Re: filmscanners: Test Imacon, Nikon.Polaroid

["Dave King" <kingphoto@mindspring.com>]

----- Original Message -----
From: Tony Sleep <TonySleep@halftone.co.uk>
To: <filmscanners@halftone.co.uk>
Sent: Saturday, July 14, 2001 9:30 AM
Subject: Re: filmscanners: Test Imacon, Nikon.Polaroid


> On Sat, 14 Jul 2001 01:17:28 -0400  Dave King
(kingphoto@mindspring.com)
> wrote:
>
> > If there are no mirrors in either, what would explain better
sharpness
> > in the Imacon (assuming flat film in the Polaroid and Nikon)?
>
> A bigger budget for the lens? ;) - but also the whole point of a
curved
> film gate is to equalise raypath lengths. There is no such creature
as a
> truly flat-field lens, and this is especially true of macros.
>
> AIUI the Imacons work rather like a drumscanner, presumably moving
either
> the film or lens/CCD relative to the film (I still haven't seen one
in the
> flesh so don't know which). The curved film plane eliminates focus
errors
> due to differing focal points along the longest dimension.
>
> Of course, to do it properly the Imacon would need the film to be
dished
> in both directions :)
>
> Regards
>
> Tony Sleep

You make an interesting point.  The Imacon, (as you know?) curves the
film in the long dimension so that it will be perfectly flat (in
practical terms) along the short dimension to a slot that the lens
"sees" through to the focal plane (or line in this case) at the film.
Since the slot runs across the short dimension of the film, unequal
distances are reduced but not eliminated.

Regarding your other comment "no creature as a truly flat field lens"
I would only say there are many lenses corrected well enough to get
the job done (again) in practical terms, but as you say, it helps to
start with longer than normal focal lengths.  Perhaps lens focal
length has something to do with the Imacons high performance levels.
If I recall Imacon uses an apo Rodogon, which are among the best
available.  The Imacon also appears to be a longer than usual light
path, and it's unfolded as well.

Dave