[photo-3d] Re: Centerline definition

[Alan Lewis <3-d@xxxxxxxx>]

David asks:
> 
> Alan Lewis in taking about Holmes viewer focal lengths says:
> "Note that when you focus your eyes through any scope that the
> distance from
> the card to the lens will be less (by 1 to 2") than the actual focal
> length of the lens, and that this focus distance will change
> slightly relative to the centerline distance of the views."
> Could someone define "centerline" for me?

The original context of this comment would help me remember it.  The
part about the focus distance being less than the f.l. of the lens is
not always true, I think I was commenting on the practical measuring of
the focus distance for me with speicific cards at the time, without
eyeglasses.  There are so many assumtions to make for each individuals'
viewing conditions that it is difficult to come up with steadfast rules.

What I meant about centerline distance of the views is the distance
measured across two identical image features on the stereoview.  You can
measure the infinity centerline (separation) or the window centerline
(separation). Just measure between two identical far point image
features on the two views as mounted and this is the infinity separation
(centerline).  You can do the same for the stereo window (near points). 
They will not be the same.
 
I find that with viewer design it works best if the lens centerline
equals or exceeds the maximum infinity separation that is used when
mounting.  Notice that one must follow certain maximum infinity
centerline distances when mounting in order for the view to work well in
a viewer.  This is true for all stereo view mounting, slide or print.