Re: Home made Lenses ( soft focus )

----- Original Message -----
From: "John Cremati" <johnjohnc@core.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Friday, March 04, 2005 11:04 AM
Subject: Home made Lenses ( soft focus )

> Examples of Tom Ferguson photos:
> http://www.ferguson-photo-design.com
>
>
>
>
> Hi Tom,
> Wow! Your depth of field on those photos is almost
> nothing! Plus
> you have center focus and then it fades to the
> edges....... I think you got
> real lucky on that lens..... Is the effect controllable?
> Is this chemical
> focus or is it mechanical?
> I bought a book published by Jay Allen called"
> Pictorial Soft-Focus
> and Portrait Lenses from the Past" ... He has been
> researching Soft focus
> Lenses since the 1980's when he found out that almost
> everyone that knew of
> them extensively had died......... In his book he lists
> the different makers
> of such lenses and the model and series number of their SF
> models.... Most
> importantly he lists the coverage of the lenses ( It is
> hard to believe that
> many of the 14 inch SF lenses do not even cover 5x7 as
> they commonly have
> a very low angle of view) All most all of his
> information came from the
> manufacturers catalogs and pamphlets.......
> He talks on the correct use of the lenses which do
> not operate as
> conventional lenses for the most part.....The differences
> in the use of
> chemical focus and mechanical focus for starters......
>
> What little I know has come from his book ( 50 pages
> Xerox copy bound
> together) ...I have been making steady progress........
> John Cremati
>
    A single cell of many symmetrical lenses will make a
soft focus lens, at least of sorts. A lens which has a
fairly sharp image at the center but becomes progressively
more blury toward the margins probably has coma. Many
symmetrical lenses depend on the symmetry to correct coma so
the single cells can have plenty of it. The Goerz Dagor is
an example. Spherical aberration is the opposite, it adds
rather than cancels so a half dagor has no more than the
complete lens. Spherical aberration causes a bluring which
is uniform from center to edge. Since the effect is of a
lens without a definite focal length the image is really al
combination of sharp and blured images from a field
surrounding the average focal length. This is what gives the
images their unique look. It is not just a scattering of
light from a diffuser.
   Coma is a variation of focal length which varies with the
image angle. Coma, by definition, does not exist near the
center of the image but becomes progressivly greater with
image angle. Coma tends to produce tear-drop-shaped blure
spots which can be somewhat unpleasant. There is an
aberration called oblique spherical aberration which
produces a similar image but comes from a different source
in the lens. All three of these aberrations are proportional
to the stop so many lenses which are quite soft when wide
open become quite sharp when stopped down.
   Dagor and similar double meniscus lenses, tend to have a
lot of zonal spherical aberration. The can be pleasantly
soft when wide open or near it. This is not so strong an
effect as from a lens designed for soft focus effect but it
can be useful where just a little softening is desired.
   Edmund scientific sells achromatized lenses. I have no
idea how well corrected they are but Edmund could probably
supply data. One would have to mount these with a stop of
some sort. For experimental purposes cardboard tubes would
probably do as a mount. I suspect that some of these might
have a nice soft focus effect.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com