Re: [Rollei] focus shift

[peter kotsinadelis <peterk727 >]

In simple terms, does this mean you agree with Bob?
Boy Richard, I fear asking you the time of day.

Peter K

- --- Richard Knoppow <dickburk  > wrote:
> 
> ----- 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
> field
> > 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
> object.
> > > 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
> coma.
> 
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk  
> 


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