Myth busted: smaller pixels have more noise, less dynamic range, worse diffraction, etc.

[Mark Elberson]

<span style="font-size: 9pt; color: black; font-family: Verdana;"]Daniel,<o></o>


<span style="font-size: 9pt; color: black; font-family: Verdana;"]Great job! Once again you have taken a complex, and quite often misunderstood, topic and made it accessible for everyone to easily digest. I think I could actually discuss diffraction and feel pretty comfortable with what I would be saying

[clemmb]

AmI right in saying that DLA is always present but the higher the density of a sensor, the sooner it becomes visible. The less dense the sensor, the less able it is to resolve the DLA?
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Yes you are correct. There is a lot of discussion about diffraction in communities that discuss telescopes. No pixels there, just the human eye. Diffraction is an issue with optics in general but it is more apparent in digital photography as pixel density increases.


Mark

[Jon Ruyle]

This all has to do with the wave nature of light- roughly speaking, when a wave meets an obstacle, it spreads out. Light does the same thing. The aperture of the lens is an obstacle which blurs the light. The bigger the aperture, the less the degree of the blurring.

There is a lot of discussion about diffraction in communities that discuss telescopes.

I think you're right Mark- thinking about telescopes is helpful.


Larger telescopes resolve more than small ones. They don't just gather more light, but they give sharper images, and the reason is diffraction. Even if a six-inch telescope has perfect optics, point light sources (like stars) blur as they pass through the aperture and becomes a disk with a diameter of a little under an arcescond. If you want to resolve details 1/10 arcsecond aparts (without deconvolving), you need something like a 50-inch telescope.


The exact same thing is happening with camera lenses. Points become disks (not exactly but who cares) and images blur. Longer focal length exaggerates the size of these blurs on the ccd and large aperture makes them smaller, so the size of the discs (in micrometers, say, on the ccd) is a function of focal length / aperture, or f number. The smaller the f number, the smaller the discs.


DLA has confused a lot of people because it makes it sound like the ccd is causing the diffraction. As Daniel has explained so thoroughly, this is not the case: diffraction has nothing to do with the ccd. What *is* true is that higher pixel densities let you *see* diffraction more easily. But arguing that a higher pixel density his bad because it makes DLA lower is like arguing that high pixel density is bad because it lets you see flaws in lenses more easily (though oddly enough, there are people who make this argument)

[Alan]

I will say it one last time...





I like cookies!
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Keith, I'm with you.....I like cookies, too.

[Daniel Browning]

This all has to do with the wave nature of light- roughly speaking, when a wave meets an obstacle, it spreads out. Light does the same thing.

Great post, Jon. Your description is correct. Sometimes people give the incorrect description, such as saying that diffraction is caused by light "bending" around the edge of the aperture. The only time that is true is when there is interaction with a strong gravitational field, such the lensing of a pulsar by a galaxy or the slight lensing of distant starlight by the Sun. Most lenses aren't quite that big (though the Canon 1200mm f/5.6 might seem like it is as big as the the Sun sometimes [].)


Wave interference is the clearest way to describe diffraction, like you did above. But there is another way that is a little more fun (although less instructive): the Heisenberg Uncertainty Principle (HUP). HUP says that the the more you know the position of a wave, the less you know about its direction of motion. A lens pins down the location of the wave at the aperture (narrower f-numbers is pinning it down to a more precise location), and that causes the direction of motion to become more uncertain: the light spreads out more and more. What's really neat is that diffraction still occurs even when there is no aperture! Interferometry systems such as phased array radar use the exact same diffraction formulas we do.

[Colin500]

The effect of "space between the buckets" is quantified through fill factor: the relative area of photoreceptive portion of the pixel: the photodiode. In CMOS, the rest of the pixel is taken up mostly by circuits. For a given design, the area required scales with semiconductor manufacturing process, which, as we know, scales with Moore's Law; which, in turn, is shrinking the non-photo-diode area faster than pixel sizes themselves are shrinking, so there has actually been a net gain in fill factor, quantum efficiency, and full well capacity for smaller pixels (at least down to 1.7 microns).

Hi, I joined this forum long after this discussion started ... and have read through most of your excellent explanations, which really helped me sort out some misconceptions, and confirm some intuitions (for example in some cases I accept what seems a noisy image because scaling down for a web site "removes" most noise).


However: If, at any given manufacturing process and any given sensor size, and assuming that the non-aperture-area per pixel has constant size, or at least constant (border) widths around the aperture-area, a sensor with less megapixels will have a higher fill-factor, and therefore collect more light?

[Daniel Browning]

However: If, at any given manufacturing process and any given sensor size, and assuming that the non-aperture-area per pixel has constant size, or at least constant (border) widths around the aperture-area, a sensor with less megapixels will have a higher fill-factor, and therefore collect more light?

Gapless (or nearly so) microlenses result in the same light collection from both. The more important factor is full well capacity, which would theoretically be better in the larger pixel. But in actual products, the FWC is better in the smaller pixels (e.g. LX3 vs 5D2); that may just be an imbalance in technology.

[Colin]

Diffraction is always there. It's always the same, no matter what the pixel size. When the f-number is wider than the DLA, it means that the image is blurred so much by large pixels, that it's impossible to see the diffraction blur. Smaller pixels simply allow you to see the diffraction blur that was always there.

I'm not sure if this has already been mentioned, as you guys have covered so much stuff that it's hard to keep track, but in case it's been missed, I would point out...


The 'limitations' of the higher pixel density in terms of DLA may seem more significant if you're looking at a 100% crop comparison. Since the resulting image is finer in detail to begin with, any blurring shown will look larger, even if it's the same blur in terms of the total image resolution.


also....


Smaller pixels, i.e., tighter sampling of the image, i.e., more resolution, actually allows smoother transitions, which can be seen as less sharp. If you can resolve the blur, it will look smooth. However, larger pixels that can't resolve the blur, will, in effect, sharpen these edges. A pixel has a structure that has nothing to do with the image. The sum of the light that was sampled by that pixel might have a certain average value, but the pixel set to that value is not truly the image that filled the pixel. All of the light is averaged, and set into a nice clean square. The edge of the pixel is not image detail, it is an artifact. When sampling a transition, larger pixels will have a more abrupt transition at the pixel edges (because there are less steps)... As such, they may appear sharper, superficially. From a nyquist sampling perspective, if you could, we 'SHOULD' filter out the edges of pixels and blend them together with surrounding pixels, because that 'edge detail' transition between the pixels is not based on image data, but is rather an artifact of the medium. The same image, with less pixels, either due to the sensor or resampling, may in fact look less noisy and sharper, but it's an artifact, not a virtue of fidelity.


Sorry if I was redundant.