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There was reference to doing copies with lights. Regular camera  
lenses, even very fancy ones, are corrected for two colors, not  
three. Normal photography done with apocromats is very different.  
Pictures of people look fairly awful and most regular subject matter  
looks flat. They are flat field lenses corrected for one to one  
though I had one that they custom made for me with a different ratio.  
The angle of coverage is considerably less than regular lenses. Also,  
they are mostly f9.5. The other place they are encountered is very  
expensive lenses for microscopes where again, the image quality is  
very different.  Copy work done with apocromats is very visibly  
superior to work done with regular lenses.

Malcolm Smith.



On Aug 14, 2008, at 9:18 AM, RA Friedman wrote:

> I don't believe a scanner uses a lens? Color reproduction is going  
> to be more dependent on how correctly profiled (calibrated) the  
> system is, bit depth and the sensor array of the scanner and also  
> how finely the stepper motor moves at each increment as it moves  
> across the image being scanned.  Please correct me if I'm wrong on  
> this.
>
> Most modern lenses are corrected for chromatic abberations across  
> the visible spectrum. From what I know of lens design, it's always  
> a compromise trying to get all wavlengths of light to focus  
> identically at the same distance from lens to film plane.  My guess  
> is that once you hit a certain price plateau, you are going to see  
> little difference from one brand to the other. The most marked  
> differences will be due to camera focus and internal processing (if  
> any) and variations in sensor array resolution.
>
> I'm pretty sure that the Goerz lenses are now in the collector's  
> realm although they are still used by some large format photographers.
>
> RA Friedman, Philadelphia
> http://rafriedman.com
>
> ________________________________
>
> From: Association for Recorded Sound Discussion List on behalf of  
> Malcolm Smith
> Sent: Thu 8/14/2008 11:31 AM
> To: [log in to unmask]
> Subject: Re: [ARSCLIST] Large-format scanner redux
>
>
>
> A couple of thoughts with respect to copying flat originals. There is
> a reference to distortion. This common problem is caused by lenses
> not being symmetrical (front elements the same as the rear elements).
>
> There are lenses designed for producing color separations. They are
> known as Apocromats. They are corrected for three colors instead of
> two. If there is any way to use them, they can probably be found
> second hand at bargain prices. They were once very expensive. One
> name to look for would Artar produced by Goertz American Optics. The
> images these lenses produce from color originals are very much better
> than those produced with regular lenses.
>
> Malcolm Smith.
>