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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.

Mark Sawyer
15-Jan-2011, 11:57
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?

Yes, they can move around a bit; it's one of the "degrees of freedom" when designing a lens. Where you put the aperture influences field curvature, coma, pincushion/barrel distortion, etc. The "best" place is usually near the rear nodal point, but can do minor shifts as other elements of the lens design change, and according to priorities of the designer. (Note: this is for "conventional lenses"; in telephoto or retrofocus lenses, the nodal points may be outside the lens.)

Leonard Evens
15-Jan-2011, 12:08
For normal lenses, both the front and rear nodal points are located close to the front of the lens board. For lenses of telephoto design, the rear nodal point is located in front of this, sometimes even in front of the first lens surface. Wide angle lenses are often of inverse telephoto design, and the rear nodal point is behind (closer to the film) the front surface of the lens board, but usually not by a whole lot.

The only way to be sure about these things is to look up the lens specifications. You look up the flange focal length and compare that to the focal length. If the focal length is greater than the flange focal length, then the rear nodal point is located in front of the lens board. If it is less than the flange focal length, then the rear nodal point is located in back of the lens board. By subtracting one from the other, you can determine the distance from the lens board to the nodal point. For example, the flange focal length for my 75 mm Grandagon-M is 82 mm. That means the nodal point is 82 - 75 = 7 mm closer to the lens than the front of the lensboard.

Note: The rear flange of the lens rests against the front of the lens board, so the distance to the flange is the same as the distance to the front of lens board.

I find it easiest just to focus on a distant object. It doesn't have to be too far away for it to be effectively at infinity.

Leigh
15-Jan-2011, 12:58
The nodal points (i.e. the two principle planes) can be located anywhere in relation to the physical lens. Only the rear nodal point is of interest to us.

On short FL lenses (wide angles) it's usually behind the lens, so you don't have to crush the bellows to focus at infinity.
On long (telephoto) lenses it's usually in front of the lens, so you don't need bellows three feet long to focus on close objects.

It's all a matter of design, at the engineer's option.

The published parameter of interest is the "Flange Focal Distance" or "Flange Focal Length". This is the distance from the film to the front of the lensboard when focused at infinity. The value is on the lens datasheet, and is typically accurate to 0.1mm.

The FFL for your lenses:
Nikkor SW 120 = 130.7mm (5.146")
Sironar-N 210 = 200.0mm (7.874")


- Leigh

Chuck P.
15-Jan-2011, 18:39
Thank you all for the good information---I have a better understanding of flange focal length now.