# Canon 10D's anti-aliasing filter question, and AA filters in general

Discussion in 'Digital Photography' started by William Wallace, Jan 15, 2004.

1. ### William WallaceGuest

I know about Nyquist's with regard to time varying signals. The
sampling theorem applies to PAM, and assumes infinite precision zero
error analog samples at a specified jitter free frequency. The
sampling theorem is useful PCM, because quantization noise is well
understood.

One real-world constraint with anti-aliasing filters is that it is
difficult to design filters that approach the ideal brick wall
approach. This is solved a number of ways, e.g., over sampling and
using an AA filter with a cut-off frequency less than 1/2 the sample
rate, say 1/4 or 1/8 the sample rate, and a step slope, say 60

But with regard to imaging, I would like to better understand AA
filters.

Do they set the cut-off frequency at 1/2 the sampling frequency?
E.g., at 2 pixels? How is this done? Is it a brick wall? Does it
operate in two directions (horizontal and vertical) or does it also
work diagonally? Are they optical or digital?

And getting to the Canon 10D, I have noticed that it does not capture
my son's eye lashes too well...sometimes, it just erases them. Is
this due to a poor AA filter design? Bad luck (e.g., maybe all of the
eyelashes are pointed straight at the camera)? A feature of the 10D
sensor? Strangely, a single fine hair laying across my wife’s
face is captured? Is the 10D using an algorithm to not filter hair of
a certain length? (A neat trick). Maybe my wife's hair is just
thicker than my son's eye lashes.

William Wallace, Jan 15, 2004

2. ### andrew29Guest

Optical. It's usaually done by stacking multiple layers of a
birefringent crystal such as Lithium Niobate.

A birefringent material splits a light beam into two parallel
components, depending on polarization. The distance between these
components is less than or equal to the spacing of the pixels. Stack
two of these crystals at 90 degree angles and each beam is split into
four components. This is quite a precise technique, and not just a
simple blur.
Well, that depends on how close the size of the hair is to the

Andrew.

andrew29, Jan 15, 2004

3. ### KenPGuest

As a different approach, the 14n uses no physical anti-aliasing
filter, simply allowing the blur circle of the lens image (such as it
is) to accomplish the anti-aliasing. This permits higher possible
resolution, but in the case of exceptionally sharp lenses and distant
picket fences (for example) the aliasing can become evident in the
image (if not on the print). Perhaps as image sensors become even
more dense, the need for anti-aliasing blur filters will disappear
altogether.

KenP, Jan 15, 2004
4. ### Stephen H. WestinGuest

Nope. Just as in audio, such a thing doesn't exist. The Fourier
duality still applies. So every AA filter is a compromise. Kodak used
to supply a removable filter for their DCS cameras; this was
apparently more aggressive than filters built in to other cameras, but
could be removed for ultimate sharpness, at the risk of aliasing. As
another poster pointed out, Kodak's most recent camera does without
any filter, but is susceptible to aliasing, e.g. on tweed jackets and
the like.
Optical; as you know, once aliasing has been introduced in sampling,
no digital processing can remove it.

Fortunately, however, the aliasing is most objectionable in the color
components (at least for non-Foveon cameras), and with some insight
into human perception and the 3-color sampling process, as well as
perceptual limitations, can often be masked or filtered out by
software.
Is it that a bright object against a dark background is more visible
than the opposite?

Stephen H. Westin, Jan 15, 2004
5. ### Don StaufferGuest

I don't see how that produces higher res than a specially designed sharp
cutoff filter. Most lens aberrations have a very gradual slope near
cutoff (slope to mtf), so are not a sharp cutoff. If the normal lens
aberrations are doing the filtering, I am surprised that this is sharper
than something designed from scratch to produce a sharper cutoff.

I must also admit I have never tried lens design with birefringent
materials, so I have no idea how hard it is to produce a sharp cutoff
that way.

Don Stauffer, Jan 16, 2004
6. ### andrew29Guest

I'm not sure this qualifies as a different approach. It's a cost
saving measure that happily ensures better performance on resolution
tests. Ideally, you want high contrast right up to the Nyquist limit,
and then a sharp cutoff. If lenses really did all the necessary
antialiasing they'd lose a lot of contrast below the Nyquist limit,
and such lenses are rightly criticized as soft. But I'm not a lens
designer; maybe lenses with the right characteristics can be made.
Perhaps, although we've a way to go yet. Besides, in many areas of
photography people are looking forward to the day when high
performance digital systems can fully replace medium- and large-format
film.

Andrew.

andrew29, Jan 20, 2004