berryman Posted September 29, 1999 Share Posted September 29, 1999 I was speaking with a freind about photography and they mentioned a type of destortion called Keystoning I was wondering if you could explain this for me. Link to comment Share on other sites More sharing options...
ellis_vener_photography Posted September 29, 1999 Share Posted September 29, 1999 Keystone distortion is what happens when you point a camera upwards at a vertical surface with the top of the film plane tilted away from the building.</P> Imagine that there are two parallel vertical lines on this surface. with the top of the film plane (which is the same as your groundglass (AKA gg) further away from that surface than the bottom of the film plane the two lines will no longer appear parallel on the gg but will appear to converge as they run towards the top of the surface as viewed on the gg.</P> This type of distortion is corrected by making the film plane parallel to the surface. Link to comment Share on other sites More sharing options...
david_grandy1 Posted September 29, 1999 Share Posted September 29, 1999 The reason it's called Keystoning is the resemblance of a rectangle to the stone at the top of an arch. This stone has sides that are similar to an upside down triangle although the angle is much less acute. Anyway this stone supports the entire arch, thus Keystone. A rectangle, like an office building, shot from below with the lens tilted up will take on this shape. <p> Lens rise will minimise or remove this distortion, but keystoning &/or "falling over backwards" can add a sense of drama and impact to a photograph of a building, so don't close your mind. Link to comment Share on other sites More sharing options...
sean_yates Posted September 29, 1999 Share Posted September 29, 1999 Keystoneing is what happens when you drive through Pennsylvannia without a map! It also is known to involve less than professional police officers with cream pies. Link to comment Share on other sites More sharing options...
robert_a._zeichner Posted September 29, 1999 Share Posted September 29, 1999 One thing to keep in mind when correcting for this type of distortion is that making the sides of the building parallel to the edges of the print may look peculiar at times. If your point of view is from a very low or very high angle, our minds eye expects to see some degree of convergence. Our everyday "seeing experience" trains us to expect things to look a certain way. When we over-correct to keep all the verticals perfectly parallel, it sometimes makes the top of the skyscraper look wider at the top. On a different note, it's common to run into keystoning problems when projecting slides from a coffee table on a screen that drops down from a high ceiling. With the optical axis of the projection lens not being perpendicular to the screen surface, the portion of the image that is further away will seem larger. The projected retangular image will look like a keystone. There are tripod mounted screens that can lean forward to correct for this. It also helps in keeping the entire image in focus. Link to comment Share on other sites More sharing options...
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