[pics] SD10 vs. 20d sample

Started Feb 21, 2005 | Discussions thread
PaulyOly Veteran Member • Posts: 7,723
No need to argue which is correct.......

Helge Hafting wrote:

Peter G wrote:

PaulyOly wrote:

processing settings. Something I find strange is the difference in
the white sheet, the strands or texture looks larger in the sigma
shot, is this from upsampling?

No it is from Aliasing. Lack of an AA filter. It is the same
phenomena that produces 5 lines from 9 on the res chart in the

Correct, but I wouldn't call it "lacking" for not having an AA
filter. Bayer cameras don't have AA filters to blur out details,
they have the AA filter for avoiding unsightly color aliasing. You
will always get something wrong when taking pictures of details too
small for the sensor. The "smudge it out" approach of an AA filter
is no more "correct" than the aliasing we get without.

but this test doesn't show an advantage of no AA filter judging by the sigma shot.

Either way, we get an image without the details the camera couldn't
capture, and then the interesting question is "what looks best".
For the cloth, the aliasing still looked like cloth and was
therefore a good thing. The AA-filtered image simply looks
unsharp. Of course one can find cases where blur look better than
aliasing too, but those are rare. Most users of AA cameras fight
the filter with unsharp masking.

As the details get smaller the actual ouput gets bigger.
This gives the impression that it is show more detail when in fact
it is obscuring the real detail and giving a bigger false pattern.

The false pattern is indeed false, but it does rarely obscure a
real pattern you cannot capture anyway. The false blurring of an AA
filter is often the bigger problem.

Sometimes people with a background in audio shows off formulas that
"prove" why an AA filter of a certain strength is a good thing - or
even necessary. But they work from a wrong assumption, the
assumption that any false signal is very bad. This assumption
holds for audio, where any false detail really kills the music. But
the assumption rarely holds for vision, except in a few
pathological cases where a chessboard pattern positioned "just so"
turns into a set of broad stripes. Patterns a bit less regular
(such as gravel, dirt and foliage) tends to alias into slightly
coarser but similar patterns that are much more pleasing to the eye
than the "mathematically correct" area of almost uniform color.
Areas of uniform color is particularly bad when sharpening is
applied later - you get a sharp line between the sky and the
distant forest, but at such sharpness the eye really expect some
approximation of distant trees and branches, instead of solid green.

As for the resolution chart, the sigma is clearly not showing a
"true" image when it merges nine lines into seven and then into
five. But at least it shows a set of black and white lines, where
AA-equipped cameras shows a single column of gray. The sigma photo
carries more information - "there is lines" where the AA photo
merely says "don't know - too high resolution for me". There was
no gray in the original resolution chart, so the gray column is

Helge Hafting

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