Re: [Rollei] front element focussing

[Richard Knoppow <dickburk >]

At 11:44 AM 01/15/2001 +0100, you wrote:
>Browsing through the archive I found this precise technical note
>(9/4/96) from Richard K. :
>
>>  An interesting feature of element focusing is that the field of
>> view of a fixed finder remains accurate. This is because the
>> focussing action actually changes the focal length of the lens,
>> shortening it to focus closely. The usual finder (not Rollei) sees a
>> wider field when close to the subject than the lens does.
>
>Hmm... I think even if the focal lenght may slightly change (like when
>you add a Rolleinar close-up lens) I would myself also invoke first
>the simple reason that the distance between the piece of glass
>carrying most of the converging power and the film simply changes (you
>usually physically move it away from the film by "unscrewing") and
>thus will easily comply with Newton's formulae for a single element
>lens: s*s'=f*f. For f=50mm (resp: 75 mm), and s=1metre (approx minimum
>object distance on classical cameras), s' (lens shift w/respect
>infinity setting) is only 2.5mm (resp : 5.6 mm) wich does not seem too
>difficult to achieve by moving the front element only. Now if in a
>real Tessar-like lens (I should check on my fathers's AGFA Isolette,
>f=80mm or on a friend's Rollei 35T f=40mm) if the actual element shift
>is smaller than that, then, yes, it would mean that the focal lenght
>also changes, and in the favourable way.
>
>-- 
>Emmanuel BIGLER         
><bigler  
>
  The focal length of a front element focusing lens does change, that's how
it focuses. Most of the power in a Tessar is in the rear component. Varying
the distance between the front and center element causes a comparitively
large change in the power of the front cell, which in turn, varies the
power of the whole lens. It is, in effect, a very elementary zoom lens. 
 Since the physical position of the second principle point stays fixed with
relation to the finder the field of view of the two agree at all distances. 
 The easiest way to achieve this for a lens which moves as a unit is the
simple wire frame finder where the frame rides on top of the lens. Other
than parallax these are very accurate finders. 
- ----
Richard Knoppow
Los Angeles,Ca.
dickburk  

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