> I'm planning to start 9x12cm sheet film photography.
> Am I right to use formula for hole diameter D:
> SQRT(Focal lenght)
> D= -------------------- [mm]
> 44
The optimum hole size occurs when the hole size is equal to the the size
of the Airy disk: the Fraunhofer or farfield diffraction pattern of the
pinhole. The equation for this condition is:
0.61*lamda*f/s = s
where: lamda is the wavelength of light
f is the focal length (hole distance from film)
s is the radius of the pinhole
This can be approximated by:
f = s^2/lamda
Assuming a wavelength of 550 nm to approximate visible light:
D = 0.047*sqrt(f) [measurements in mm]
with D being the optimum hole diameter for a given film to hole distance
(f). The multiplier 0.047 comes from 2*sqrt(lambda) so it will vary
depending on the wavelength of light for which your hole size is
optimized. It looks like your equation with a multiplier of 1/44
optimizes the hole for a wavelength of 129 nm. That would make a pinhole
based on your equation work quite well for ultraviolet. Visible light is
in the range from 700 nm to 400 nm.
- Wayde
(wallen@boulder.nist.gov)