RE: AF and lens correction based on focal length

[Rolland Elliott <rolland_elliott@xxxxxxxxx>]

On 21 Jan 99 at 15:14, Rolland Elliott wrote:
> I'm sure one of the reasons no one sees lots of
action  IR images with
> shallow depth of field is because of the inherent
problems of focusing
> IR light accurately and quickly with the shallow
depth of field
> associated with long telephoto lenses usually used
for action
> photography.

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.  

Sincerely Rolland Elliott

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