Re: [Phys-l] half lens

[Roger Haar <haar@physics.arizona.edu>]

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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