Sigma/Foveon Questions
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From: Alfred Molon on 30 Apr 2010 08:32 In article <hrdtf0$i2g$1(a)news.eternal-september.org>, David J Taylor says... > The amount of correlation will depend on the lens MTF - what the > point-spread function is, and how the anti-alias filter spreads the image. > That's independent of scene content. In the example you cite, any > atmospheric effects over the period of the exposure time would also > contribute to blurring of distant flowers. > > When viewing a print with "distant flowers", the eye can't resolve the > colour detail in any case, if by distant you mean "covering a small number > of pixels". I don't think that anyone is saying that Bayer is the perfect > solution - it isn't - but it is one which works very well in practice. But the question is, what the performance of the sensor is. It may well be the case that the lens sucks and the AA filter additionally contributes to blur everything, but this is a separate issue. The key question is, can the sensor resolve pixel-level changes of colour. A Bayer sensor *cannot*, while a Foveon sensor or other full- colour imaging device which does not have to interpolate the data from adjacent pixels can. And by the way, with Bayer cameras you are forced to design the AA filter for the lower colour resolution of the sensor (1/2 of the sensor resolution) if you want to avoid colour aliasing. With a full-colour sensor you can instead design the AA filter for the full resolution of the sensor. -- Alfred Molon ------------------------------ Olympus E-series DSLRs and micro 4/3 forum at http://tech.groups.yahoo.com/group/MyOlympus/ http://myolympus.org/ photo sharing site
From: Alfred Molon on 30 Apr 2010 08:34 By the way, nospam, do you realise that a Bayer sensor has an additional issue: sensitivity. Each pixel only captures either red, green or blue photons, thereby throwing away 2/3 of the incoming light. A sensor capable of capturing and processing all photons has overall a much higher sensitivity. -- Alfred Molon ------------------------------ Olympus E-series DSLRs and micro 4/3 forum at http://tech.groups.yahoo.com/group/MyOlympus/ http://myolympus.org/ photo sharing site
From: nospam on 30 Apr 2010 09:18 In article <MPG.2644ec47875d2aba98c2c2(a)news.supernews.com>, Alfred Molon <alfred_molon(a)yahoo.com> wrote: > The key question is, can the sensor resolve pixel-level changes of > colour. A Bayer sensor *cannot*, while a Foveon sensor or other full- > colour imaging device which does not have to interpolate the data from > adjacent pixels can. no sensor can accurately resolve pixel level changes due to aliasing.
From: nospam on 30 Apr 2010 09:21 In article <MPG.2644ecd95ab0ed9798c2c3(a)news.supernews.com>, Alfred Molon <alfred_molon(a)yahoo.com> wrote: > By the way, nospam, do you realise that a Bayer sensor has an additional > issue: sensitivity. Each pixel only captures either red, green or blue > photons, thereby throwing away 2/3 of the incoming light. strange, how despite that, foveon has a roughly 3 stop noise penalty. > A sensor capable of capturing and processing all photons has overall a > much higher sensitivity. not necessarily. foveon is much worse. anything else is pure speculation.
From: Martin Brown on 30 Apr 2010 09:29
Alfred Molon wrote: > In article <290420101341351528%nospam(a)nospam.invalid>, nospam says... > >> there is no perhaps about it. the accuracy can be measured. delta-e >> (colour errors) for foveon is higher, or less accurate. >> >> or just look at the images. they're full of weird colour casts, >> including yellow skin, cyan skies, etc. and it's not consistent even >> among multiple foveon cameras. since there's a huge variability, it >> *can't* be more accurate. > > The big issue with Bayer is the effective resolution which is way lower > than the nominal one, because a Bayer sensor only captures 1/3 of the > needed information. It doesn't only capture 1/3 of the information. It is actually quite tricky to put an exact number on the amount of information that it does capture. But based on the 0.30, 0.59, 0.11 weights for R,G,B a Bayer quad captures (0.3+2*0.59+0.11)/4 = 0.40 or 40% of the information. Experience has shown that as long as the luminance signal has close to full resolution (and Bayer demosaicing ensures that) the lower resolution chroma information is invisible to the human eye except in a few special test cases. TV broadcast has exploited this for decades. >> so is bayer. 1/3 the data for an almost perfect reproduction. > > No - Bayer is not a data compression method. And reproduction is not > almost perfect. Actually it is after a fashion almost optimal. It measures the essential components needed to reconstruct a full colour representation of the image in an efficient manner. >>>> foveon is the only existing full colour sensor. >>> So? Full colour cameras exist, which is what matters. >> but not a full colour sensor. > > Wrong. Foveon is a full colour sensor. > > <snip> > >> many drawbacks for very little gain. > > 50% more effective resolution is a huge gain. Not when the sensors are so much smaller and seem to have nasty artefacts if there are specular highlights in the FOV. > >> interpolation is *not* guessing. > > It is, because the value in the middle does not depend on the > neighbouring values. It does depend very strongly on the adjacent pixels that is true unless the image is undersampled which is what the antialias filter prevents. > >>> which often are wrong. >> what is the error rate? exactly how often is it wrong? > > You can see the impact on the resolution: the Foveon 4.6MP performing as > well (resolution-wise) as an 7-8 MP Bayer sensor, according to the > reviews. Only on the resolution of colour detail. Bayer is not prefect but it is nowhere near as good as the Foveon marketeers would have you believe. You seem to swallow their all their PR bullshit hook, line and sinker. > >>> You cannot calculate the value of a random variable. >> it's not random. > > The colour of one pixel does not depend on the colour of neighbouring > pixels. And there are scenes with pixel level colour changes (meadow > with flowers at a distance is an example). But after it has been through the lens and formed an image on the sensor it is already limited by the MTF of the lens. It is theoretically possible that sub pixel colour detail that was resolved by the lens and lands on the wrong cell will be lost. > > In practice, real world scenes often have large areas of (almost) > homogeneous colour, which is why the performance of Bayer sensors is not > as abysmal as the lack of 2/3 of the needed information would imply. That isn't it at all. The lens produces an image which is continuous over the length scales of pixels. The adjacent pixels are a very good predictor of the value in the middle. > > Which is why the effective resolution of a Bayer sensors averages > somewhere arounc 60%-70% of the nominal, which is very good given that > just 1/3 of the needed data is captured. > > However there is the huge issue of colour aliasing and the need to use > AA filters tuned to 1/2 of the pixel count resolution, which is why > Bayer images are so soft at the pixel level. Bayer images are not soft at the pixel level. Certain manufacturers of P&S deliberately oversharpen their images because they know what sells. Same is true of TVs in shops with visible ringing on edge transitions and heavily posterised colours. That is apparently what customers want! You are flogging a dead horse with Foveon. It had its chance and did not make the grade. WHat it does is largely irrelevant unless you spend your life photographing colour test charts intended to prove how good it is. The human eye doesn't see fine colour details very well and if the luminance signal is good the perceived sharpness is dominated by that. Regards, Martin Brown |