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Re: [Phys-l] pinhole camera



Hi,

I tend to agree with Michael Porter that a pinhole camera does not form an image in the sense that a lens does. Images formed by a lens or mirror has a location, The "image" from a pinhole is everywhere from the pinhole to infinity. The depth of field of a pinhole is infinite. Reducing the size of the hole sharpens the "image" until diffraction starts being important.

We do a pinhole camera in all of our freshmen level labs ( 300-400 students per year) that teach optics. Often the lectures are behind and and may not have started optics yet. So it is kind of a filler. One of the introductory activities we do is a simulated pinhole camera. We use a two-meter stick to repress the light rays. One student, the object, stands about 2 meters from the white board, another holds a "pinhole", a third guides the stick over the head and shoulders of the object, and a fourth student traces the motion of the other end of the stick on the whiteboard. We also use a bigger "pinhole" to show how that increases blurring.

We also use a "standard pin" to make the holes, and for the first exposure everyone photographs the same object. Students are assigned exposure times and after these are developed, the photos are put up on the board so students can see the effect of exposure time on the image.

Thanks
Roger Haar
U of AZ

Michael Porter wrote:
On 4/12/07, Anthony Lapinski <Anthony_Lapinski@pds.org> wrote:
...
I am thinking it would be a good way to show students that light travels
in straight lines, and to introduce them to real images (rather than begin
with virtual images in a plane mirror).

Could you make the argument that what is formed by a pinhole camera
isn't really an image (real or not)? There is no coherent
recombination of light rays in a pinhole camera, just divergence. What
appears on the screen is more like a complicated shadow than an image
in terms of how it is formed.

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.

Sorry that doesn't help you with your query (others have already
helped there... I'm hungry for oatmeal cookies now!).
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