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Diffraction is a property of the trignometry involved with the angle at which the light is being 'fracted". Not sure of its origins, but I think of diffraction as a word that means "diffusion" caused by "fraction" i.e. diff + fraction = diffraction. Fraction referring to a fracturing or dismantling/dispersion/portion of something that was once whole.
In the case of photons, the change in angle requires the light to be spread farther apart which both reduces the density of photons and the angle of incidence. From a trignometry/physics perpsective, that means the photon is now striking the sensor (or film) with less energy (glancing blow) than if the light path were to remain perfectly straight/perpendicular. While we can compensate for the density/volume of photons reaching the sensor by allowing the shutter to remain open longer, we are not changing the angle of impingement that the photons are striking with ... i.e. "direct hit" vs. "just grazed him".
Light is a vector quantity. Most of us realize that AI=AR (Angle of Incidence = Angle of Reflection). Change the angle that light is striking, and you change the amount that if being reflected. However, the unmentioned corollary to that is ... change the angle of the light striking, and you change the amount that is being ABSORBED.
All surfaces exhibit some degree of reflectance and absorption. A surface that absorbs 99% of the light energy (i.e. black) only reflects 1% of that energy that is striking it. With more of the light entering at angles that create less absorption and more reflection, the amount of absorption is reduced. The remaining amount that is reflected, most of that light gets captured by the black internal components of the camera. That helps keep it from 'bouncing around' too much. But that still doesn't do anything to improve the LESS absorption aspect that is caused by the more oblique angles. When we are using apertures that induce diffraction, having a straighter incoming light path (think spot/grid) can help mitigate the effect of diffraction (as a trigonometric function).
From that it is simply that the more direct the light hits the sensor, the more absorption that occurs. In lighting, in general, we know that a soft box or umbrella disperses and changes the angles to much more obtuse ones, whereas a grid of a spot changes those angles to more direct angles. You can get a properly exposed image using either one, but the level of contrast will be significantly different. It is the same with light striking your sensor/film, as it is with it striking your subject.
Simply put ... light is light.
Whether you are shooting on film or digital, the change in angles caused by your choice of aperture remains to have the same effect on the projection. There might be a difference in refelctance/absorption between film vs. digital, but the light that you are SENDING to it remains the same. Adjusting the aperture, impacts the angle of the light path ... and I simply sum it up like this:
Straighter light = more absorption & more direct reflection = more contrast. Diff - fracted light = less absorption & more reflected away = less contrast.
(This is readily seen by simply changing the beam pattern of a flashlight as you shine it on an object from a constant position.)
Light is light, whether on the subject, through the lens or in the camera. And while most people will NEVER go throught the physics & trigonometry involved, this is a VERY LOOSE explanation of how/why diffraction is involved and one would have to know the distance between the aperture and the next element, as well as the shape of the next element to know which aperture is optimal by design to generate the least amount of loss. Suffice to say that whether it is 5.6 or 2-3 stops down from WO (fairly typical) ... the FARTHER you move away from the optimal aperture, the greater its effect.
Long, OT and a hodge-podge of science/analogy, but hopefully it helps more than it confuses.
BTW ... my physics instuctor was the co-inventor of night vision for the Pentagon ... suffice to say light was his thing.
Edited on Feb 24, 2012 at 06:22 PM · View previous versions