John wrote: >I just tested a bead-in-hole shadow sharpener and five pinhole sharpeners ranging in diameter from 2mm to 6mm. I used the shadow edge of my house's roof (which is about one tenth the height of one of Kitt Peak's side styles). The best image of the half disk was using the smallest pinhole held at a distance of about 1 meter. The larger holes produced fuzzy images that were larger and less precise.<
Hi John, Sounds like you got the right figures by experimentation! However you might like to be able to calculate something near to the 'best' size of hole for a particular imaging distance. The natural focal length (f) of a pinhole is related (for wavelengths of visible light of around 550nm) to the hole diameter (D) by the relation D= 0.047*SQRT(f) where D and and f are expressed in millimetres Since the sun's rays are nearly parallel the image will occur at (or near!) a distance from the hole equal to the focal length. So, the theory says that for a 'best' image at one metre you would need a hole of diameter 0.047* SQRT(1000) which is 1.5mm. Incidentally since optimum pinhole diameter increases as the sqaure root of the focal length you can improve the 'detail' of the image by scaling everything up. So if you quadruple the focal length you get the same field of view whilst only requiring a doubling the pinhole diameter This results in a doubling of resolution. However, nothing is free and doing this means that the image is less easy to see. Naturally there is an optimum point to be selected by trial according to the conditions and you are right to take several hole sizes with you!. You do have to be careful though with pinholes since it's possible to see 'spurious' resolution which is caused by the blurring being so bad that two elements of the image overlap and create an apparent improvement in resolution which is not true and certainly would affect the measurements that you want to make. >The bead-in-hole was absolutely useless!.< This is what's called a pinspeck effect device (I think it was discovered by X-Ray scientists in the 1970s). It produces a very low contrast negative image with several times poorer resolution than the pinhole so I am not too surprised you found it useless! Hope your measurements go OK Patrick ------------------------------------------------------------------------- E-Mail: [EMAIL PROTECTED] Web: http://ourworld.compuserve.com/homepages/Patrick_Powers/ Lat: N 51d. 49m. 09s: Long: W 00d. 21m. 53s -
