Could you please explain what diffraction is in simple terms, or point me to a source where it's explained? I have a grasp on what diffraction is, but I don't understand the role it plays in this.

The smaller the aperture, the more diffracted incoming light rays. Once the 'circle of confusion' or airy disc exceeds the size of a single pixel on the sensor, you begin to lose lens sharpness. If you imagine two strips of paper, one green, one blue, directly adjacent. Now imagine photons from either side of the adjacent edge passing through a lens which slightly diffracts them.

This results in an obvious merging of the blue/green strips. The more the diffraction the more they merge, the less sharp the images become.

Current APS-C sensors at 18 megapixels have pixels small enough that an aperture of around f/11 is the minimum size before this effect begins. As sensors get more and more dense, the required aperture size gets larger and larger. F/8 is considered a 'normal' aperture and once sensors hit that density then the payoff from increased resolution is significantly less and can impact image quality adversely.

This is why science missions use 2mp CCDs that they know the characteristics of, rather than some 40 megapixel phone sensor.

edit: I've just realised I've typed all this out with the wrong idea. You wanted a simple explanation of diffraction. Imagine a water tank with waves being generated from a source at one end. Put a wall with a narrow hole in the middle of the tank. If the wavelength of the waves is significantly less than the width of the hole, they will for the most part pass through unaltered. As the hole gets smaller, more significant changes to the waves occur. They begin to 'spread out' or 'diffract'. An intuitive explanation is difficult but this is a practical one that makes sense to people.

Science missions use 2mp CCDs mostly because of the sensitivity. The larger is the individual sensor ("pixel") the more photons it can absorb thus more electrical potential it can generate compared to the noise level.

There's also technological barriers for creating large megapixel CCDs as opposed to CMOS. Large CCDs are usually created by "stitching" multiple sensors together, which increase their price as a square of the size.

You can shoot fullframe (35mm) camera at ISO 25600 and get decent results, but if you crank up signal amplification on iPhone to 25600 you'll get nothing but random color dots :-)

> Science missions use 2mp CCDs mostly because of the sensitivity

It would be foolish to suggest there was a single motivator for inclusion of a sensor. In reality it is a combination of our answers. Large sensors allows lower gain and better noise performance, enables smaller aperture use at acceptable sharpness, takes less resources to process, transmit etc.

You're not wrong in mentioning it though. Although FF at 25600 is going to be piss poor even if it's Nikon / Sony.

edit: corrected stupid misspelling

You've got very high standards in image quality :-)

Nikon D600 at 25600: http://www.imaging-resource.com/PRODS/nikon-d600/nikon-d600G...

Not even close to "piss poor" in my book.

That's about the noise level and sharpness I have on my EOS 40D with ISO 1600. I think I'm going to cry in a corner a little.

That being said, you don't go that high unless you're very desperate and the sensor itself can go up to 6400, the rest is software.

Sample image if needed: http://fc07.deviantart.net/fs71/f/2010/032/f/e/City_Lights_b...

That was f/32. The rings around the street lamps are diffraction artifacts.

Thank you, for both explanations, that was very informative.

Simple layman's explanation (from a layman): lenses spread light out a bit as the light passes through, so the red wavelengths hit the sensor in a slightly different spot than the blue wavelengths. You need more expensive lenses to reduce this effect, but you're not going to get those in consumer-grade mass-market 1/8"-across lenses.

As a result, higher pixel densities will cause light from a single point to be spread across multiple pixels, with different wavelengths going to different pixels. Software can probably reconstruct the image somewhat, but I'm sure you'll hit a limit where there is too much interference to get the image any sharper no matter how many pixels you squeeze in.

> Simple layman's explanation (from a layman): lenses spread light out a bit as the light passes through, so the red wavelengths hit the sensor in a slightly different spot than the blue wavelengths.

What you've described is chromatic aberration.

Diffraction refers to the spreading of waves around an obstacle -- such as the aperture blades in your camera. No lens need be involved -- you can get diffraction with a pinhole camera.

there is no simple explanation, or rather the simple explanation is your photos will be slightly blurry, from interference patterns due to the mysterious world of quantum mechanics. If you want a better explanation, ask a photon to make up its mind whether it's a wave or a particle.

It's the photon shot noise.

The higher your resolution compared to your aperture, the less photons you get per pixel.

Sensor size doesn't matter at all.

More advanced materials with lower noise characteristics can theoretically produce perfectly reasonable images from single photon impacts.

Diffraction cannot be solved as easily.

Photon shot noise (http://en.wikipedia.org/wiki/Shot_noise#Optics) is as fundamental as diffraction - it's a property of light, not a property of the sensor.

If your sensor counts a mean of 100 photons per pixel, then you'll see shot noise with standard deviation of 10 photons in each of those, for a signal-to-noise ratio of 10. If you quadruple your pixel size and now measure 400 photons per pixel, then your SNR goes up to 20.

This is why bigger sensors (more captured photons for same light level and exposure time) are fundamentally better at image capture.

Ok this is getting pretty convoluted. I didn't mean to imply that photon noise was not a real thing, just that diffraction is a more direct concern. Photon noise can be amortised by a number of different processes which aren't applicable to the case I was trying illustrate. You can't amortise diffraction errors by longer exposure or better photon capture. Pixel size is a primary concern regardless of its noise characteristics.

Think about shooting with a shotgun at a target. If you have a thousand pellets, the distribution of pellets at the target will be quite smooth. Now, shoot through a narrow aperture that takes out 99% of the pellets. Now you have ten pellets. The distribution can not be smooth. The sampling frequency (pixel size) at the target does not matter much. It is the aperture size.

I realize that is a very flawed analogy but it's the quickest I could come up with.

Here's more: http://en.wikipedia.org/wiki/Shot_noise#Optics

To get such less noise you not only need to utilize advanced materials, but also cool down the sensor considerably. To register single photon impacts you'll need to cool down to 0K I guess :-)

Not at all, just look at the difference in the current sensors on the market. There's a wide range of performances due to engineering restrictions. Only the pixel size and aperture size are really relevant to diffraction so it occupies a different class of problems.

Even if you had 100% efficient sensors, photon shot noise would still be an issue. You are sampling the photons emitted by the target. After having reasonably efficient sensors, it's a baseline you can't go under.

I'm not sure about that minimum, but I do know noise (dark current signal non-uniformity for those in the know) increases considerably with temperature. So yes it matters.

FWIW, the rods in your own eye's retina are able to register single photon impacts.

This doesn't put a limit on the resolution, though. It just means you need better optics, or larger sensors. The problem is that these cameras generally have inferior optics with a resolution much worse than what the pixel size would indicate.

You're mistaken, the optics are irrelevant. It is the existence of an aperture through which the rays must pass that causes diffraction. You are correct in that resolution does not matter so much as density, but practical density is already hit and sensors are still quite large.