Physics and digital camera design limits
Physics and camera design limits
Looking at how fundamental aspects of physics could limit some aspects of camera design
Some update notes on our original article about physical design limits
Some of the Physical limits in Digital photography
We’ve just published a lengthy new article on the site. This short introduction dates from when we didn’t have comment features on articles, so its comments are here.
- Original article link: Physical Limits in Digital Photography
- Updated article – partly based on the comments and the Author’s response here. Extended article: Physical digital photography limits
How does the physics of light limit camera design?
This article represents a bit of a sea change for our site, in that it’s by a guest author – David Goldstein from San Francisco.
We’re looking at expanding the range of photographic / technical articles and there is a limit to how many I can write and run Northlight Images’ photography business – or as I sometimes call it the ‘real work’.
Have a read of David’s article (the extended PDF version is worth making the effort to get through the bit of maths, even if it’s not your strong point) and do feel free to add a comment here with your opinions.
Update 30th June 2009
David has been following the comments on the article, and will be writing an updated version of the full PDF article addressing some of the comments below.
In the meantime he’s said:
“I want to thank all of the commenters for their thoughtful and thought-provoking technical responses. I have analyzed them at some length, and will provide a revised version of the paper in response. This revision will most likely still reach the same numerical conclusions but will be clearer about both the conditions and caveats that underlie them.
My efforts to develop responses to the comments led me to the conclusion that there are some significant additional factors that should be taken into account in explaining the basis of the broad results in the article. The most important clarification is that the limits imposed by diffraction as I have specified them are not the hard limits, but rather refer to the onset of visible reductions in image quality of ordinary photographs. These problems get worse as apertures decrease beyond the thresholds I identify, but diffraction still allows resolution or contrast data to be gained as the sensor increases from ~24 megapixels potentially to the ~200 megapixel limit suggested by John Green. (However, the last factor-of-two [100 to 200 megapixels] makes at best a very small difference unless we are looking at a signal that is a very deep green and beyond the color gamut of sRGB and Adobe RGB).
The conclusion on the practical limit to megapixels will likely be revised to say that above the stated levels we will see rapidly declining returns to higher megapixel counts rather than allowing the reader to infer that more megapixels are useless.
The primary calculational basis for this conclusion is the case where we are looking at a signal in which two points are separated by two pixels—the Nyquist frequency—and we want the first minima of the two Airy disks to coincide. This case (which is already in the paper) will be presented first to clarify that it is the most interesting or relevant one. It is most interesting because its predictions are corroborated by real pictures, both the ones displayed in the article and other ones that I have taken casually or as tests. It also agrees with observations other photographers have published on line (for example, and numerous lamentations among camera reviewers and bloggers about the megapixel race in pocket cameras).
I will add clarification on the issue of noise. Noise is important not only to prevent grainy appearance but because it eliminates visual information from the picture that cannot be recovered with accuracy by post-processing. This clarification embraces both noise generated by photon statistics and noise generated by point subjects being rendered as reduced-contrast blurs.
I will add more detail and examples related to color. Color is a complex issue, because it is not the same thing as wavelength of light. All pure wavelengths are a mix of at least two primary colors, and all but one, or perhaps three (depending on which version of the chromaticity diagram you use) particular wavelengths are a mix of all three. Obviously any mixture will therefore contain all three colors. Since at the level of diffraction limits, the color of pixels changes from one to each of its neighbours, this consideration now seems more important than I (or the commenters, or anyone else I have read) thought.
This observation, which is missing from my current draft, appears to lead to some very interesting and complex interactions between the Bayer filter, the anti-aliasing filter, and the sensor’s resolution limit and artefacting problems. These do not seem to have been discussed quantitatively before now, so it will take me a little time to analyze them. It seems that I will essentially be reverse-engineering an anti-aliasing filter. And of course I will welcome comments on the derivation when I present it: since it appears to be new information (or a rediscovery of information not in the public domain), the potential for error is higher than usual. The preliminary result is that a strong anti-aliasing filter will create severe artefacting for strongly colored (in the sense of the dominant color coordinate being on the order of .7) subjects, while a weak one will usually avoid this problem while also mitigating the color-insensitive aliasing. This result would imply that the AA filter must allow some information content at above-Nyquist frequencies to pass through.
Once again, thanks to everyone for the comments. I hope they will lead us all to new insights.”
BTW If there’s anyone reading who feels like writing an article, then do let me know what you’re thinking of and we can see how it fits in with what’s already on the site (see the writing an article page for a bit more info) This site had over 5 million visitors last year, so your article will certainly get noticed – We’ll sort out web layout etc. You just need to supply text and some images. Of course, all submitted content remains copyright the author.
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