FF sensors: is 80MP needed?

[A complimentary Cc of this posting was sent to
acl
], who wrote in article :
OK. If by impulse you mean the filter we need to apply to the fourier
transform of the signal (*), then I agree: if it's positive everywhere,
no ringing.

Actually, the question is two-headed; so there must be two answers,
not one.[*]

But the point is that this filter must be of infinite extent
(or extend? Hmmm...) in frequency space to avoid overshooting: it cannot
vanish above some frequency.

This was already shown to be false: Cesaro filtering gives no
overshooting.

I mean, for example: Take a delta-fn in
position space, FT it; you get a constant. Then, multiply this constant
by a "filter" exp(-k^2/D^2), and FT back; you'll get something
proportional to exp(-D^2 x^2), ie, no ringing, like you said.

So far, correct.

But the moment you truncate the filter (ie make it vanish above some
frequency), you'll get ringing (in real space), no matter how high
the maximum frequency included. Try it.

I do not think this is true - but cannot be ABSOLUTELY sure right
now. Anyway, we are not discussing "Gauss multiplied by a cut-off
step function", but a more general case of a function with finite support.

All of the above applies also to edges (as opposed to points)
[*] Here comes the crucial distinction when one consider
"point-sources" of light (e.g., subpixel speckles), and edges
(e.g., disk-like sources of light). Positive PSF will not get
ringing near edges, or any overshoots. However, if the PSF has
"ringed" structure (consider one of diffraction), there will be
rings about point-like sources.

In pictorial photography, point-like sources are so rare that
the ringing about them does not matter (much).

Yours,
Ilya

[A complimentary Cc of this posting was sent to
acl
], who wrote in article .com:
I expect it would be very education to listen to the opinion of
^^al

somebody who "understands probability" (I presume you meant yourselves
there) on what is deviation(A+B) when B = -A... Please go ahead.

No, I misunderstood. I did not realise you meant that there is
....

Who are "ourselves", though? I'm just one.

Consider them just a figure of speech. ;-)

Ilya