Re: Pentax K10D beats (sharpness, detail) Canon 40D?

David J Taylor <david-taylor@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Wolfgang Weisselberg wrote:
David J Taylor
<david-taylor@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Floyd L. Davidson wrote:

But a 43.1 KHz tone is extremely easy to filter out.

Of course, I was trying to make the numbers easier, and to
demonstrate how something could alias right down into the wanted
band.

But a similar image frequency is far harder to filter out.

As I already mentioned, in audio oversampling is typically used to
ameliorate these issues, but as far as I know, oversampling has not
yet been applied to DSLRs.

Oversampling in audio, unless I misunderstand, would be sampling
with a much higher frequency than the target 44.1kHz frequency.

In DSLRs that would mean using 4 (2x oversampling) or 16 (4x
oversampling) pixels where there is now one. For a 6MP resolution
you'd need 24 million pixels at just 2x oversampling in width
and height. For physical reasons so many pixels need large (and
expensive) sensors or must deal with small full well sizes and
photon noise.

On the other hand, the new sRAWs may be downsampled from ordinary
RAWs, and thus count as oversampling + downsampling. Of course
you can get the same effect by resizing your photo intelligently
from 8MPix to, say 2MPix (2x) or 0.9MPix (3x)

-Wolfgang

Wolfgang,

Yes, the oversampling allows the first (analog) filter to be simpler, and
the subsequent filtering to be done digitally. Final samples are
delivered at 44.1KHz (or higher in studio work).

If oversampling were used for digital cameras, I don't think that the
photon noise and dynamic range would be significantly worse, as the same
area of silicon is used per output pixel. In your 2Mpix or 0.9Mpix
analogy, it would mean that you could use an anti-aliasing filter with an
equivalently strong cut-off (i.e. less sharp 8Mpix images).

There remains one problem with digital photography though, as someone
mentioned recently, that the very simple anti-aliasing filter used does
not have a sharp cut-off just before the Nyquist frequency, it has a more
gentle slope, meaning that there will be some compromise between what some
call "resolution" and the damaging effects of aliasing. Different cameras
will behave differently, and different users may prefer different results.

That was the point I was trying to get at in my original
question. Many DSLR users already accept that different images will
respond differently to different levels of sharpening, and that for
best effect sharpening should be applied at whatever level of
resolution the final print or screen image is to be viewed at, which
may not be the same as the original camera image resolution.

Hence most cameras now come with menu-settable changes in the degree
of sharpening applied to their jpgs, and to what level of image
resolution. Since the extra power and sophistication of large computer
processors compared to those in the camera mean that image editors can
employ more sophisticated sharpening than the cameras, many DSLR users
now prefer to do the minimum in camera and most of it later in their
own preferred image editor.

What I don't understand is why the same kind of flexibility is not
applied to AA filtering. I can't see any technological barriers to
it. Given the current state of camera technology it would only be of
benefit to those using expensive high quality kit, but the fact that
some camera makers (such as Leica, Sony, & Fuji, IIRC) are already in
their top models edging in the direction of less optical AA filtering
in order to exploit more of the native resolution in their technology
it seems to me that camera technology has now got to the point where
this has become relevant.

--
Chris Malcolm cam@xxxxxxxxxxxxxxxxxxxx DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]

.

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