Re: closeup lens focus

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

OK, I've got it figured out.  Both ways are correct.  The problem 
was, we were talking about apples and oranges.  The easy way to 
look at it is as Dick Koolish and Michael Gordon suggested.  But 
that only works precisely in the one special case where the prime 
lens is set at infinity and the object is at the front focus of 
the auxiliary (close up) lens.  What I was talking about, and 
temporarily forgot I was talking about (an unfortunately 
increasingly frequent event), was the combination of two lenses 
becoming one equivalent lens.  In that case, as it turns out, you 
measure from about the junction of lens and SLR camera body.  
 
The first way of looking at things only works when the prime lens is 
set at infinity.  The reason I made the spreadsheet (now at T3D's 
web site) to calculate the one lens from a combination of two was to 
allow a person to use the (limited) focussing ability of the prime 
lens to shift the combination lens into focus.
 
So let's compare the two methods in the special case mentioned 
above.  If we plug into the spreadsheet a +1 (1000 mm) lens located 
50 mm from the prime lens, then we find that the combination lens 
also has a focal length of 50 mm and that the front principal point 
is unmoved (it is still at about the junction of lens and body).  
However, the back principal point has moved forward 2.5 mm.  So its 
distance from the film is now 52.5 mm rather than 50 mm.  The object 
is located at 1000 mm from the front of the combination which is 
1050 mm from the front principal point, since the front principal 
point wasn't moved.  Now we check these figures:
 
  1     1     1
  -  =  -  +  -
  f     s1    s2
 
  1     1      1
  -  =  -   +  -
  50   1050   52.5
 
And it checks.  So either method gives the same result in the 
special case.  The easier method just doesn't apply in any other 
case so you're stuck and have to go to the spreadsheet.
 
Now let's look at a case where this additional complication would be 
useful.  A 50 mm prime lens for an SLR ought to have at least 4 mm 
of adjustment; it probably has more.  So what happens when we put on 
the auxilliary lens and focus out 4 mm?  Where should the object be 
now?  The new focus is 56.5 mm from the rear principal point (if we 
still use a +1) and the lens combination focal length is still 50 
so the object must be located 435 mm from the front principal point 
(juncture of lens and body) or 435-50 = 385 mm from the front of the 
auxilliary lens.  Since I want to make sure, I go to to the optical 
bench and set up the experiment and find...  Yup, it's about 385 mm.
 
John B


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