Toothwalker wrote:
I have to disagree. You don\'t prove that the crispness (detail) is due to aliasing. The detail is due to perfectly rendered signal components up to the Nyquist frequency.
The fall-off of the anti-aliasing filter, that starts well below the Nyquist frequency, prevents this in other cameras. You don\'t like aliasing. You like the benefit of having no filter for the rendering of detail and accept the drawback of aliasing. (Visible in the blinds as well as via color artifacts in several other places.)
As I said, it doesn\'t really matter which way it is - if I object to anti aliasing or if I like aliasing. It ultimately comes down to the same thing. AA as digital signal processing isn\'t much better than the optical variety. If you truly want to eliminate aliasing you\'ll have to ruin the image first by excessive low pass filtering. And if you don\'t sharpening will restore it (there\'s a question of degree of course, but it doesn\'t detract from the principal argument). I\'ve shown as much in my previous post.
However, I\'m not willing to say that I don\'t want aliasing. I still maintain that more information is preserved even when a signal is subsampled at a frequency below the Nyquist limit.
Consider for instance the function t*sin(t)
An aliased (sampling frequency below the Nyquist limit) reconstruction will still reflect the increase in amplitude rather than the flat 0 an anti-aliased solution would present. You\'d preserve both the information of periodicity (not the period, but that it is periodic) and the increase in amplitude. That\'s real information.
Now in a real world image things are more complex where you have a blend of multiple spatial frequencies. And in effect you\'d be blending the poorly reconstructed signal with a properly reconstructed one. So which is worse, sabotaging the good one or throwing away information from the poor one? I don\'t know and I have so far not heard any convincing arguments either way.
Since the pixel density is high the transfer function of the lens already acts as an anti-alias filter and the artifacts are not too overwhelming. I don\'t think that you would like a sensor with a low pixel density and without AA filter.
Ken brought up this point as well, but I think you underestimate the lenses in question. For instance I used a Zeiss 25/2.8 ZM in a previous example which is diffraction limited at f/4 and resolves up to 400 lp/mm center frame. It is way beyond the resolving power of any 35mm sensor. The crop above was shot with a lens that is around 90% MTF at 40 lp/mm, so you can rest assured that it outresolves the sensor by a large margin. So I very much doubt there is any AA effect from the lens.
This however gave me an idea of a rather simple experiment to perform. One could point a high resolving lens on an AA-less camera at a subject where you know moire patterns will be evident. Then by very slight steps you move focus and for each move take a shot. At some point the moire pattern will disappear as the misfocused lens will act as an AA filter. Unlike the camera based AA filters this will be a very high quality AA filter. At that point where the moire disappears one should check how much detail has been lost (if any) in other regions where there was no apparent aliasing. If tuned precisely the lens should at one point cut off at the nyquist limit and in theory there should be no loss of detail according to the \"aliasing is bad\" theory. Correct?
Aug 16, 2011 at 06:20 PM
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