squaring the circle of confusion

Discussion in 'General Technical Discussion' started by Iliah, Oct 5, 2005.

  1. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    In the heat of FF vs. DX debates the fact that it is about digital sometimes is forgotten. Statements like "smaller sensor needs more magnification" sound very confusing.

    But there is yet another aspect of the problem - demosaicing. Here is a typical example of how DoF on film is explained: "Imagine the very tip of a pin with a size of exactly zero, located precisely in the plane of perfect sharpness, that means, it is imaged to the film with a size of exactly zero, not widened by any blur. Now, move this pin toward the camera and watch the diameter of its tip increase by blurring. When it has reached 30 μm, halt the pin! It is now right at the inner border of the depth of field. Now, do the same in the opposite direction. Beyond the plane of perfect sharpness you will reach the outer border of the depth of field." (from Zeiss newsletter CLN #1, p.3, article "Depth of Field – An Insider’s Look Behind The Scenes")

    Film has very fine grain, but often grains are overlapping, forming nearly continuous sensitive surface. Not so with digital sensors. Moreover, there are substantial gaps between cells, and 5mkm cell can have only 3mkm sensitive surface. Demosaicing interpolate from the cell to all neighboring cells, making luminance and colour data to distribute to at least 8 cells (3x3 square). That means our imaginary 5mkm sensor cell allows to record data in 15 μm linear. (Digression: to complicate things even more, influence of data from a cell is attenuated in demosaicing depending on the distance, and as a result we have blur. The larger are the cells, the more blur we have. This is one of the reasons why smaller pixel count on a smaller sensor can provide images sharper then bigger sensor with greater pixel count, even with AA filters being taken out.)

    Now, if we try to apply the explanation given above to the digital sensor... Let's assume the tip of a needle is lucky to be projected right into the center of a pixel. When we start moving it, the tip immediately jumps to effective size of the pixel, and thus is demosaiced in 8 surrounding pixels, forming 3x3 square - linear size of 15 μm. Next jump will be to the size of 3 pixels, and demosaicing will extend it to 25 μm. If the tip is located not at the center of a pixel cell, but between cells, its first race will be 10 μm, and it will be blurred by demosaicing into 20 μm.

    If no pixel-level sharpening is appied to the image in post-processing, and we want our CoC in the resulting image to be 20 μm (30 μm/1.5, that is 4 pixels of the sensor), for taking the image we need CoC of 10 μm.

    If we apply sophisticated demosaicing, which restores luminance information in each pixel using multi-pass approach, or even simple pixel-level USM (400..500/0.2..0.3/0..1), we are getting pretty much to cell size of CoC.
     
  2. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  3. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Paul,

    I do not remember 14/2.8 behaving any better on my Kodak SLR/n, which has no AA filter and no micro-lenses.

    AA filters on Nikon cameras introduce blur smaller then 1.3 pixels, and RML demosaicing reaches Nyquist frequency (independent data) of Nikon sensors calculated without AA filter.
     
  4. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  5. cwilt

    cwilt

    Apr 24, 2005
    Denver, CO
    Iliah,

    I have no idea where you are headed with this, but you have my attention. :smile:
     
  6. JeffKohn

    JeffKohn

    Apr 21, 2005
    Houston, TX
    How does diffraction fit into all this? I thought I had read that once you hit the point where the "airy discs" from diffraction were larger than your CoC you were essentially spinning your wheels. So what aperture are we going to have to limit ourselves to in order to really get benefit from a CoC of 10 μm?
     
  7. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  8. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Guys, may I suggest a small landscape-taking tripod-working session (optionally using different dSLRs :)). Get a lens with DoF markings, set your aperture to f/8, set f/8 DoF mark to infinity, shoot; set f/5.6 mark to infinity, shoot; set f/4 mark to infinity, shoot; set f/2.8 mark to infinity, shoot. Compare the sharpness at the distant objects. I bet you will see that calculator-predicted DoF is not quite right, and you will see some interesting consequences in image appeal other then just DoF.
     
  9. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Paul,
    "None of the materials I looked at could adequately model the results I'm emperically observing (with the D2x)." - right, and what are they? BTW, seems you consider them to differ from some other camera, am I right? :)
     
  10. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  11. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  12. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Paul,

    The answer for me is CoC of about pixel size. I tried to explain why in my initial post.
     
  13. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Jeff,

    D2X is diffraction limited at f/11, so it seems to me and some others.
     
  14. Does that mean that shooting larger than f/11 only increases DOF?

    Good thread by the way.
     
  15. OK, so what happens in a macro situation when you go beyond F11, say even F22 or so, mainly thinking on the D2X situation?

     
  16. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  17. Here is a good article about the same subject: http://www.rmimaging.com/information/depth_of_field.pdf

    I'm thinking about putting Excel to work to work out similar tables for my lenses and the D2X.

    Most important learning for me in this article is that we tend to be overcritical in our observations of 100% pixel size pictures on a monitor, when our eyes in rea life on real life print sizes would not be able to see the difference...

    Everyday I realize how little I know about Photography :) looks like I will be busy learning for the rest of my life
     
  18. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  19. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Thank you, Paul - I missed that Thom's post, actually. To add to what he is saying, nowadays Zeiss as a company repudiated original reasoning somewhat.
     
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