From: jacobson@cello.hpl.hp.com (David Jacobson)
Subject: Re: Nodal point on lens.
Date: Wed, 2 Sep 1992 23:36:36 GMT
Organization: Hewlett-Packard Laboratories

In article <4860@daily-planet.concordia.ca> honna@cs.concordia.ca (SEGEL honna) writes:
>In article <1992Aug28.041539.1840@cello.hpl.hp.com> jacobson@cello.hpl.hp.com (David Jacobson) writes:
>>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?

I don't have my original posting at hand, but there was supposed to be
a paragraph that basically said to let the light come through the lens
backwards and find the focal point as the light emerged from the front
element of the lens, then measure backwards one focal length.  The
problem is that with some lenses, for example my 24mm Sigma, you see a
somewhat out of focus image on whatever target you use, say a piece of
paper.  You try to focus, and discover that the paper hits the front
element before focus is achieved.

>>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?

Right.  Only a typical lazy-susan would be much too big, unless you
were doing a large lens.

>>
>>                       ---------------------- mirror
>>
>>
>Should the mirror and slitted screen be plane-parallel?
A line parallel to the table pivot axis should go thorough the slit and a
similar one should line in the plane of the mirror.  The slit is
approximately a line.  The back of the slit should be approximately
parallelel to the mirror (in the horizontal plane), but it's not
highly critical.  A line from the slit through the pivot should be
approximately normal to the mirror.  It is actually useful to start
off with this not being true, so the image is not right on top of the
slit.  


>>
>>
>>                                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?  

Right.  Note what it said above: "Arrange the lens on a pivoting
mini-table. (It pivots about a vertical axis.  You are looking down in
this diagram.)"

>>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'd get the image of the slit in focus first, then worry about making
it not move when the lens is rotated.

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

Oops, that was supposed to be "until the image of the slit is sharp."
When will they invent read-my-mind spelling checkers? :-)


>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)?

You could do this for the front nodal point, except you can't put the
mirror on the focusing rails; it has to be fixed.  You can't do it at
all for the rear nodal point, as the slit would have to be right at
the film plane.  That's all ok, as its the front nodal point you
usually care about.

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

This is actually all quite easy to just hack together for demo
purposes.  I just did it moments ago on my desk.  Using an exacto
knife I cut a crude slit in a business card, then I put my swingable
desk lamp behind it.  As a test lens I used a magnifying glass I had
in my drawer.  And I borrowed a compact from the secretary for the
mirror.  I did't actually have a rotating table, but with a simple
magnifier the nodal point is virtually in the center, so just rotating
it on its edge left the image in a fixed point.  

  -- David Jacobson