[alt-photo] Re: two alt listers with big big big

[Richard Knoppow]

----- Original Message ----- 
From: "Loris Medici" <mail at loris.medici.name>
To: "'The alternative photographic processes mailing list'" 
<alt-photo-process-list at lists.altphotolist.org>
Sent: Monday, June 14, 2010 11:22 PM
Subject: [alt-photo] Re: two alt listers with big big big


> Hi Christina,
>
> Not that beautiful (and showy) as those old portrait 
> lenses, but, lensbabies
> also give a similar effect (due severe astigmatism?) it 
> you like it...
>
> Regards,
> Loris.
>
    The "swirl" effect in the photos is strange looking. It 
may be due to some defect in the lens but it looks more like 
a digital effect.
    Petzval lenses are actually pretty good, their limit is 
having a narrow field of view. Petzval lenses are still used 
as projection lenses where the a wide coverage angle is 
generally not necessary.
    Most soft focus lenses make use of uncorrected spherical 
aberration. Spherical is not dependant on image angle, that 
is, its the same all over the image. One effect of SA is 
that highlights have soft haloes around them. Cooke Triplet 
type lenses are often used for soft focus especially where 
it is desired to have it adjustable. That is because the 
spherical aberration is strongly dependant on the spacing of 
the center element. By making it variable the softness can 
be adjusted. Most other soft focus lenses are just 
undercorrected and rely on the f/stop to vary the amount of 
SA. SA varies with the stop becoming smaller as the stop is 
made smaller.
     Some other aberrations, notably chromatic aberration 
have been employed in some soft focus lenses but in general 
they are not as satisfactory as residual SA.
     Another common aberration is coma. Coma is similar to 
SA but is dependant on image angle. It results in tear-drop 
shaped blurs of highlights, the blur becoming worse with 
angle. The narrow end of the blur can point either toward or 
away from the image center depending on the sign of the 
aberration. It is usually considered an unpleasant 
aberration. When very severe it can give something like the 
swirled effect.
     Astigmatism in a camera lens is a different aberration 
from the opthalmic astigmtism. In a lens it is the 
difference in the focal plane between light rays entering 
the lens along a radius and that entering along a tangent. 
It has the effect of giving two points of focus for a point 
of light, one results in a line tangent to the optical axis 
and the other a line radial to the axis. the point of best 
focus is between these and the size of the blur spot is a 
measure of the amount of deviation between the two fields.
     Before the invention of "Jena" glass in the late 19th 
century it was thought impossible to produce a lens which 
was simultaneously corrected for astigmatism and chromatic 
aberration. That turned out not to be the case but it was 
not discovered until after the new glass types were 
available. Previous to this time lenses often had fairly 
large amounts of astigmatism and often the field was curved 
delibrately by the designer to average the two stigmatic 
fields around the focal plane. The Rapid-Rectilinear lens is 
an example. If stopped down enough the depth of focus will 
give sharp focus despite the astigmatism. Modern lenses are 
corrected astigmatically so that the two fields converge at 
the center and at some point in the field. Depending on the 
lens type the deviation of the two fields may be rapid or 
slow beyond the stigmatic point. Examples are the Goerz 
Dagor where the deviation is slow and the lens can have a 
quite wide angle by stopping down and the Goerz Dogmar or 
Artar where the deviation is very rapid so that the image 
quality becomes very bad very quickly beyond the limiting 
angle.
     BTW, the Dagor has a lot of zonal spherical aberration 
and has a pleasant mild soft focus effect until stopped down 
about two stops beyond wide open.

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