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Ivon Visalli

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  1. Ivon Visalli
    Hi David, It's tough to answer why, but I have a thought exercise that might help. If I shoot a test chart that has tiles numbered 1 through 8 from white to black that cover 7 stops of exposure and I have a camera with a sensor that can see all 7 stops, but I encode it with 2 bits (four values: 00, 01, 10 and 11), the camera data will come over as 00, 00, 01, 01, 10, 10, 11 and 11 OR white, white, gray, gray, dark gray, dark gray, black, black. The dynamic range captured by this camera is 7 stops. It saw all 7 stops of EV, but the image would not be considered faithfully rendered. Bit depth limits how well an image can be rendered, but it does not limit the dynamic range of the captured image. I think some people would say the image produced is 3 stops, therefore the bit depth has limited us to 3 stops. But 3 stops of camera DR would mean it saw tile 3 as white and tile 6 as black or white, white, white, gray, dark gray, black, black, black. I'm making some assumptions about the position of middle gray and the exposure settings, but I think you get the idea. I also apologize to the bit heads that know that generally 00 is black and 11 is white, I've reversed that for this example. Edit from my original post: 8 tiles would actually be 7 stops of DR. I have corrected the example.
  2. Ivon Visalli
    Yep, it's a good point. I should not have implied that the ADC is applying the log function (or any mapping). The ADC is likely just doing a linear mapping and the log function is being applied in processing in the digital domain. I just wanted to make two points in this thread: 1) bit depth does not equal dynamic range and 2) the log function of a camera is not as a result of converting values to binary, it's an actual mathematical translation and 3) log becomes necessary as DR increases and bit depth remains constant (or grows only a little) if you want a nice image. Okay -- that's three points. I was never very good at math, I'm always counting like "1, 1.4, 2, 2.8, 4...." :-)
  3. Ivon Visalli
    Carl, I don't seem to be able to get the quote function so I'm going to just address the point that the relationship between photons and electrons is linear, just as you say, but I don't think your assertion that the count between electrons and bits is a non-linear equation. It isn't. It's perfectly linear. You are correct that binary is a log based representation, but that log only occurs when I type 1011 to represent 11 which represents eleven ticks on the prison wall. The log function of a camera changes the spacing between lower counts of electrons and higher counts of electrons. Both linear and non-linear functions are represented in binary form in digital storage, but the linear spacing is remapped using a function. So assuming my ADC counts 1 electron as 1... if I didn't use a log function, 1 electron would be 00000001 and 127 electrons would be 01111111. Using a log function 1 electron is represented as 00000001 and 127 electrons is represented as 10011001 (the number 153). The same binary system is used to store both linear and log, but the spacing is compressed as you move up in value in log. If we had all the bits in the world to work with, it would work just fine to keep the linear relationship between electrons and storage, but real world limitations -- like the bandwidth of cables and the write speed of disks -- create better images by mimicking how out eye/brain works, which is a non-linear relationship. It's not the binary that's encoding the non-linear relationship it's an actual mathematical translation that's doing it. That's what the Alexa curves that Tom provided yesterday show.
  4. Ivon Visalli
    Hi Carl, Forgive me for butting into the conversation you are having with Phil, but isn't a digital film sensor a photon counter? From my point of view, that's the basic premise of this thread... that this linear photon counter needs log applied to properly mimic how our eyes operate. A camera's sensor responds linearly to the light striking it (equally proportional to the photons striking it). So, the value output by a sensor does increase the same amount going from 1 to 2 photons, as it does going from 1000 to 1001 photons. I hope we all understand that this is an example, I doubt any sensor can actually perceive 1 photon or the difference of 1 photon. In the days when the dynamic range of sensors was limited, just reporting that proportion was good enough. But, as DR has increased and the number of bits used to report the light values has only grown by a few bits, it's become more important to apply a log function for images to "look right". Is that your understanding or am I missing your point to Phil?
  5. Ivon Visalli
    The problem, as someone has already stated, is the initial assumption that bit-depth determines the dynamic range. It does not. Bit depth determines the precision or granularity of the light values being recorded, not their overall range. The dynamic range is determined by properties of the photosites of the sensor. As the OP stated, the ADC quantizes the voltage output of those photosites into a digital value. Here's an example. Let's say you have two cameras, Camera 1 with four stops of dynamic range and Camera 2 with ten stops. The lower dynamic range of Camera 1 means at a certain point, it can no longer distinguish between darker and darker values. Everything goes to black. On the other end, lighter and lighter values above a certain value go to white. Assuming both cameras have photosites that output a voltage range between 0 and 1 volt, and both use an 8-bit ADC, I've attached a table that shows what values from both cameras might look like. These are made up numbers, just to illustrate the point. Log curves in cameras are used to extend the dynamic range, not to account for ADC conversions but because the eye is less sensitive to light changes with brighter values than darker. Therefore cameras don't need to record changes in light values as accurately in the brighter areas as they do in the darker.
  6. Ivon Visalli
    Thank you, Kyryll.
  7. Ivon Visalli
    "Stranger than Paradise" is a film I use all the time to contrast technical prowess and the art of filmmaking. The film has negative scratches and is not well exposed in places... Jarmusch "crosses the line" (supposedly intentionally)... and yet it's a wonderful film to watch. I enjoy technical discussions, but it's interesting how most of the cinematography forums contain threads on technology and gadgetry and not so much about how to employ the technology to tell stories -- engaging stories. How much lens sharpness, resolution, color science, dynamic range went into "Stranger than Paradise" (yes, allowing it's b&w film not color digital -- and short ends to boot). Here's to Mr. Jarmusch.
  8. Ivon Visalli
    Miguel, thank you so much for the response. If you get the chance, I'd love to see a photograph of your cover, especially detail around where you are attaching the viewfinder and matte box.
  9. Ivon Visalli
    I'd like to know your thoughts on rain coats for cameras. Do most of you keep some sort of rain cover in your kit? Do you find yourself shooting exteriors in the rain often or is it a rare thing? What do you think is important to look for in a rain cover? Thank you.
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