With respect to lenses, the only lenses that I know of that have adjustable
elements for compensating for field curvature and producing effective,
although not complete, flattening are flat field copy lenses and true macro
lenses. I will not comment on silicon sensors except to say that no matter
how flat they may be they certainly will be effected to one degree or
another by the optics of the lens in the digital camera or scanner in the
case of CDD scanners.
> -----Original Message-----
> From: filmscanners_owner@halftone.co.uk
> [mailto:filmscanners_owner@halftone.co.uk] On Behalf Of gary
> Sent: Thursday, July 05, 2007 8:29 PM
> To: laurie@advancenet.net
> Subject: [filmscanners] Re: film and scanning vs digital photography
>
> I thought the lens design has elements to compensate for field
> flattening. In any event, the predictably flat silicon focal plane has
> to be better than the lottery of film.
>
> Tony Sleep wrote:
> > On 06/07/2007 Arthur Entlich wrote:
> >> Does anyone know if there is a chart which shows depth of focus at
> the
> >> film plan versus aperture of lens used?
> >
> > No, but the plane of focus itself is not flat, it's usually a section
> of a
> > sphere that is only part corrected to flatness. This becomes an issue
> when
> > focussing wideangles at wide apertures, especially. If you use a
> focus aid
> > or AF at the image centre then re-frame to put it near the edge,
> it'll be OOF.
> >
> > I used to do enough of this that with a 24mm f2 that I bought a plain
> > matte screen without any focus aids so I could focus as framed. It
> can be
> > quite a handy property since edge of frame close objects can be in
> focus
> > at the same time as more distant central ones, without having to stop
> down
> > to provide as much DoF as expected.
> >
> > If you photograph a flat wall with such a w/a, you can see the
> problem;
> > the edge-of-wall to lens distance can be substantially greater
> (nearer
> > infinity) than the centre ditto. This would mean the lens needs to be
> > racked in further for the edge image to be sharp, more extended for
> the
> > centre.
> >
> > Constant subject-lens distance d implies a part-spherical plane of
> focus
> > of radius equal to d. The back focus of the lens b is also a
> > part-spherical surface of radius b. For longer lenses with narrower
> angle
> > of view none of this is really noticeable, as the smaller section of
> a
> > sphere is near enough flat and DoF hides the effect.
> >
> > We need spherical film or sensors - but the radius would be
> different for
> > each focal length dammit.
> >
> > --
> > Regards
> >
> > Tony Sleep
> >
> >
> >
>
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