diffraction and pinholing.

Disclaimer: The intent of the following theoretical diatribe is NOT to club an artist over head with a physics book.
   
  Uninterested? Please skip to The Last Part.
   
  You can ignore it all or read it as desired, but every time I read about diffraction assumed to be a limiting factor in pinhole photography, I feel the need to remind artist-explorer hybrids we pinhole photographers end up being, that in pinhole imaging, difraction is the only physical phenomenon producing an image. There is no refraction to speak of (I can't argue about air vs vacuum pinholes) and no reflection (other than imperfections in pinhole aperture edges). We have just covered the three ways for an image to be physically produced (maybe one could argue virtually also).
   
  I would argue that pinhole images are produced purely by diffraction, so I doubt 'diffraction limiting' at the aperture is the reason for quality problems. Maybe some physical property in the sensor (even crystal diffraction?) , maybe the smaller camera geometry (object/image ratios w.r.t aperture) results in a mix of Fraunhofer AND Fresnel diffraction, maybe scanning/software algorithms in the camera implementation. Diffraction theory (texts) begins with an analysis of diffraction patterns for the 'classic' simple geometries of a hole or a slit. The Airy Disk and Rayleigh's resolution limit are borne of this diffraction theory. The mathematical model for diffraction is significantly different for a round hole vs a square hole of slit, so hole complexities may be another drift away from predictive results.
   
  The way pinhole images are imaged in a sensor is alot different than on paper, film, or a classroom diffraction 'screen'.
   
  There are a host of assumptions and approximations from theoretical derivations that give us the simple equations that allow us to optimize a pinhole camera that 'works' well enough. It all ends up being a pretty good approximation but a wide range of solutions work to recreate an image that is far more complex than a single wavelength hitting an aperture on-axis. Real world images are far more than that, yet the analog pinhole camera still works in the face of the math not being able to fully describe how it works.
   
  A digital camera has another whole different mess of engineering compromises that interact with wave/particle impingement than the dark/light result a chemical sheet (film/paper) produces to be received by the eye.
   
  THE LAST PART:
   
  If diffraction were not the physical explanation for pinhole imaging your intended scene onto a receptor surface, you would only have a small dot of light. Refraction is the desired method with a lens glass and diffraction is be avoided, but you cannot have a pinhole image without pronounced diffraction effects. The reasons a pinhole camera can be optimized come from the derivation of diffraction theory.
   
   
  Stand (or sit, as you like) proudly as a DIF-fractive photon painter, superior in sensibility and sensitivity to RE-fractive 'appliance' operators who need glass to give them confidence!
   
   
  No photographers were harmed in the writing of this post (so don't take it the wrong way).
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Received on Fri Mar 30 15:23:30 2007