surface. Barring that, here's the same solution using a friend and very simple algebra: You need a late evening or a moderately moonlit night, a car with a reasonably accurate speedometer, a large, empty parking lot such as are found in front of shopping centers, and a very bright flashlight. A friend will drive the car at a constant rate at right angles to your tripod-mounted camera while another friend aims the flashlight directly out the side window of the car without moving it from side to side. You'll have to determine by experimentation what speed is going to give you a good, long blur on your prints: the longer, the more accurate your results will be. 1. Station yourself in front of a series of parking stripes: two of these will be used as a reference distance. Your friend will be driving at right angles to the stripes so that part of the stripes can be photographed along with the flashlight. Note that it's important for the flashlight and the stripes to be in the same vertical plane when you shoot. Measure the largest distance in inches between stripes that will appear in the photo. The farther apart the stripes are, the less any errors will affect your results. 2. The driver makes 5 passes at right angles to the camera with the flashlight pointing at the camera. The camera must not be moved during this operation, and each pass must be made at the *same* *constant* speed. Be sure to note the speed carefully. 3. The shopping center security team is coming! Run like hell! OK, seriously, I'm sure if you check with them first, they'll be happy to let you conduct this 10-minute experiment in an out-of-the-way end of the lot. 4. Carefully measure the lengths of the flashlight images in the 5 pictures and average them; this assumes the car was traveling at the same rate each time. Use any units for this. 5. Carefully measure and average the distance between the parking stripes in your pictures. Use any units, as long as they are the same as the units you used in step 4 and the stripes are the same ones you measured on the ground. ------------------------------------------------------------------- The formula you need to solve is: Distance = Rate x Time or D = RT Since you are looking for the time (your shutter speed), T = D/R ------------------------------------------------------------------- To get the Distance in inches, it's a simple proportion: D(actual)= Actual (average) distance traveled by the flashlight while the shutter was open D(image) = Average length of the 5 flashlight images in the photographs L(actual) = Actual distance between the parking stripes on the ground L(image) = Average distance between the same parking stripes in the photographs D(a)/D(i) = L(a) / L(i) or, D(a) = [L(a)*D(i)] / L(i) Plug in your results and you'll know how far the light travelled. Next, calculate the rate of the car: R = Average speedometer reading * 17.6 inches per second Now you have both the numbers you need. Just plug them into the formula above (T=D/R) and you'll get a decimal number. Convert it to a fraction and there's your shutter speed. I *think* that'll do it, anyway!! :-) Dan
