From: honna@cs.concordia.ca (SEGEL honna)
Newsgroups: rec.photo
Subject: Re: Nodal point on lens.
Date: 2 Sep 92 20:06:39 GMT
Organization: Computer Science, Concordia University, Montreal, Quebec

>There are two nodal points.  The rear nodal point is one focal length
>in front of the film place when the lens is foused at infinity.
>
>This is not always possible, since on some lenses the image is
>actually behind the front element.  In that case you can use a macro
>lens to find the point of focus.

How, pray tell?
>
>Another fancier scheme that doesn't depend on knowing the focal length
>is the "nodal slide".  Arrange the lens on a pivoting mini-table.
>(It pivots about a vertical axis.  You are looking down in this diagram.)
By "pivoting mini-table" do you mean something like a Lazy-Susan?
>
>                       ---------------------- mirror
>
>
Should the mirror and slitted screen be plane-parallel?
>
>
>                                lens on table 
>
>
>                        white
>                       ------------ --------------- 
>                                      slit
>
>                                  lamp
>
>Now light will pass through the slit, through the lens, reflect off the
>mirror, and come back onto the white back side of the slit.  When you
>rotate

rotate about the vertical axis, presumably?

>the lens the image will move.  Adjust the distance from the
>pivot of the table to the slit and the position of the lens over the
>pivot until the image of the slit is share and does not move when the
>lens is rotated a little.

This looks suspiciously like I have to adjust Two things: slit-pivot
distance, and lens-pivot relation...Is there an easy algorithm for deciding
which to do first, and how to determine the direction of the adjustment?

I'm not sure what you mean by "until the image of the slit is share"...

Can this whole procedure be simplified so that I can determine my slide-
side lens nodal point while the lens is On the camera body?  A mirror on the
film rails, perhaps, the camera on one of those cheapo flash-brackets
(with the sliding 1/4 - 20 screws); these together on a level tripod
head( such that the bracket's Fixed 1/4 - 20 screw is secured to the
tripod head)?

Adding to my own confusion... The fixed screw serves as the pivot point
(via the tripod head's swivel) and the sliding screw serves to position
the camera lens wrt that pivot.  

>  The distance from the pivot to the slit is
>the focal length and the pivot is over the slide-side nodal point.

Flash bracket looks something like this:

_______________________________________                                       
|              ____________________    |  
|   X 	( )   (____________________)   |
|______________________________________|                                        
                                                        _________________ 
 Flash shoe at X; fixed screw at ( ); sliding screw in (_________________).                                       


HS/Michael

-- 
***************************************************************
honna segel
department of computer science
concordia university  *****************************************


-- 
***************************************************************
honna segel
department of computer science
concordia university  *****************************************