FreeLists / opendtv / [opendtv] Re: Red camera lenses

[flyback1 <flyback1@xxxxxxxxxxx>]

Read about Telecentric lenses:
http://www.lhup.edu/~dsimanek/3d/telecent.htm
http://www.computeroptics.com/telecentric.html


johnwillkie wrote:
> Question wasn't rhetorical, and it remains avoided.
>
>  
>
> My photography training from a few decades back (I did most of the 
> math) "led me to believe" that to image a scene on film (or a camera 
> obscura) required the remote image to be rendered on the close plane, 
> and to do that, one needed to:
>  
>
>    1. prevent extraneous liget from hitting the focal plane
>    2. have to establish the focal point between remote scene and the
>       focal plane (which limits the light on-scene, not unlike 1, in part)
>    3. have to control the exposure.
>
>  
>
> One nice point is that the remote scene is rendered on the focal plane 
> upside down. 
>
>  
>
> I don't see how one can accomplish the above without a lens.  That 
> "lens" can be a piece of finely ground glass, a pin hole in a piece of 
> card-board, a glass shard, a magnifying lens, a coke bottle, the hole 
> in a magnetic torus, etc.  Each has a focal point (okay, a glass shard 
> and others might have more than one)
>  
>
> But, unless I've missed something very new, this is how imaging has 
> been done for at least 500 years, perhaps longer.  
>
>  
>
> If I'm wrong, I'd love to see links to lenses that provide usable 
> images yet don't have focal points (and don't use lasers, since 
> collimated light is another matter.)
>  
>
> John Willkie
>
>  
>
> ------------------------------------------------------------------------
>
> *De:* opendtv-bounce@xxxxxxxxxxxxx 
> [mailto:opendtv-bounce@xxxxxxxxxxxxx] *En nombre de *dan.grimes@xxxxxxxx
> *Enviado el:* Wednesday, June 13, 2007 1:09 PM
> *Para:* opendtv@xxxxxxxxxxxxx
> *Asunto:* [opendtv] Re: Red camera lenses
>
>  
>
> Though your question might be rhetorical, there are lenses that do not 
> have focal points.
> If you are talking about a "lens" as a single element, there are 
> lenses that do not have a focal point where the light diverges but 
> rather converges.  But the divergent lens would have a negative "focal 
> point" even thought the light never converges.  An example would be a 
> biconcave lens.  So one might argue that this still has a focal point. 
>  Ultimately, if the positive or negative focal point is infinite, then 
> you would have a "filter" and might not define it as a lens.
> As far as a system lens, a lens made up of many elements, there aren't 
> too many lenses that produce an image that do not have a focal point. 
>  There are conversion optics that do not have a focal point, but they 
> do not make an image by themselves.  There are some rare designs that 
> do in fact "image" without a system focal point, but I don't think 
> there are any in the broadcast industry.
> Dan
>
>
> From: "johnwillkie" <johnwillkie@xxxxxxxxxxxxx>
> Subject: [opendtv] Re: Red camera lenses
> Date: Tue, 12 Jun 2007 11:27:48 -0700
>
> "In some lenses, light may radiate from a central point within the lens
> and travel in non-parallel lines to the sensor."
>
> Seems to me that glass without this characteristic cannot be called a 
> lens.
> Is there a lens without a focal point?
>
> John Willkie