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Re: [Rollei] front element focussing
- Subject: Re: [Rollei] front element focussing
- From: jerryleh
- Date: Tue, 16 Jan 2001 17:02:41 -0800
- References: <18.104.22.168.20010115102057.0088cce0 >
On this matter, doesn't the Rolleimagic I and II focus
BTW, the Super Ikonta B is a superb camera. My
pics are every bit as good as my Rolleiflexes!
Richard Knoppow wrote:
> 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
> 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.