Re: [Rollei] front element focussing

[Richard Knoppow <dickburk >]

At 10:08 AM 01/16/2001 +0100, you wrote:
>From Richard K. : 
>>   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....It is, in effect, a very elementary zoom lens.
>
>Thanks very much Richard. So I was completely wrong by thinking that
>most of the power in a Tessar is in the front element. Consequently
>the amount you have to move the front element for focusing is
>certainly much smaller that is required when moving the whole lens
>according to Newton's formulae. Now I'm sure our RUGger friends would
>all like to get a comprehensive set of MTF charts for the
>front-focused 40mm f/3.5 Tessar at various focusing distances before
>making a major decision of purchase : a 'R-35 T/TE', or a 'R-35 S/SE'?
>;-);-) (The collector's answer : get'em all!!!)
>
>-- 
>Emmanuel BIGLER         
><bigler  
>\
  The Tessar, like the Protar from which it was derived has most of the
power in the rear component. The front is composed of a fairly high power
positive and negative element with a low combined power. Most of the
correction is done in the front half of the lens, that is, its used to
generate aberrations which cancel those of the rear lens. the cemented
surface in the rear is used mainly for chromatic correction. Because both
of the air spaced elements have considerable power, which is nearly
cancelled changing the spacing between them a little has a rather large
effect on the combined power, which is slightly negative. This, in turn,
changes the focal length of the entire lens. Front element focusing can be
used in any lens where ther is a balance of power between two elements such
that moving one of them a little makes a magnified difference in overall
focal length. Really a rather clever design. 
  In the Zeiss Super Ikonta cameras the rotating front of the lens is
coupled to one side of a split rotating prism, making a simple but very
accurate rangefinder. I think this rangefinder accounts in large part for
the reputation these cameas have for sharpness.  
- ----
Richard Knoppow
Los Angeles,Ca.
dickburk  

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