Chuck P.
15-Jan-2011, 10:05
In The Camera it says that for a lens to focus at infinity, the lens has to "be located at its own focal lenth from the film"; this distance is then measured from the film plane to the rear nodal point of the lens, which it states that the rear nodal point is located close to the aperture plane.
I was checking infinity focus with my Rodentsock 210 Sironar-N, so 8.25 inches from the film plane to the rear nodal point should be infinity focus. Well, after focusing on very distanct subjects, the measured distance from the film plane to the aperture plane (if that is the rear nodal point) falls short at 7 3/4 inches---whereas, a distance of 8 1/4 inches comes to a location on the lens barrel where I cock the shutter for this lens. For this lens, the aperture plane "seems" to be right in line with the plane of the lens board.
Infinity with my new Nikkor 120SW would be 4 3/4 inches from the film plane to the rear nodal point, and, for this lens, this distance from the film plane is pretty much at the lens board.
To summarize, finding infinity focus is not a problem, but I was just wandering about the rear nodal points of large format lenses----are they located about at the aperture plane or can it vary appreciably from lens to lens?
In case someone is wandering why---with my Horseman monorail, once I find the infinity focus, I simply measure the inside millimeter distance between the standards and make note. For example, with the 210, I can quickly set inifinity focus with the standards set 109mm apart; in finding this distance with my new Nikkor 120, I determined them to be 39mm apart.
I was checking infinity focus with my Rodentsock 210 Sironar-N, so 8.25 inches from the film plane to the rear nodal point should be infinity focus. Well, after focusing on very distanct subjects, the measured distance from the film plane to the aperture plane (if that is the rear nodal point) falls short at 7 3/4 inches---whereas, a distance of 8 1/4 inches comes to a location on the lens barrel where I cock the shutter for this lens. For this lens, the aperture plane "seems" to be right in line with the plane of the lens board.
Infinity with my new Nikkor 120SW would be 4 3/4 inches from the film plane to the rear nodal point, and, for this lens, this distance from the film plane is pretty much at the lens board.
To summarize, finding infinity focus is not a problem, but I was just wandering about the rear nodal points of large format lenses----are they located about at the aperture plane or can it vary appreciably from lens to lens?
In case someone is wandering why---with my Horseman monorail, once I find the infinity focus, I simply measure the inside millimeter distance between the standards and make note. For example, with the 210, I can quickly set inifinity focus with the standards set 109mm apart; in finding this distance with my new Nikkor 120, I determined them to be 39mm apart.