Beams and beaming

[P3D Neil Harrington <nharrington@xxxxxxxxxxxxx>]

Larry Berlin writes:

>> Neil Harrington writes:
>>Light is not "beamed back to the camera" in any reasonable sense of that
>>word.  You might say light is beamed by a flashlight, ....... That doesn't
>at all describe the
>>situation with light from any ordinary subjects.  I cannot remember ever
>>having taken a picture of any kind in which the light was _beamed_ into the
>>lens(es) of my camera.
>
>***************  Then you never took a photograph???!!! 

Well, over the last 40-odd years I've taken a few.

>Light had to enter
>your camera or the film woudn't have been exposed. ; -)   

Yep.  But what I don't think I've ever done is had light ** BEAMED ** into
my camera.  "Beam," the verb, conveys the idea, as I said, of light being
directed at something in a concentrated way, like a headlight beam . . .
"beamed back to the camera" suggests that such a beam is aimed specifically
at the camera.  I don't think I've ever had occasion to point my camera at
anything that did that, and I can't really imagine ever wanting to.

It's probably possible to get into yet another word-hassle over "beam," but
I'm reluctant to do so.  I'll settle for pointing out that "beam," the verb,
does not really carry the same meaning or implications as "beam," the noun.
It is correct to speak of sunbeams, for example, but the sun does not _beam_
light in any one direction as a headlight does; it radiates pretty equally
in all directions.  A beam of light, whatever the source, is a directional
thing practically by definition, but that doesn't mean or imply that the
source was directional.

I've always photographed subjects that scattered light in nearly all
directions rather than beaming it.  Everyone else I know who takes photos
does the same.  That part of the light which enters my camera may indeed be
properly regarded as a beam, or more correctly a very large number of beams,
coming from different directions and following different paths.  

When they enter my camera all those beams are unsplit, though, even if
they've been routed through a mirror stereo device.  Unsplit, uncut,
unchopped, unsliced, undiced, undamaged in any way.   ;-)

>I agree that it is
>stretching definitions a bit but there is a specific cone of incident light
>that enters the lens and falls on the film. In a single lens camera there is
>a single cone of that light. With a mirror attachment on that single lens
>device, the usual single cone (loosely called a *beam* for purposes of

Whoa, Larry, that's too loose for me!  "Beam" to me implies something much
more directional than "cone."  You could have a cone with an included angle
of well over 100 degrees in the case of a wide-angle lens, which is pretty
hard to see as a beam.  Can we agree that your "cone" is an imaginary
formation which includes all the beams of light entering the lens?

>discussion) is formed into two beams which enter the single lens providing
>the two images of a stereo pair. 

But in this case there never was your "single cone" to begin with.  With the
mirror attachment there are _two_ overlapping, but distinct, cones, each
eventually producing its own half of the stereo pair.  Two (imaginary)
cones, two (imaginary) apexes.  There is no such thing as one cone with two
apexes.

>****************  If you include all incident rays of light reaching the
>camera lens as a single *beam*, then the description does make sense and
>this normal single *beam* is essentially split into two paths. Yes, in a
>technical sense they are now separate paths and could be considered two
>separate beams, but the normal for a single lens device is only one such
>beam. Hence, the mirrored stereo attachment splits the single beam into two
>beams directed into the single lens.

Whoa.  They are two separate cones when they first enter the stereo
attachment, each cone containing its own set of beams.  There isn't any
single beam that's split into two beams; that's the whole point.  Trace any
one ray from subject to film.  Either it enters at the left mirrors and ends
up on the right side of the frame, or t'other way around.  It never splits.

>>Then for your money, any stereo camera is itself "a beamsplitter pure and
>>simple," since it does what you have just described just as much as any
>>mirror stereo attachment does.  Not for my money, though.
>
>*****************  Not true, since a  stereo camera has more than one lens
>it could not be described as a beam splitter. 

Please read the original message again.  He said "two separate lenses" in
his description of the so-called beamsplitter.  I repeat, his description of
"a beamsplitter pure and simple" was in fact a working description of a
stereo camera.

>It records a *two-beam*
>situation instead of one beam. 

As already pointed out, the mirror attachment does the same.

>Besides, if you wanted to be totally accurate to the terminology, you could
>devise an arrangement of reflecting surfaces with actual optical beam
>splitters which would sandwich the two images through a single lens. 

Seems to me that'd be going to a lot of trouble simply to justify calling
the contraption a beamsplitter, though.    :-)


Neil


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