----- Original Message -----
From: "Joao Ribeiro" <jribe...@greco.com.br>
> When the light enters the camera, it enters in an angle the depends on
> the pinhole diameter or this angle is constant?
> Imagine I have a pinhole of 0.5 mm. If I make a bellows camera and set
> it to say 50 mm focal distance using an 8x10 film I'll have a very wide
> angle image. But if I enlarge the bellows distance to 500 mm I'll then
> have a telephoto image. Well, actually the image "cone" will be the
> same, I'm just choosing a section of the cone farther away from the
> pinhole or origin, and I am also selecting part of this cone to be
> recorded. How can I calculate the cone angle? Will changes in the
> pinhole diameter make any difference in this angle or it will always be
> constant?
Joao,
Geometrically/mathematically speaking, the angle changes when the
pinhole diameter changes, the change is so small tho, that in practice you
can dismiss it. Since you want to calculate the "cone angle", otherwise
known as "angle of view", here is a formula I just derived that takes the
pinhole diameter into consideration:
Cone angle = ArcTan [ (D+P) / (2 * B) ]
Where"
D = Diagonal of your film format
P = Pinhole diameter
B = Bellows extension (or focal length)
As you can see, the effect of adding P to D is very small, i.e., for 8x10,
"D" would be equal to about 325mm if you add to that a "P" of 0.5mm, you get
325.5mm, again, not a big change. The same happens if you change the
pinhole diameter.
Guillermo
