RE: AF and lens correction based on focal length

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On  1 Feb 99 at 9:59, Rolland Elliott wrote:

> WJ in response wrote:
> ... high-end tele's are more likely to be IR-corrected than normal
> focal lengths, hence the argument is less
> valid
> 
> My response on 2/1/99:
> Are you sure about this?  What is the theory behind this?  With
> visible light the longer the focal length of the lens the more
> problamatic color fringing becomes (color fringing is the result of
> the optical glass scattering the different colors of the spectrum due
> to their different wavelengths; much like a prisim breaking up white
> light into a rainbow). Thus the need for many lens manufacturers to
> use low dispersion glass and emphasize the APO characteristics of
> their longer telephotos.
> 
> I would assume this color fringing phenomenom would continue into the
> IR range.  

Yes, but perfectly corrected APO lenses, also corrected into the IR 
spectrum, are more common in the tele range than in the range 
20-150....with the exception of a Leica 100mm macro APO....
So the argument of focus-difficulties (tele wide open vs wide angle 
stopped down) isn't as valid as it seems, as tele's have a higher 
chance of being perfectly corrected....

Note though that this level of correction varies with manufacturer,
as well as the specific lens, *especially* when true APO is not
claimed, like with Canon and Nikon....some of these are corrected,
some others are not....but when Leica or Zeiss says 'APO', then it
definately works for IR. 

--                 
Bye,

Willem-Jan Markerink


      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]
*
****
*******
******************************************************
*  To remove yourself from this list, send:          *
*         UNSUBSCRIBE INFRARED                       *
*       to                                           *
*         MAJORDOMO@xxxxx                            *
*----------------------------------------------------*
*   For the IR-FAQ, IR-Gallery and heaps of links:   *
*  http://www.a1.nl/phomepag/markerink/mainpage.htm  *
******************************************************