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Thread: Please explain diffraction in lenses

  1. #71
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    Re: [digression!!] Please explain ... the relevance of MTF curves for the human visi

    [QUOTE=Emmanuel BIGLER;1242851If you include the human vision in 'imaging systems', then tell me what the relevant linear correspondence between input and output and we can continue the conversation [/QUOTE]

    For the purposes of perceived sharpness and detail, it's really not as complicated as you're making it out to be. The basics are pretty basic. And with some clever homegrown photoshop experiments you can demonstrate them to yourself and to anyone.

    Our visual cortex determines sharpness and "image quality" almost exclusively with contrast in the range of 1 lp/mm to 5 lp/mm. Anything higher frequency than that range is essentially inconsequential. And anything lower frequency than that range is practically irrelevant, since any visual system that does an adequate job at the high frequencies will do a more-than-adequate job at the low ones.

    There are now piles of research showing this. And I've demonstrated it by incorporating these lessons into sharpening routines. I've shown many people inkjet prints that look more like contact prints to them than actual contact prints from the same negative.

    I agree with everything you're saying about audio. Human psychoacoustics seems like a much less mature and much more mysterious field than human vision.

    Although one caveat: I don't get your point with the beat notes of the organ pipes. That's a very simple, completely linear phenomenon. If you add two sine waves that are slightly off from each other, you'll see aliasing in the form of a non-signal, low frequency wave. Both it and visual moiré patterns are simple, easily calculable phenomena.

    The examples of visual and auditory illusions play more to neurophysiological quirks, and both more interesting and less easily quantifiable with simple physics.

  2. #72

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    Re: Please explain diffraction in lenses

    I don't get your point with the beat notes of the organ pipes. That's a very simple, completely linear phenomenon

    Paul, the problem, from an academic point of view, is exceedingly simple.
    Let's listen to two sinusoidal sound waves at a slightly different frequency.
    Now consider that out ears+brains consist only in a linear detector.
    And propose a model where the difference in frequencies could appear in this 100% linear detection scheme.
    The difference in frequencies comes out immediately if the detector performs some non linear operations with respect to the amplitude of sound vibrations. But a purely linear detector, by no means can yield different Fourier frequencies in output.

    Aliasing comes out from a sampling detector. To the best of my knowledge, our hears are purely analogue.

    And regarding human vision, I agree 100% that a certain range of relatively low spatial frequencies contribute to what is perceived as sharp or not. And I agree that Fourier/ linear post-processing techniques can yield very interesting improvements in image sharpness.
    But this does not prove that our vision system can be modelled only with a purely linear scheme.

    Regarding complexity in modeling human vision, you probably know all examples of classical visual illusions as well as E. Land's "Retinex" models explaining the perception of color and contrast. I stand corrected: this is not so simple.
    Are you sure that all secret pre-processing algorithms in DSRLs are 100% linear and spatially invariant?
    The two mathematical conditions of linearity and spatial invariance are required, otherwise MTF models are not valid.

  3. #73

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    Re: Please explain diffraction in lenses

    And to elaborate on Emmanuel's OT remarks about organs and churches, I've heard that it wasn't uncommon for smaller/poorer churches to build organs with two short pipes which sounded like one 16 foot (or maybe 32 foot) pipe. I play Tuba and in fact there is almost no energy at the pedal tones, but people are good about perceiving the low note based on energies at the higher harmonics. I think there's something similar about the acoustics of bells.

  4. #74
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    Re: Please explain diffraction in lenses

    Quote Originally Posted by Emmanuel BIGLER View Post
    But this does not prove that our vision system can be modelled only with a purely linear scheme.
    I'm not trying to prove that our visual systems are purely linear (I'd have to see this defined to even have a hunch one way or another). But what I've seen, in lots of research and in personal experience, is that MTF models subjective impressions very reliably. If you know how to read the charts. And while I'm sure MTF models our vision imperfectly (what model is perfect?), I have yet to see discrepancies that were important.

    On the other hand, there are gigantic discrepancies between traditional models of image quality (extinction resolution, etc.) and subjective impressions. This is why MTF is such a breath of fresh air: it's the first model to come along that actually corresponds with what things look like.

  5. #75
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    Re: Please explain diffraction in lenses

    Quote Originally Posted by Emmanuel BIGLER View Post
    Aliasing comes out from a sampling detector. To the best of my knowledge, our hears are purely analogue.
    It would have more accurate to equate musical beating with moiré patterns. These are analogue phenomena and are easily modeled. There is no psychoacoustic component; the beating would register the same to any audio instrument that has adequate frequency response.

