Re: [Phys-l] half lens

[Bernard Cleyet <bernardcleyet@redshift.com>]

What's going on here, I think, can be apprehended by noticing that the 
eye has a stop that reduces the image brightness w/o vignetting *.  Then 
remember that one may block view of a portion of an object w/ one's 
finger held in front of the eye.  (I don't recommend putting it anywhere 
else.)   This duplicates the two cases discussed;  a half lens and the 
"photo' divider".    

Corollary:   note the position of the f stop  [f/ x.x]   for various 
camera lenses including the simple one in "box" cameras, and the extreme 
telephoto' catadioptric ones.

* I can't think of the term to mean blocking the object -- BTW stopping 
down reduces vignetting! (Correct use of the term.)

bc, who thanks JD for introducing him to the concept in the technical 
term "lens center".

Roger Haar wrote:

> Dear Anthony,
>
> 	Please note the following is a preliminary
> answer.
>
> 	I just did a quick test first looking through an
> old telephoto lense and then with a simple
> magnifier.  In both cases covering half of the
> lens cut off half of the image.  Now, both systems
> have an additional lense, the eye.  A camera
> lense, is almost certainly not a simple lense, but
> several in series with one or more stops.
>
> 	Try drawing the following:
>
> L  A simple Lense  
>    ( include the optical axis ) 
>
> F  Focal points
>
> O  An Object Extending above and 
>   below the axis, placed a little 
>   outside the focal point.
>
> S   A Stop after the lense about 
>    1/2 focal length from the lens, 
>    with the diameter of the opening 
>    1/2 the diameter of the lense.
>
> B   A Block blocking the lower half 
>    of the lense.
>
> 	Below is about what I mean:
>
>  O                       L        S
>  O                       L        S 
>  0                       L         
>  O-----F---------------- L ------------------
> F    
>  0                    B  L        
>  O                    B  L        S
>  O                    B  L        S
>
> 	When you do a ray tracing on this, 
> many rays from the bottom of the object are 
> stopped by the Block then the Stop is 
> sized and positioned to stop the rest.  
>
> 	I have not fully convinced myself 
> that this works in three dimensions, but 
> I think it must.
>
> 	With lenses in series, the size 
> of the second, third, . . . lense(s) may
> act as a stop(s). 
>
> Thanks
> Roger Haar
>
> **********************************************************************
> Anthony Lapinski wrote:
>  
>
> > If you cut a plastic lens in half (through the diameter), you can still
> > form real images. Compared to the original (full) lens, the images are the
> > same size (same focal length), but appear half as bright.
> >
> > So I asked my students a follow-up lab question: If you put black tape
> > over half a camera lens (the old SLR type), how would the images be
> > affected? Obviously, they would be intact but half as bright, no matter
> > which half of the lens was covered. But a few students, who are also in a
> > photography class, said you would only see half the object/scene. They do
> > this as a project using a "photo divider," which attaches to the front of
> > the lens and can rotate. So one can do double exposures of half a scene at
> > a time. Wow!
> >
> > I had never heard of this before. And it seems to contradict the half lens
> > demo I did in class. Can someone tell me how these camera attachments
> > work? Do the objects have to be "very close" to the camera?
> >
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