Re: [Phys-l] Image of object placed at focal point

[John Denker <jsd@av8n.com>]

Rick Tarara wrote:

> But they don't focus on a true, mathematical infinity to which my comments 
> referred.  Billions of light years may be damn close to infinity and the 
> width of the object may be damn near zero on a relative scale, but they are 
> not mathematically infinity or zero.  ;-)

I stand by my previous remark, and extend it to include not just
astronomers but also landscape photographers.  Almost every camera-lens
I've every seen has a "focus=infinity" setting on it.  This is useful
in a wide range of practical situations, because of the physics of
"depth of focus".

Also, what is the definition of "mathematical infinity"?  One could
argue that infinity means sufficiently far away that farther wouldn't
make any difference.  We formalize this in terms of you-pick-an-epsilon
and I-pick-a-delta.  In physical optics, physically-significant values
of epsilon are limited by the resolving power of the lens (and by the
pixel size of the film or CCD).  This means that infinity may be a whole
lot closer than billions of lightyears.