Re: [Phys-l] pinhole camera

[Bernard Cleyet <bernardcleyet@redshift.com>]

There was an additional criterion posed by Michael P. viz.  that the eye 
substituted for the screen does not see the object:

"With an actual real image formed by a lens, you can remove the screen
and still see the image if your eye is lined up with the object and
the lens. You cannot do this with a pinhole camera."


I presumed he had done the xpt. in confirmation of the above.   Since 
prysbiopia, I often use stenopaeic vision and know one obviously sees 
the object.  However, I had always used the "pin hole" -- usually a 
forefinger, rolled up in the thumb and finger angle, close to my eye, so 
I didn't "know" whether magnification results when the aperture is 
distant from the eye.  I just tried it, curtains closed, using our front 
door's faux stained glass window.  Wow!


Incidentally JD didn't mention in detail a very important "aberration" 
most peculiar to pin hole lenses.

I posted it only on tap-l, as it was the subject of a previous thread on 
PHYS-L -- anyway here it is again:

-----------------
Au contraire, to properly discuss pin hole (stenopaeic) imagery one must 
discuss diffraction.  For example Ditchburn ("Light" 2nd ed. p. 280) 
asks:  "... obtain an expression for the optimum value of the size of 
the hole in a pinhole camera."   This has been discussed on PHYS-L.

bc, enjoys the Exploratorium's portable camera obscura.

p.s. a specific answer:  Assuming a distant object, the hole-screen 
distance is the hole's "focal length", and using the Rayleigh criterion:

hole d = ~ 2 Lambda f/

cf. Rayleigh: "Scientific Papers"  I, p. 513


[in response to:]

"When I teach optics in my (high school) course, I traditionally discuss
reflection leading into plane and curved mirrors, then refraction leading
into lenses. Does anyone incorporate pinhole cameras when they teach this
topic? It has been in the back of my mind for years, but there is
little/no math involved and "higher level" textbooks rarely mention them."

cut

---------------

bc, doubting Thomas.

Edmiston, Mike wrote:

> It seems to me whether the pinhole forms an image depends on your
> definition of image.
>
> If your definition of "real image" includes that light falling on a
> particular point in the image has converged on that point, then I guess
> a pinhole does not form an image.  But that's not my definition of real
> image.
>
> My definition of real image would be more like... Light passing through
> a particular point on the image came from a particular point on the
> object.  That is, I see the object-image relationship as a one-to-one
> mapping of light from one point on the object to one point on the image.
> A pinhole does that.  A pinhole does not do that perfectly, but as John
> Denker said, a lens doesn't do that perfectly either.
>
>
> Michael D. Edmiston, Ph.D.
> Professor of Chemistry and Physics
> Bluffton University
> Bluffton, OH 45817
> (419)-358-3270
> edmiston@bluffton.edu
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