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Re: [Rollei] focus shift

- ----- Original Message ----- 
From: "Bob Shell" <bob  >
To: <rollei  
Sent: Thursday, January 01, 2004 3:51 PM
Subject: Re: [Rollei] focus shift

> I think all lenses have focus shift, but the amount of
shift probably
> varies with lens design.  The good news is that depth of
> increases as you stop down so focus shift is generally
covered by this
> and not noticed.
> Bob
> On Thursday, January 1, 2004, at 04:50  PM, Feli di
Giorgio wrote:
> > Can someone explain this term?
> >
> > If I understand it properly it works like this.
> >
> > You have the lens set at lets say f2 and focus on an
> > You decide to change the aperture to f5.6, but unless
you refocus
> > the shot will be soft.
> >
> > Do all lenses do this? Some more than others?
> >
> > Inquiring minds want to know.
> >
> >
> > Thanks  and happy new year to everyone.
> >
> > Feli
> >
> >
> >
> >
> > _______________________________________________________
> > feli2  
> >
> >
> >
  Focus shift is due to spherical aberration. while good
quality lenses are highly corrected for spherical some still
have a residual of it. Spherical aberration is a fundamental
property of spherical surfaces. The effective focal length
of a lens with spherical surfaces varies continuously from
the center to the edge of the lens. The focal length is
greatest for light entering near the center of the lens,
what is called the paraxial region, and gets shorter as one
moves toward the edge. A hand magnifying glass will
demonstrate the effect of spherical. Most of these
magnifiers are bi-convex. If one focuses to get an image
with one the image will be blurred and have no definite
point of sharp focus. In fact, it is an overlay of many
images focused at a coninuum of distances from the lens.
Spherical is corrected by combining positive and negative
spherical surfaces. A corrected lens has a plane from which
the image appears to originate called the second principle
plane. In fact, its not a plane but a curved surface of
approximately paraboloidal shape. A lens can be corrected
for spherical to any desired degree but the more highly it
is corrected the more complex the lens must be. The use of
even a single aspherical surface can reduce the spherical
aberration very considerably.
  The correction for spherical in practical lenses is done
so that there is no spherical in the paraxial region and the
spherical is again brought to a minimum at some point toward
the edge, ususally at the edge. This leaves a "zone" of the
lens, typically at 0.707 the diameter, where the residual
spherical is at a maximum. This is called zonal spherical
aberration and is the primary cause of focus shift. When the
lens is used wide open a sharp image is projected by the
center and edge of the lens, but there is also another range
of images focused at a closer (usually) distance by the zone
of the lens. If this residual spherical is large enough the
image will appear to be slightly soft. When wide open the
best sharpness, especially when judged by image contrast,
will appear to be somewhat closer to the lens, a compromise
distance between the center and edge distance and the zone
distance. As the lens is stopped down the zonal spherical
has less contribution to the image so the point of best
focus will seem to move further away from the lens to the
paraxial focus position.
  Zonal spherical is minimised by, of course, improved
spherical correction, but also by the adjustment of the
distance the point where the aberration curve crosses the
zero point in relation to the lens diameter. A lens
corrected this way will ave less focus shift although it may
not be a sharp as one corrected in the usual way. The
compromised correction is often used for lenses which are
used in rangefinder cameras since the range finder can not
correct for focus shift. The slight loss of maximum
sharpness is justified by the position of best focus always
being near that indicated by the rangefinder. This is also a
requirement for use on a TLR camera.
  Occasionally, lenses are compromised in the other
direction. This results in a smaller blur spot when the lens
is used stopped down. It can be used for lenses which are
usually used stopped down and where resolution is important.
  Meniscus lenses like the Goerz Dagor have inherently have
large zonal spherical residuals, so they exhibit focus
shift. They must be focused when stopped down enough to
eliminate most of the zonal aberration and resulting focus
shift. for a Dagor this is around f/11.
  In general, the kinds of lenses used in rangefinder and
TLR cameras do not have lot of zonal spherical. Tessars can
be designed to have good spherical correction and lenses
derived from the double Gauss type, like the
Planar-Biotar-Opic type can be very well corrected for
spherical even without aspherical surfaces. That is one
reason that these lenses are popular for rangefinder cameras
and TLR's. Modern lenses wtih aspherical surfaces can have
virtually perfect correction for spherial aberration.
  BTW, since lens aberrations are interacing some residual
spherical is often left because it helps with correction of

- ---
Richard Knoppow
Los Angeles, CA, USA