Pete,
I am not Guy but thought to input my 2 cents worth of opinion.
You asked:
> sorry for the silly question. But can't a camera be both pin hole and zone
> plate?
A pinhole and a zoneplates are 2 different kinds of "lenses". Unless a
camera has a turret that allows it to have more than 1 lens and then select
the lens you want to use by rotating the turret, each camera can have just 1
lens mounted on it. You can not have a camera that is a pinhole and a
zoneplate camera at the same time.
You also asked:
> as I understand it the image is formed on the plate after coming through
> the pin hole. I don't know much about zone plates and view them
> something like filters.
As mentioned above, a pinhole and a zoneplate are 2 different kinds of
"lenses". The former, as you know, is just a minute hole. A zoneplate as
we used them in photography, is a set of alternating black and clear
concentric rings (very much like a bulls-eye), the width of those rings gets
smaller and smaller at such a rate that all the rings cover equal areas.
Zoneplate rings for an specific focal length are calculated using a math
formula. To make a zoneplate, we usually draw on paper a (let's say) 30
times bigger zoneplate and then take a picture of it using a very high
contrast film, usually lith film. By the way, the paper zoneplate usually
has a black inner ring and then alternating clear and black rings, this is
so when we take a picture of it, the result is a zoneplate on film that has
a clear inner ring. When you take a picture of the paper zoneplate, you
have to place the camera at such a distance that the resulting zoneplate on
film is the exact size as calculated by the formula. Once you have the
zoneplate on film, we usually mount it on a piece of metal foil for mechanic
protection. To mount them, we just make a hole on the metal foil a little
bit bigger than the largest outer zoneplate ring. A zoneplate's inner ring
is actually a pinhole, that is why you could think of a pinhole lens as a
zoneplate lens with just one ring. A zoneplate focuses light by virtue of
diffraction, which happens to light skimming the edges of the rings.
Zoneplates let lots of light go thru undiffracted in between the clear
rings, that light is largely responsible of creating the characteristic
"glow" in the highlights of zoneplate photographs and the reason why Guy
knew your SALOON image was made using a zoneplate lens. Since zoneplate
focus light, ideally, they have to be installed at a distance from the film
plate equal to the focal length they were calculated for. The more clear
rings a zoneplate has the "faster" it will be. Zoneplate lenses are usually
manufactured so they are 3 stops faster than the "optimum" pinhole for the
same focal length.
I wrote a small article that basically explains the same I explained above,
you could read it here:
http://members.rogers.com/penate/zoneplate.html
Hope it helps. Let me know if you have questions.
Guillermo
