 Joseph R. Ward |
nodal point of a lens I bought a pano head to take panoramic shots and to limit post shot work. I was hoping someone knew the easiest way to find the nodal point of my lens. I use a nikon D70 with a 18-200mm Nikon VR lens. if anyone has a quick and easy method let me know. if there isnt a quick way then pleae let me know the long and arduous way. thx so much all. joe
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John G. Clifford Jr |
Well, usually lenses are marked as to where the nodal point is. Sometimes it's a circle with a line through it, sometimes it's the serial # block. If not, you can use the plane of the aperture blades as a close approximation, or you can test each lens by setting the camera up on the pano head with what you GUESS is the nodal point above the point of rotation, and then rotate the camera with a close object and a far object almost lined up. If you have the nodal point set correctly on the pano head, you will see no apparent movement between the close and far objects as you rotate the camera from side-to-side. If you see movement, then you're not at the nodal point. Move the camera back (or forward), and try again, until you get no movement.
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Alan N. Marcus |
Your question is extraordinary; you want information on the location of the nodal point. We use the location of the rear nodal point to calculate the focal length of a lens. The actual position of the read nodal can only be obtained when the lens is mounted on an optical bench. It lies within the lens for most however it can fall forward of the lens on a true telephoto and behind the lens on an inverted telephoto which is a short wide-angle lens designed with retro-focus. The retro-focus design is required when a short lens must be mounted. The retro-focus design gives extra clearance distance so the rear element will not obstruct with the mirror travel path on an SLR. I think what you are really asking is how can I find the true focal length or where is the location of the focal plane? Anyway the easy way, without an optical beach: We need to focus the lens and achieve unity. This is a life-size image same size as real life sometimes called 1:1. You can make-shift optical bench by dismounting the lens and placing it on a book or other rest. Ahead of the lens is placed high contrast printed matter (black on white print). Behind the lens is ground glass (tissue paper can also serve). Illuminate the print with a spotlight. Place the ground glass at some distance behind the lens. You must experiment with both object distance and image distance. After moving the printing matter back and forth and the ground glass back and forth you will see an image projected by the lens on the ground glass. Now using a ruler adjust distances to achieve unit (life-size). If you have two copies of the printed matter you can temporarily lay on lens side of the ground glass. This will be helpful to see if you have achieved unity. Carefully measure distance from printed mater to ground glass. At unity this distance is 4X the focal length of the lens. Thus the rear nodal of this lens is exactly half way between the ground glass and printed matter. The circle with line symbol is the postion of the focal plane of the camera body. Alan Marcus (dispensed questionable techno babble)
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William Schuette |
Joe, go to reallyrightstuff.com. They have tutorials on the procedure for finding the nodal point of any lens. The process involves lining up two objects so that one appears right in front of the other and then adjusting the point at which you rotate the lens/camera combination until the two objects stay aligned when you rotate the camera. For most panos you really don't need to worry about it. It's more important to make sure the camera travels as level as possible. Bill
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