-----Original Message----- From: Godfrey DiGiorgi [mailto:[EMAIL PROTECTED] Sent: Sunday, September 11, 2005 6:05 PM To: [email protected] Subject: Re: First non DSLR digicam with 10MP APS sensor- contradiction
On Sep 9, 2005, at 2:27 PM, J. C. O'Connell wrote: > I did a quick sketch to clarify what I said: > http://www.jcoconnell.com/temp/rearanglediagram.jpg Your sketch is misleading: it exaggerates the relative sizing of the sensor target compared to the lens and also does not indicate where the nodal point is. In a typical Cooke triplet, it's the distance from the nodal point to the imaging plane that determines the deviation from the orthogonal as you approach the edge of the film/ sensor format, not the distance between the rear element and the film/ sensor. ++++++++++++++++++++===================++++== My sketch was a simple sketch ( that obviously not a real lens design with two convex lenses) and I explaied I was showing the ACTIVE area of the rear element so it does not matter where the nodal point because if the entire active area gets closer to the sensor then the angles to the corners of the sensors get further away from perpendicular/ideal. ========================================= The point of having a lens designed for a digital sensor that has its rearmost element very close to the sensor plane is that the rearmost elements of the lens performs correction designed to orient the light path from the nodal point (placed sufficiently far forward in the lens) such that the ray trace to the photosite plane is orthogonal, not that you'd place the nodal point further rearwards in the lens. This is quite similar to what a condenser enlarger head does: it positions a collimating lens group very close to the film plane in order to make the light pass evenly through all points of the negative, right to the corners, and oriented orthogonally through the film so that a flat field imaging objective (the enlarging lens) will exhibit very little light falloff at corners and edges. ======================================= I totally disagree with the englarger light house because the output of an enlarger condensor assembly is PARALLEL light rays going to the film form a point light source. A camera, digital or otherwise has a POINT SOURCE image formed at the film/sensor from a point source REAL OBJECT, in other words the output of a camera lens is an image of the real object formed on the film/sensor while the output of an enlager condensor lamphouse is completely different, its NOT forming an image of the enlarger lamp, its forming a cylinder of parallel light rays instead of an image at the film. ====================================================================== A large diameter element at the rear of a lens designed for the digital sensor helps in promoting this even illumination of the entire sensor area. Placing this rear lens group close to the sensor, relatively distant from the nodal point, allows the strength of the elements to be lower and thus promotes less distortion from the correction. ============================================================== You are overlooking that the "diameter" of the rear element is not "fixed" and it gets smaller in its active area ( optical path), quite small in fact at small fstops like f11/16 so that is changing with lens settings and cannot be maintained constant...So if the advantage of the large rear element is there its not constant and the angle at which the light rays hit the sensor corners is worse when the lens is stopped down. JCO ==========================================================
