What is Diffraction?

I just received some concerning news, it turns out my beloved Photos are showing signs of being Diffraction Limited. Are you one of the many Digital Photographers or Videographers suffering from Diffraction?  The test is easy and can be preformed in the privacy of your own computers so please help stop this growing epidemic and get your images checked.

Squint your eyes down, smaller, smaller still, until they are just barely open. What you notice is that the smaller your eyes get the darker and softer your vision becomes. You have just observed a crude example of Light Diffraction.  For the quick solution don't stop down past F/8 then use ND Filters to protect your Photos from Diffraction, it's like how throwing a pair of sunglasses over your eyes get you a darkened image without nearly as much loss of sharpness.

As a Photographer I felt the need to better understand Light and although I am not a Physicist I do know that the 'what', 'how' and 'whys' of Light are still under debate and that a conversation about Particle-Wave Duality is best left to the Quantum Physicists. What a Photographer dealing with Diffraction needs is to understand Light as a Wave. A Light Wave emits from it's source in all directions; what has helped me is to imagine Light as a growing bubble of Photons moving out as a tightly shaped sphere that is immediately followed by another wave and moving faster than the eye can register. They race away until they crash into an obstacle, as the surface of the obstacle is bombarded with photons each point of contact becomes a new point of emission sending another wave of photons back in the other direction. 

Light moves outward as a Radial Wave so when it rushes passed an edge it not only hits one side and reflects back but also around the corner to the second side of the edge reflecting waves across as well. When Light encounters an obstacle with multiple edges in close proximity, like your nearly closed eye lids or the inner edge of a Lens Aperture, the light waves reflect across from one inside edge across to the other and back again sending waves back and forth across the small opening. The opposing waves overlap and interferes with the portion of the wave that made it past the edge cancelling out and redirecting some of the light that passes near the edge off to an extreme angle reshaping the wave that emits out of the obstacle into a wider angle reducing brightness and sharpness that make its way to the sensor.

A Digital Photographic Sensor, like a Georges Seurat pointillism painting, is made up of tiny "Pixel-Sensors".  An Image is projected from the back of a Lens onto the Sensor like a tiny movie theater inside your camera. Then each of these microscopic Pixel-Sensors record one microscopic point of the projection and save it as one pixel of the digital photograph.  However these Pixel-Sensors also have their own edges where the light diffracts, the smaller the Pixel-Sensors the more susceptible they are to diffraction caused by the lens aperture.  A modern day "Full Frame" Camera has Pixel-Senors of about 6.25 microns and an "APS-C" Camera of today has 4.3 microns.

Now 

The solution to avoid diffraction is to keep your Iris settings wide and use the largest and/or highest resolution sensor you can; but understanding why and what your particular camera's diffraction limit is important to getting the sharpest images possible from your lenses.

Basically Diffraction is the lens not getting enough light to the sensor's individual photodiodes to properly resolve your image.  A high resolution sensor has more physical photodiodes, but they have to squeeze them into the same amount of space so the photodiodes need to be smaller which means they have more trouble saturating and when closing down the aperture reducing the light through the lens.  When a pixels next to each other are made from photodiodes that are under-saturated it translates to lower contrast and creates a softer image. That's the easy explanation anyway.

Technically the lens is an obstacle for light to squeeze through, it forces the light down together and then spreads back out at a weaker intensity.  Think of light-rays like waves of water moving through an obstacle, turning strong aggressive waves into a weak gentle ripple.  In a camera the light also needs to squeeze through the aperture and the smaller the aperture the more narrow the flow which cuts down more light-rays that can get through, the light-rays that make it through are then spread back out to cover the sensor area, which results in wider yet weaker light-rays. Modern day 'Bayer Pattern Array' CMOS Sensors are actually a grid of microscopic sensors called photodiodes or 'Pixel Wells'. These 'Pixel Wells' are very small, measured in microns, and each 'Pixel Well' should record a different ray of light than its neighboring 'Pixel Well'; together they are resolving an image down to their microscopic micron's size, which we preserve as fine details, micro contrast or sharpness.  When the individual light-ray are wider than the sensor's microscopic micron sized pixels then each pixel will start to record part of the light-ray of its neighboring pixel, the more they overlap the more similar each pixel is which effectively lowers your pixel count (resolution) and the softer the image appears, a "fully diffracted image" may render 9 individual pixels as 1!  Turning an 18 megapixel image into a 2 megapixel one.  That is called the diffraction effect.

The higher the resolution of a sensor the smaller its 'Pixel Wells' need to be so that they can physically fit on the sensor and the faster they'll reach a diffracted state, called the diffraction limit.  This is why for higher resolutions it is important to use a physically larger sensor; although an argument can be made that higher resolution sensors are still higher resolution when fully diffracted (9 pixels act as 1) than a lower resolution sensor and that is true but only in very extreme cases, for example a fully diffracted 60 megapixel image would be 6.6 megapixels which is still higher resolution than the sensor of my first Canon DSLR, the EOS D60 (6.3 megapixels) which I was still able to make passable 8 x 10 prints with.  Right now we are at about 20 megapixels on most DSLRs and those diffraction limits (that when the diffraction effect starts) are f/11 for Full Frame and f/8 for APS-C; the higher we go the less exposure choices we'll have, one day we might even be diffraction limited at f/2.8... but that's quite a ways off.

Cambridge in Color has a great description of diffraction with very detailed examples and even an interactive 'diffraction calculator', I recommend going back there to carefully read the article and try out their calculators to get a thorough understanding of this effect.

"Diffraction is an optical effect which limits the total resolution of your photography — no matter how many megapixels your camera may have. It happens because light begins to disperse or "diffract" when passing through a small opening (such as your camera's aperture). This effect is normally negligible, since smaller apertures often improve sharpness by minimizing lens aberrations. " -http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm