Message text written by INTERNET:[email protected] >John said: ...but do you think your formula could help determine the optimum size of the gap between the bead and hole of a bead-in-hole sharpener?<
Yes it can help but it's not the same! Strictly the process is different but the formula used as I suggested (where you place the pin hole at distance f for a hole of diameter D) instead gives the MAXIMUM distance of the pinspeck device from the screen or image. You should actually choose a distance that is substantially less than that given by the formula if you want a pinspeck device to work. The formula I sent for the pinhole gives the details for a specific optical situation that lies between two other forms of so called imaging which happens to be the best for the purposes of a pin hole camera (or shadow sharpener). Art is right that image formation is not achieved in the same way as it is in a lens but if you choose to define focusing as 'a means by which points in an object can be proportionately and spatially translated into corresponding points of an image' then the pinhole and the lens do the same thing - and you can consequently still talk of things like focal length too. That's why I (like some people in the literature) do use the same terms for both. But you do have to realise that it's not the same... The pinspeck - that's the 'bead-in-hole' device - operates somewhat differently. Here the bright points in an object cast shadows of the pinspeck itself (that's the bead) onto the screen or floor and it is these points of shadow that you see. It therefore looks like a negative image. Here the key things are: 1. That the bead must itself be large enough to cast a shadow at the distance it is from the screen. 2. The distance of the screen from the bead has to be less than (s^2)/L if the effect is not to be marred by other optical effects like diffraction.. So the formula gives a distance that you must be well within. 3. The thing needs light to work so it doesn't work well in half light and being placed in a penumbra effectively switches it off. Also if you are that close then the image size is usually too small and indistinct even in good light. Because, with a pinspeck, every other point than the one causing a shadow also illuminates the rest of the screen the contrast of the resulting negative image is very poor - much less than that of the positive image obtained with a pin hole. The pinspeck 'camera' does however have a greater light gathering capacity than the pinhole so it can be useful for imaging simple objects. That is why it is used (or used to be used) for setting up X-ray tubes etc. However I would imagine that it would be near useless when you take it into shadow because the contrast would vanish. Having said all this I must confess to never having properly experimented with a pinspeck device so I really ought to shut up at this stage!! Theory is all very well, but.....! Maybe others who have tried this device can comment? Have I confused you or is that of any help? (Sorry about the strange use of ^ to indicate a number raised to a power (as in 2^3=8) - it's something that comes from computer programming but it's very convenient when trying to write formulae in e-mails!! Some people use two asterisks for the same thing (as in 2**3=8) too) Patrick ------------------------------------------------------------------------- E-Mail: [EMAIL PROTECTED] Web: http://ourworld.compuserve.com/homepages/Patrick_Powers/ Lat: N 51d. 49m. 09s: Long: W 00d. 21m. 53s -