    Some of the other phenomena you and Jim mention, like phantom fundamental frequencies, are psychoacoustic in nature. They are quirks of the listening organism. But these are not directly analogous to the visual phenomena we're talking about. I tend to think hearing is more mysterious than seeing. The book hasn't even been written yet on how to measure loudness.

  6. #76

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    Re: Please explain diffraction in lenses

    Quote Originally Posted by Jim Andrada View Post
    And to elaborate on Emmanuel's OT remarks about organs and churches, I've heard that it wasn't uncommon for smaller/poorer churches to build organs with two short pipes which sounded like one 16 foot (or maybe 32 foot) pipe.
    This is correct. The lowest octave of a 32' rank of a flue stop is sometimes simulated (to varying degrees of success) using open fifths an octave higher in a 16' rank, and this is often done where either space or cost (or both) are prohibitive. Playing a 16' C with G above it, adds a "virtual" 32' C. The stop name is usually something like "Resultant" or "Resultant Bass". Because our ears are fairly lousy at differentiating frequencies that low without higher harmonics, this usually works ok, although the relative volume of the fifth above the fundamental, and whatever else is going on at the same time, determine the extent to which we hear the fifth. Ideally, the fifth is played quieter than the fundamental, for example. The lowest notes in a resultant 32' octave might use principal 16' pipes for the fundamental and a softer rank for the fifths. You can do this on a piano. Play a loud bottom C, then add a softer G a fifth higher, and you'll get an additional C an octave lower (16 hz).

    Sorry this was way off topic.

  7. #77

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    Re: Please explain diffraction in lenses

    Apologies to the moderators if we are going off-topic but I really appreciate the comments regarding low-frequency organ pipes.
    Actualy I know the effect only due to my reading in the last century of a classical French textbook on the subject:
    Émile Leipp , "Acoustique et musique" - isbn 978-2225801969, Masson Ed. (1984)
    I have found no English edition of this book, unfortunately. The text was written a long time before heavy digital processing and modelling took place in music and acoustics, but I believe that some principles are still valid today.

    ------------------------

    Regarding moiré patterns for two overlayed gratings observed in transmission mode, since the transmission factor of the overlay is the product of the transmission factors (the optical density is the sum of both optical densities), all cross-products of the elementary Fourier components of both individual gratings (I mean: Fourier components of the transmission factors, not Fourier components of the optical densities) will show up in the product of transmission factors. The effect is of course richer than a beatnote between two sounds, since we operate in a 2D space.


    These are analogue phenomena and are easily modeled.

    Hey, Paul (just pulling your leg again), so easy that I'm still waiting to see how a purely linear sound detector can create the cross-product, hence the sum & difference of frequencies between two purely sinusoidal signals linearly summed up

  8. #78
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    Re: Please explain diffraction in lenses

    Emmanuel, I may not even understand your question. If you're talking about something besides out-of-tune pipes creating a lower "beating" frequency between them, then I get it. And that's something that any sound detector will hear. If you're talking about something else, then I don't know what you're talking about. But I'm betting it doesn't have anything to to with the relationship between MTF curves and subjective sharpness!

    Off-topic or not, I'm fascinated by audio and optics, and psychoacoustics and visual perception (psycho-optics doesn't seem to be a real word, for better or worse). I love it when parallels between the two fields are useful. And I see a lot of examples where the weirdnesses of hearing and the weirdnesses of seeing are too different to compare meaningfully. It's important to check the analogies for relevance.

    Before trying to quantify the shortcomings of MTF in modeling sharpness, it would make sense to see if they exist, and to what degree. I'd be very interested to see an example of MTF failing miserably in predicting subjective impressions. I haven't yet.

  9. #79

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    Re: Please explain diffraction in lenses

    Interesting discussion.
    It is understandable that difference components are generated in a non linear transfer function, eg the ear or a diode modulator/detector.
    We treat the em propagation as linear (I mean media properties regarded as invariant with intensity as in a vacuum or a glass lens etc)
    But what about sound in air?
    Are there artifacts by the compresssion of air iself in sound progagation?
    Would a perfectly linear microphone detect difference frequencies and their harmonics when placed close to a loud organ?
    I suspect so.
    I did a search and so far found nothing about sound wave distortion by gas laws etc.

  10. #80
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    Re: Please explain diffraction in lenses

    But what about sound in air?
    Doppler effect - regardless of medium

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