Nodal point of lensgreenspun.com : LUSENET : Large format photography : One Thread
I have now started building a panoramic roll film camera, and am in need of one piece of info. I have a 90mm Rodenstock f6.8 lens, and need to know to what point on the lens (nodal point I think they call it), from the film plane, I need to measure to give me 90mm, so that the lens will be focussed at infinity.
As I will not have a ground glass, and no bellows, I will be using a solid aluminum cone, I need to have the film plane to the nodal point of the lens as accurate as possible. Do I understand it correctly, that when the lens is focussed at infinity, in this case at 90mm from the film plane,the lens will be in focus. I addition, how does one determine the closest point that will be in focus.
Thanks for your help. Karl Beath
-- Karl Beath (email@example.com), May 28, 2002
Get a sheet of frosted glass and measure it, or ask Bob.
Then you need to know the hyperfocal distances for each f stop, and the extensions that give you that focusing distance.
I believe that the hyperfocal distance is the distance of the plane of sharpest focus from the lens when (at some specified circle of confusion) Infinity is just in focus, giving you optimum DOF back from infinity.
-- Dick Roadnight (firstname.lastname@example.org), May 28, 2002.
The relevent measure for your use is the "flange focal distance". This is the distance from the lens flange (where the lens mounts to the board) and the film plane, (at infinity focus). According the the data sheets Bob sent me years ago (thanks Bob!) this will vary on your lens from 93.6mm to 91.5mm. ...So with your lens you will need to 'calibrate' your focus. If you are using this for fixed focus' you may want to set it to hyperfocal for f/11 -f/16 or so to maximize DOF.
-- Gary Frost (email@example.com), May 28, 2002.
Rodenstock recommend f22-32 as optimum working aperture for this lens (assuming it's a Grandagon-N). And yes, the flange focal distance is given as 91.5mm / 93.6mm. Check http://www.rodenstockoptics.de/rodenstockoptics/index.htm - I'm sure they can give you the correct distance from the serial number.
-- Ole Tjugen (firstname.lastname@example.org), May 28, 2002.
You don't really need to know the nodal point of the lens, you just need to know the distance from the film plane to some reference on the lens, like the mounting flange. But in any case, I doubt you can position the lens accurately enough by dead reckoning alone.
Why not make things easy for yourself, and incorporate some threaded adjustment for the lens, or make the cone slightly undersize and use shims to fine-tune the focus?
A temporary GG across the film plane, during construction of the camera, will allow you to set the lens accurately in position.
BTW, wouldn't it be better to set the lens at the hyperfocal distance for, say, f/8, rather than infinity? A 90mm lens doesn't have that much depth-of-field.
-- Pete Andrews (email@example.com), May 29, 2002.
I agree with the suggestion that you use a temporary groundglass and determine the focus empirically.
It would be interesting to know where the nodal point of the lens is if you want to make multi-frame panoramas. Your frames will line up better if the camera rotates around the nodal point of the lens, so if you know where the nodal point is, you can decide where to put the tripod socket, based on the design of your tripod head.
-- David Goldfarb (firstname.lastname@example.org), May 29, 2002.
Mount the lens on a LF camera, focus at infinity on the ground glass, measure the distance from the lens board to the film plane. You can measure it from the outside or remove the lens board and measure it from the inside via the lens board opening....
-- dan n. (email@example.com), May 29, 2002.
It's a nice thing to understand the relevant problem in a question before the answer :-)
Obviously, you are correct in asking the distance from film plane to nodal point.
Unfortunately, I have not the answer, as Rodenstock doesn't offer comprehensive data sheets on the Internet as Schneider does.
-- Dominique cesari (firstname.lastname@example.org), May 30, 2002.
For a "panoramic" camera you need to determine the length of the conical tube needed. Do this by experiment: Arrange the lens to be looking at a distant object and move a ground glass until an imaage focuses clearly. Measure the distance between the frosty side of the glass and the place on the lens where it will bear against the tube you are designing. Some make such a tube using a threaded insert which can be used to "tweak" the length and thus avoid critical measurement.
The "nodal point" is only useful in the case of a "panning" (not panorama) camera. In a panning camera the film is arranged inside a cylinder and the lens is placed so that it is at a distance to focus on the film. The nodal point is the axle around which the lens is turned so as to pan around and make a travelling image painted onto the film, usually thru a vertical slot.
This point (nodal point) can also be determined by simple experiment: Arrange the lens so that it focuses at infinity on a ground glass. The point at which the lens can be rotated (swuing about a vertical axis parralel to the film) and the image on the ground glass will not displace either left or right is the nodal point. Finding this point makes the trick of the panning slot work properly. Its usually right near the iris.
-- Steve Grimes (email@example.com), June 02, 2002.