[rollei_list] Re: OT - Bokken (was: Question on operatingand on screen for Rollei GX and Hood)
- From: "Ruben" <ruben@xxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Thu, 26 May 2005 06:32:52 +0200
thanks Richard - your answer is a keeper in the archive for me - put some
things straight - thanks again! ruben
----- Original Message -----
From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, May 26, 2005 3:16 AM
Subject: [rollei_list] Re: OT - Bokken (was: Question on operatingand on
screen for Rollei GX and Hood)
>
> ----- Original Message -----
> From: "Ruben" <ruben@xxxxxxxxx>
> To: <rollei_list@xxxxxxxxxxxxx>
> Sent: Wednesday, May 25, 2005 4:42 PM
> Subject: [rollei_list] Re: OT - Bokken (was: Question on
> operatingand on screen for Rollei GX and Hood)
>
>
>> Richard
>>
>> Is there a reason why you do not se planars faster than
>> 2,8 and summicrons
>> faster than 2,0 ? or is a summilux 1,4 just a fast
>> summicron ?
>>
>> I am asking questions here in a field I know nothing about
>> so please excuse
>> me if there are faste planars etc.
>>
>> best
>>
>> Ruben
>
> These days the name Planar is used by Zeiss for lenses
> on medium format cameras. There is no reason it couldn't be
> put on a lens faster than f/2.8 but Zeiss choses not to do
> so. The five element lens is not used for faster lenses than
> f/2.8 because adding an element makes it easier to correct
> the lens for faster speeds. At f/2.8 it is superior to a
> Tessar, and if fact it has some advantages over the Tessar
> anyway, essential symmetry being one of them. FWIW a
> symmetrical lens is free of lateral color, coma, and
> geometrical distortion. While these cancel completely only
> when the lens is perfectly symmetrical and when the subject
> and image distances are equal, in other words with the
> entire optical system is symmetrical, the cancellation is
> substantial even when the object distance is infinity. By
> making the lens somewhat asymmetrical by shifting some power
> from front to rear cell, the degree of cancellation of these
> aberration can be improved for infinite distances.
> The generic Planar/Opic/Biotar is a six element lens
> derived from what is called a Gauss double lens after its
> inventor, the famous mathematician Carl Friedrich Gauss.
> Gauss discovered that if a telescope objective was made with
> two air-spaced meniscus lenses. This arrangement has the
> advantage that its spherical aberration is constant with
> wavelength. By mounting two of these lenses back to back
> around a stop the advantages of symmetry are gained. Paul
> Rudolph, a genious lens designer (Protar, Tessar, Planar,
> Plasmat, and others) discovered that if he compounded the
> inner (negative) elements and made the negative element of
> the cemented pair thick he could obtain exellent correction
> for spherical aberration and for chromatic aberration.
> Rudolph's lens was an f/4.5 lens at which speed it works
> well. However, in order to raise the speed its helpful to
> make some changes. These were tried by H.W.Lee who change
> the design to a somewhat asymmetrical lens and also
> introduced the use of higher index glasses.
> The basic six element lens works well at f/2.0 but there
> are problems with the correction of marginal rays for
> faster lenses. One way of reducing the effect of these rays
> is to split one or more the the elements of a lens. The idea
> is the reduce the maximum angle of incidence of the rays of
> light with the surfaces of the lens elements. If the bending
> of the light can be split into stages the angle of the light
> rays is reduced and the intoduced aberrations reduced with
> the angle. Various schemes have been tried. Generally, it
> has been found that splitting front elements helps more than
> splitting rear elements but both schemes have been used in
> sucessful lenses. There are a large number of f/2 and faster
> lenses of the general Planar type with 7 or more elements.
> It is also possible to increase speed by compounding
> another lens element. The cemented surface can be useful in
> reducing certain kinds of spherical aberration, for
> instance. Its also possible to introduce elements which are
> not split or compounded from the original. For instance, the
> Leitz Summilux, an f/1.4 lens, is, essentially, a six
> element Opic with the addition of a thin positive meniscus
> lens. I don't know exactly the purpose of the lens but I
> have not read the patent which might explain (USP
> 2.975,673).
> Double Gauss lenses of the four element air spaced type
> were widely used for medium wide angle lenses. An
> outstanding example of this is the Kodak Wide Field Ektar.
> Lenses of this type have coverage approaching 90 degrees but
> not larger. Kodak called their lens a Wide Field lens to
> distinguish it from a true wide angle lens, which is
> generally considered to have more than 90 degree coverage.
> These were f/6.3 lenses.
> I should point out that the earliest successful designs of
> faster than f/2 were the Ernostar and the later Sonnar of
> Ludwig Bertele. These were compounded Triplet lenses. While
> they share some problems with the Triplet type they have the
> advantage of having relativly few glass-air surfaces and
> hense low flare. The f/1.5 Sonnar for the Zeiss Contax
> camera has seven elements but only six glass-air surfaces.
> The f/2 Biotar/Opic has eight glass-air surfaces and some
> modern fast lenses of this type even more. Since flare goes
> up fast with the number of surfaces the difference between
> six and eight is significant for an uncoated lens. Probably
> eight surfaces is the maximum number that can be tollerated
> in a general purpose lens that is uncoated. One of the most
> important changes in lens design over the last century is
> the development of effective, rugged, and low cost lens
> coating. This makes it possible to use glass-air surfaces in
> lenses practically without restriction, greatly expanding
> the lens designer's choices in design.
>
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@xxxxxxxxxxxxx
>
> ---
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