[rollei_list] Re: OT - Bokken (was: Question on operatingand on screen for Rollei GX and Hood)
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Wed, 25 May 2005 18:16:55 -0700
----- 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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