Re: Any thoughts /news on Foveon sensors?

Edmund <nomail@xxxxxxxxxxx> writes:

>I have to admit I don't know what that Nyquist frequency means in
>pictures. I do know the about Shannon/Nyquist in digital audio.

It's almost the same. In digital audio, the independent variable is
time, the sampling rate is in samples/second, and the Nyquist limit is
the point at which the signal frequency is half the sample frequency.
In other words, you need more than two samples per cycle to resolve
a signal without aliasing.

In imaging, the independent variable is space, and the signal is 2D
instead of 1D. So the sample rate is in samples per mm or samples per
inch, and the Nyquist limit is the point at which the signal frequency
is half the sample frequency. In other words, the wavelength is twice
the pixel spacing at the Nyquist limit, or you need more than two pixels
per cycle to resolve a signal without aliasing.


>As we expected identical results to the SD9, and with good reason considering
>the design of the sensor and the fact that the pixel count hasn't changed.
>That's to a lack of a color filter the X3 sensor doesn't suffer from color
>moire at the limits of resolution.

The X3 sensor doesn't suffer from colour moire on B&W images, and that is
a real advantage. But it *does* suffer from B&W moire on fine B&W patterns
because it lacks an anti-aliasing filter, and that's just incorrect.

>Because the X3 sensor doesn't employ an anti-alias (low pass / blur) filter it
>continues to deliver detail past Nyquist. Taking the vertical resolution bar as
>an example we can count all nine lines up to our 'absolute resolution'
>measurement of around 1550 LPH, after this point lines begin to merge and by
>2000 LPH we can count five obviously combined lines. In a real image this could
>be the detail of leaves on a distant tree or bricks on a distant wall. At the
>time of my SD9 review there was much debate over the 'validity' of this extra
>detail. My opinion on this matter is that this detail is at least of
>photographic merit, it is the correct color and represents detail that the
>human eye although not able to distinguish perfectly would also see as a broken
>texture, it's certainly better than the blurred area we would get from the
>anti-alias filter of a Bayer sensor camera.

That's a matter of taste. I, at least, would prefer that a camera
render detail too fine to resolve as the average colour, not as some
different detail that the camera would have been able to resolve had the
image contained that. I never want 9 pickets in my picket fence imaged
as 5 boards - ever. The eye blurs detail it can't resolve, and so does
film. The aliased images from the Sigma camera just look unnatural to
me.

>I can understand the frequencies relative to the pixel spacing part,
>but how about the bayer interpolation that has to guess the color of
>the pixels?
>Like here http://www.foveon.com/article.php?a=70

Calling it "estimating" would be more accurate than "guessing". But
Bayer colour estimation works remarkably well in practice for most
subjects on a modern camera; it requires some effort to find images
where it will fail.

>I just wonder if there is a theoretical advantage, the whole
>concept make sense to me. Maybe it needs to be refined a bit.

There is a theoretical advantage, if the 3 layers of the sensor have
similar colour response and sensitivity to the filtered cells in a Bayer
sensor. And in fact some video cameras use 3 CCDs and a prism colour
separation block, and Foveon themselves once sold such a camera. This
design is expensive but works very well. The Foveon X3 is an attempt to
get the same advantages from a single flat sensor - but so far it has
not worked very well.

Dave
.

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