Re: Nishika Camera Modification

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

John Vala writes:

>  I sent this to Photo 3-D by mistake yesterday, it was meant
> for this forum.  I thought I'd bring it back here anyway in
> case there were those who don't keep up with all the miscellanea
> of P3-D.

You're right; there are several people who subscribe to T3D but
not P3D and so I too will repost for their benefit:

John Vala writes:
> Does this all seem right to you?

I'll do a calc the other direction and by my methods to see if we 
get the same answer.  Should make for a good check.

With no information, I would guess that your Nishika is focused 
at 15' so that it can reach from 7' to infinity, more or less.  
That would mean your 34 mm lens was focused out 0.25 mm.  A fine 
point, perhaps, I'm not sure.

First we combine a 500 mm lens with a 34 mm lens using a 
separation of the principal points of 10 mm as you specified.  
New focal length is 32.44 from f = (f1*f2)/(f1+f2-d).  On the 
image side the infinity focal point moves in 2.21 mm from where 
it was which was already out 0.25 mm.  So the film plane is now 
at x' = 2.46 mm outward from the new image-side infinity focal 
point.  The Newtonian form of the lens equation says xx' = f^2 
so x = (32.44^2)/2.46 = 428 mm from the front focal point.  The 
front focal point is 23 mm in front of the first lens' primary 
principal point so we add that to the 428 to get about 450 mm or 
17 3/4" from the first lens' primary principal point to the 
object which is in focus.  I expect the 500 mm lens will be a 
planoconvex, plano side to the Nishika lenses, so its primary 
principal point will be right at its front surface (vertex).

Are we allowed to talk like this on P3D?  

John B


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