-----Original Message----- From: Godfrey DiGiorgi [mailto:[EMAIL PROTECTED] Sent: Sunday, September 11, 2005 7:39 PM To: [email protected] Subject: Re: First non DSLR digicam with 10MP APS sensor- contradiction
On Sep 11, 2005, at 3:51 PM, J. C. O'Connell wrote: > 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. This makes no sense. In a lens with a set of rear elements designed to correct light path to orthogonal, a large percentage of the rear element of the lens is *always* active. That is the point of the design: direct the light to be orthogonal to the sensor. If the rear elements are close to the imaging plane, and far from the nodal point, the correction is small and most of the rear element is being used. If the rear elements are far from the imaging plane and close to the nodal point, a smaller percentage of the rear elements are being used and the correction possible is reduced. >> ... 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. The use of a condenser enlarger as example is illustrate simulating a point light source at infinity such that the ray trace over the area of the film would be parallel. This is indeed the way light coming from a point source at infinity would be oriented. In the camera lens/ sensor system, the point source can be seen as the lens' nodal point. >> 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. See above. Perhaps I'll draw a diagram or two for you. > Secondly, I totally agree that increasing > the nodal point away from the sensor while > maintaining the same focal length will help > the digital sensor / lens interface with respect > to keep the lens rays more parallel incidence > at sensor plane but there is a heavy price > for that , the retrofocus lenses that do that > are far larger, heaver, worse optically, and > more expensive than if you don't need to do that. > That's why the Pentax and other cameras that > use APS sensors in old FF 35mm body designed > lenses are at a disadvantage, the 45.5mm sensor > plane to lens flange distance is way too large > relative to the small format (APS), I'm not exactly sure what you're trying to say here. Yes, inverted telephoto designs are typically more complex, heavier and more expensive than non-inverted-telephoto designs. They are a result of the need for more clearance with SLR bodies as focal length is reduced. On the other hand, evenness of illumination is typically better with inverse telephoto designs. As far as I'm aware, nearly all modern 35mm and shorter focal length lenses designed for 35mm and digital SLRs are inverse telephoto designs. Most of the better, modern wide angles used for rangefinder cameras are as well, because the even illumination is useful. Only a few are not, and those generally demonstrate corner/edge falloff to a greater degree. The register distance could be shorter in dedicated lenses for a DSLR due to the shorter mirror required, but the whole point of using the current register distance is to enable use of existing lens and mount designs. IF, however, you're designing a mount and lens from scratch for a digital sensor, you'd use a wider diameter mount with a shorter register. This lets you place large diameter, corrective rear elements closer to the sensor without having to go as far with moving the nodal point forward through inverted telephoto design. Godfrey
