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*Subject*: Re: [Rollei] Re: Rollei panorama head, and "nodal points"*From*: bigler@ens2m.fr*Date*: Mon, 12 May 2003 10:27:01 +0200 (CEST)*References*:

> > Carlos Manuel Freaza: > > > About the frames a bit closer, I think it depends > > > subject and distance to subject. > > Richard Knoppow : > >...Note, that since the lens moves with regard to the swivel > point of the adaptor when the distance of focus is changed > that the match with the nodal point will not be as good for > close objects as for distant ones. The question of identifying the best rotation point for panoramic stitching is a FAQ but the answer is not so obvious. Usually "nodal points" are invoked, but this, pardon me Richard, is not always true. And since we are in the Temple of Optics here, I dare to add the following comments. In panoramic cameras with a rotating lens drum and fixed film w/respect to the landscape, like a Noblex, a Horizon or an Alpa rotocamera (360degs), yes, you should rotate the lens around its rear nodal (or principal) point. This for keeping the image almost at rest on film, for sharpness issues while the lens rotates. But for conventional panoramic stitching like with the Rollei TLR or any conventional camera, where the whole camera is rotated w/respect to the landscape, the proper rotation point to avoid parallax effects is the __entrance pupil__. This fact is stated by Kingslake and some other old textbooks, but is almost never explained in detail. The basic reason why the pupils play a role here is that parallax effects occur between out of focus image projections. An out of focus image is built from small circles, each one being the projection of the exit pupil on film. May be there is one object plane exactly in focus, but when taking pictures of a flat object, you do not have to care for parallax effects. Rotating lens drum panoramic cameras do not respect the 'no-parallax' condition, since the entrance pupil hardly ever coincides with the rear nodal point. A detailed analysis of this is fascinating but would be far off-topic here, but it suffices to say that those panoramic cameras usually have a wide-angle lens, and that they are equivalent to stitching many vertical strips of very narrow width, hence parallax effects are small. Out of focus image points are slightly stretched under the form of an oval shape, so conventional parallax effects for out of focus images are somewhat blurred in an unconventional manner. So the proper rotation point for panoramic stitching is the entrance pupil. In quasi-symmetric lenses like a R-TLR lens, the entrance pupil is located very close to the front nodal point of the lens. Another source of confusion with nodal points !! So in order to minimise parallax effects with a R-TLR, simply try to place the rotation point roughly under the lens. A Rolleifix will do that pretty well. My understanding is that some, but not all, R-TLR panoramic attachments move the screw under the lens near the entrance pupil. No what happens with non-symmetric lenses, like retrofocus wide angle lenses or telephotos. For such lenses the entrance pupil is _not_ located at the front nodal point, nor at the rear nodal point. Basically this depends on the degree of asymmetry of the design, more precisely and very simply in terms of mathematics, it is given by the amount of pupillar magnification of the lens design. This is tabulated in manufacturers' data sheet (I mean : good manufacturers, obviously the only ones where RUGgers buy their lenses ;-);-);-). In German datasheet, the acronym EP for "Eintrittspupille" is used, AP for "Austrittsspupille". EP is a fairly obvious acronym for English-speaking readers. EP is often measured with respect to the front lens vertex. datasheet usually mention the diamter of the pupils, so it is easy to compute M_P and check what follows. Let M_p be the pupillar magnification factor equal to the ratio of (exit pupil diameter / entrance pupil diameter), the distance between the front nodal point H and the entrance pupil E_p is given by : HE_p = f*(M_p - 1) / M_p. where f is the focal length. When M_p = 1, the entrance pupil is equal to the front nodal point. This applies to all true symmetrical lens designs such as classical apo-chromatic process lenses (apo-ronar, G-claron). Most view camera lenses are quasi-symmetric, hence the entrance pupil will be located somewhere in the front half of the glass, close to the front nodal point. For a retrofocus lens like Zeiss Distagons, we have M_P > 1. For such a retrofocus, the entrance pupil will be located ahead from the front nodal point, probably in the air in front of the lens. For a telephoto, we often have M_p < 1. Here the entrance pupil will be located behind the front nodal point. Since the front nodal point is in fact behind the rear nodal point (H and H4 are crossed in a telephoto design); it happens that the entrance pupil for telephoto is located in the film plane !! this is actually something very hard to figure out ; however the consequence of this is that you do not have to add a long rail in order to properly set the right rotation point for panoramic stitching with a telephoto ! for a 35 mm camera, the tripod screw is close to the film plane and a small adjustment on the slot of your tripod head will do the job !! However aficionados of astronomical telescopes or "Telyt" non-telephoto design of long focal lengths (without the usual negative rear element of a telephoto), they'll have to rotate their camera under the front part of the lens (usually a cemented doublet or triplet), just under the glass like if it were a single lens element. For, say, a 300 mm non-telephoto lens or a 300mm process lens used as a long focal lens on a view camera, no way but rotating at 300mm ahead from the film plane. Not a problem with a monorail view camera, but this requires to use an accessory rail on a 35mm SLR. - -- Emmanuel BIGLER <bigler ------------------------------

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