Archived posting to the Leica Users Group, 2004/06/17
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]The relationship between perspective and viewpoint gets 'em every time. You are standing 2 metres from a wall covered in 0.1m diameter circles looking through a 21mm lens at the wall. The film plane is absolutely parallel to the wall and the lens is 100% distortion free. The angle of view of a 21mm lens is 92 degrees. Which means you can see 4.1421m of the wall across the diagonal. Assuming the centre of the furthest circle is 4m off centre, the near edge will be 4.38m away and the furthest edge will be 4.56 m away from the optical centre of the lens. A fixed length of line on the circle's circumference at the far point will appear on the film to be 4.38/4.56 or 96% of the same arc at the near point. Casual observers will note that on most circles 1 degree of arc on one side is the same length as 1 degree on the other. Or: Buy the best 21mm lens you can get and stand 3 people up close but in a line parallel to the film so that one's face is in the middle and the other two are in the corner. The one in the middle will have a huge nose but the outline shape of his or her head will be normal. The ones at the corner will look as if they haven't finished teleporting from the deck of the enterprise. The only way to get all the circles on your lovely ornate wall to render as circles is to shoot from so far away that the ratio of their distances from the film plane approaches 1. With thanks to Pythagoras. I've really got to do some work now. Kyle, I shot five rolls of Provia recently so I feel I've earned this. Neil -----Original Message----- From: Jean Louchet [mailto:jean.louchet@inria.fr] Sent: Thursday, June 17, 2004 12:53 PM To: Beddoe, Neil Cc: 'Leica Users Group' Subject: RE: [Leica] laws of optics... On Thu, 17 Jun 2004, Beddoe, Neil wrote: >> "Second example, let's imagine a vertical wall with lots of circles >> painted on it (let's call it the Vasarely wall).If you take a picture >> with the lens axis perpendicular to the wall, then ALL the circles will >> print as circles on the picture. If the camera is oblique, then all the >> circles will show as ellipses." > > Not quite true. With anything other than a long telephoto, you'd have to > get close to the wall and this would mean that you'd be viewing the outer > circles from an oblique angle which would render them as ellipses in your > photograph. This has nothing to do with lens distortion and is just the > normal effect of perspective. > > Neil > Sorry Neil, this is wrong. I could have said in simpler terms that as long as the object plane is parallel to the film plane, the picture will be an exact (non distorted) scale reproduction of the object, without any distortion. This is obviously true with a pinhole camera and remains true with all "homographic" lenses, even wide angle. Thus if the lens axis s perpendicular to the wall, all circle (even far from the lens axis) will project as circles, not ellipses. > Not quite true. With anything other than a long telephoto, you'd have to > get close to the wall and this would mean that you'd be viewing the outer > circles from an oblique angle which would render them as ellipses in your There is a confusion here about what you define as an "oblique angle". As long as the lens axis remains perpendicular to the wall's plane all the circles will be seen as circles, even if your eye has to turn obliquely to see it. Then, if you turn the camera then the camera axis is oblique and the circles project themselves as ellipses. What only counts is lens orientation. Jean ------------------------------------------------------------------------------ This message is intended only for the personal and confidential use of the designated recipient(s) named above. If you are not the intended recipient of this message you are hereby notified that any review, dissemination, distribution or copying of this message is strictly prohibited. This communication is for information purposes only and should not be regarded as an offer to sell or as a solicitation of an offer to buy any financial product, an official confirmation of any transaction, or as an official statement of Lehman Brothers. Email transmission cannot be guaranteed to be secure or error-free. Therefore, we do not represent that this information is complete or accurate and it should not be relied upon as such. All information is subject to change without notice.