|MadSci Network: Physics|
Hi, Linda, lucky you - having inherited the stroboscope! You can have a lot of fun experimenting with this. I’ll try to answer your question, but you will have to make some experiments to get the ideal set-up yourself, as some things will depend on the type of camera you have, the field of view of its lens and things like that. Anyway, I’m going to assume that what you are trying to do is produce a single photograph which contains multiple images of your milk drop as it falls including just one shot of the drop hitting the plate. That last bit is probably the trickiest bit… The first thing you need to work out is roughly how long the sequence will take. Newton springs to our help, with his equation: distance = (initial velocity x time) + (.5 x acceleration x time squared). In this case, initial velocity is 0, acceleration is 32 ft per second per second, so it is easy to derive that the time is (roughly) one quarter of the square root of the height (in feet) through which the drop falls. For example, if the drop falls four feet it will take half a second, if through two feet, 0.35 seconds, and if one foot only a quarter of a second. Next, you need to decide how many images of the drop, including the one of it hitting the plate you want to have showing in your picture. Clearly you want them reasonably separated, but if you have too big a gap it is going make it more difficult to stop exposing after the drop has hit the plate, before the rebound splashes blur the image. I’m going to assume 1 foot as an example, but as I say, it will all depend on the camera and the focal length of lens available to you whether you will be able to fill your frame with good focus and decent sized images of the drops. With a quarter second drop-time, and assuming you might want 10 images of the drop in the picture, you can see that the frequency of the flashes needs to be 10 per quarter second or 40 per second (40 Hz.) 5 images – 20hz will do. In general, the higher the flash frequency the closer and more abundant your separate images will be for a given exposure time and the faster the subject moves the fewer images you will capture between the beginning and end of the action. You could imaging shooting only the last foot of descent of the droplet falling from a greater height. This would mean a faster speed for a given frequency – another way of changing the number of images in the picture. I don’t know what range of frequencies are available on your stroboscope, so you may need to change the drop height or the number of images depending on how it works out. The set-up is important. Because you are exposing the same frame multiple times, you don’t want fixed objects to be exposed correctly each time – or they will end up being very over-exposed. However you do want the drops correctly exposed. The fixed objects you must have are the plate and the background. If you can make these matt black, they will reflect hardly any light and should not show up very much and provide a good contrast for the white milk. A largish piece of black velvet would be ideal for the background, and you will have to improvise something non-absorbent, but matt black to serve as the plate. To make the exposure, you have some choices – but again it may depend on your camera and particularly the type of shutter. SLR shutters have focal plane shutters which generally send a slit sort of scanning across the film. This makes flash a bit tricky, and the simplest way to overcome this is by using the bulb setting which is where the shutter remains open as long as you hold the shutter release button down. (For this a cable release is best to avoid vibrating the camera.) If on the other had, you have a shutter in the lens, you can probably (in this example just set the shutter speed at ¼ second. You want to do this in the dark or very dim lighting so that the flash dominates the exposure – and in fact by varying the level of the background lighting you can get a nice effect where the flash causes distinct images of the drop in a streak of fainter motion blurred exposure caused by the background light. Now the trick bit – synchronisation. You need to release the drop, fire the shutter and flash simultaneously, or if using bulb, close the shutter just as the droplet hits the plate. If you can use the camera via a flash socket to trigger the flash, that will be easiest, and you can adjust the height by trial and error to get the drop just hitting the plate as the shutter closes. If you have to use bulb, then open the shutter with a cable release in darkness, and arrange to get the drop to fall as you fire the flash. (Not easy). You will also want to release the cable, closing the shutter as the drop lands – (again, not easy, but not impossible.) Providing you are prepared to experiment and burn a bit of film you should get some good images. As I have said the hardest thing in all of this is synchronisation, as you may be dealing with a pretty short time – a fraction of a second, and if you are really clever you might be able to design your own system for achieving automatic synchronisation. There’s a lot of fun to had trying to do these things – usually more fun than in looking at the result!! Good luck!
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