[rollei_list] Re: Lens recommendation
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 9 Aug 2010 12:39:06 -0700
----- Original Message -----
From: "Elias_Roustom" <elroustom@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, August 09, 2010 4:40 AM
Subject: [rollei_list] Re: Lens recommendation
If you have removed the front cell, you should be looking
at the rear cell through the aperture.
I removed the front cells? element? and there was still
glass between me and the aperture, and glass behind that
still.
What's an element and what's a cell anyway? A six element
lens has six pieces of formed glass, or six surfaces?
I think my lens looks like this in cross section: ( (
: )
Elias
The glass in a lens is described as either _elements_
or _components_. An element is a single piece of glass, a
component can be either a single piece of glass or two or
more cemented pieces, each of which is an element. A
six-element lens has six individual lenses in it but some
may be cemented into components. Most modern enlarging
lenses are variations of a type called a Plasmat. This is a
six-element lens in four components. It was derived from the
Dagor by air spacing the inner lens. The Planar is also a
six element lens with four components except that it is the
outer elements which are air spaced and the order of power
is different.
The Plasmat has become very popular for enlarging and
for large format cameras because it has a wide angle of
coverage and relatively flat field. By air spacing an
additional surface is obtained for correction so that the
relatively large amount of residual spherical aberration
typical of the Dagor can be mostly eliminated. The near
symmetry allows good correction of coma, lateral color, and
geometrical distortion, all important for an enlarging lens.
There are variations of this type with more elements or
groups and some other lens types have been used for
enlarging purposes. Both the Schneider and Rodenstock lenses
are of this general type. Not sure of the Nikor lens.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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