Page 2 of 3 FirstFirst 123 LastLast
Results 11 to 20 of 25

Thread: 8 bit linear value conversion to Log Density? FILM

  1. #11
    8x10, 5x7, 4x5, et al Leigh's Avatar
    Join Date
    Dec 2010
    Location
    Maryland, USA
    Posts
    5,454

    Re: 8 bit linear value conversion to Log Density? FILM

    Quote Originally Posted by ic-racer View Post
    Quote Originally Posted by onnect17 View Post
    The 255 is the "zero" density.
    There is no a priori relation between the two. That is an arbitrary assignment.
    Assigning the highest intensity impinging on the sensor (i.e. zero density) to the 255 A/D output value is normal engineering practice.

    I would expect that to be adjustable in most designs.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  2. #12
    ic-racer's Avatar
    Join Date
    Feb 2007
    Posts
    6,736

    Re: 8 bit linear value conversion to Log Density? FILM

    Quote Originally Posted by Leigh View Post

    The other 255 values are fixed ratios of 2 from that assigned value.
    They could be that or they could be any number of things that one might want them to be. There is not physical truth binding optical density to any particular numerical sequence.

  3. #13
    8x10, 5x7, 4x5, et al Leigh's Avatar
    Join Date
    Dec 2010
    Location
    Maryland, USA
    Posts
    5,454

    Re: 8 bit linear value conversion to Log Density? FILM

    Quote Originally Posted by ic-racer View Post
    Quote Originally Posted by Leigh View Post
    The other 255 values are fixed ratios of 2 from that assigned value.
    They could be that or they could be any number of things that one might want them to be.
    There is not physical truth binding optical density to any particular numerical sequence.
    It appears you don't understand how an analog-to-digital (A/D) converter works.

    Each value is related to adjacent values by a factor of two, exactly (at least theoretically).
    That's the definition of a linear conversion function.

    Devices that produce unique outputs when the inputs are varied by a factor of 2 are called "monotonic".
    That's a desirable feature in commercial A/D converters, and is emphasized in their advertisements.

    You can play with the design of the circuitry between the sensor and the A/D converter if you want some other conversion function, like logarithmic. But that's external to the A/D.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  4. #14
    ic-racer's Avatar
    Join Date
    Feb 2007
    Posts
    6,736

    Re: 8 bit linear value conversion to Log Density? FILM

    Quote Originally Posted by Leigh View Post
    Assigning the highest intensity impinging on the sensor (i.e. zero density) to the 255 A/D output value is normal engineering practice.

    I would expect that to be adjustable in most designs.

    - Leigh
    I'm not sure what you mean by that. What SENSOR?

  5. #15
    8x10, 5x7, 4x5, et al Leigh's Avatar
    Join Date
    Dec 2010
    Location
    Maryland, USA
    Posts
    5,454

    Re: 8 bit linear value conversion to Log Density? FILM

    Quote Originally Posted by ic-racer View Post
    I'm not sure what you mean by that. What SENSOR?
    The light coming through the film hits a photo detector of some sort. That's the sensor.

    Your questions suggest that you know little or nothing about the design of electronic equipment.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  6. #16

    Join Date
    Dec 2001
    Location
    Chapel Hill NC
    Posts
    319

    Re: 8 bit linear value conversion to Log Density? FILM

    I have a section in my digital negative monograph posted on my website where I discuss the digital zone system, and have a figure showing zones, %K and corresponding pigment values. This may be what you are after.

    Regards,
    Mike

  7. #17

    Join Date
    Dec 2009
    Location
    Pacifica, CA
    Posts
    1,710

    Re: 8 bit linear value conversion to Log Density? FILM

    Mike,

    How have you fared reading the densities of a negative using a scanner and how well did it correlate to the same negative read on a densitometer?

    The problem I see is most scanners work to fit the range of the subject to nearly the entire positive scale 0-255, so a thin negative and a contrasty negative both give full-scale positive on-screen when the levels are adjusted. It's as if you had printed the thin neg on Grade 4 and the contrasty neg on Grade 2. But you know the densities of the two negatives aren't the same so there isn't a direct relationship from the 0-255 to a negative density from 0.00-3.00.

    So to correlate a negative density to scanner result, you can scan a known grayscale and a test negative with the same parameters and compare different spots on both... There are probably other ways, and possibly some systems can directly provide a density reading.

    But I don't see how you can directly relate 0-255 with 0.00 to 3.00 (or any other arbitrary density range) without some kind of calibrating.

  8. #18
    8x10, 5x7, 4x5, et al Leigh's Avatar
    Join Date
    Dec 2010
    Location
    Maryland, USA
    Posts
    5,454

    Re: 8 bit linear value conversion to Log Density? FILM

    Quote Originally Posted by Bill Burk View Post
    The problem I see is most scanners work to fit the range of the subject to nearly the entire positive scale 0-255, so a thin negative and a contrasty negative both give full-scale positive on-screen when the levels are adjusted. It's as if you had printed the thin neg on Grade 4 and the contrasty neg on Grade 2. But you know the densities of the two negatives aren't the same so there isn't a direct relationship from the 0-255 to a negative density from 0.00-3.00.
    Bill,

    You're assuming that the scanner is going to do scaling, which is a post-scanning step.

    The (few) scanners that I've worked with operate in raw mode, with the software doing any post-scan scaling.

    In raw mode you certainly can correlate a specific level to a specific density.
    The density range depends on the hardware, and will will vary from one device to another.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  9. #19

    Join Date
    Jun 2014
    Posts
    4

    Re: 8 bit linear value conversion to Log Density? FILM

    Quote Originally Posted by Leigh View Post
    Bill,

    You're assuming that the scanner is going to do scaling, which is a post-scanning step.

    The (few) scanners that I've worked with operate in raw mode, with the software doing any post-scan scaling.

    In raw mode you certainly can correlate a specific level to a specific density.
    The density range depends on the hardware, and will will vary from one device to another.

    - Leigh
    Thanks for all the ideas and opinions. Yes, one must first develop a standard in how the film is "always" scanned, and that alone will take some calibration using a step wedge, etc. For this to work with any level of accuracy, the scanner must to set NOT to scale the dynamic range of the negative automatically. That would throw the entire process of this off. My scanner is calibrated manually, using manual settings of black point, white point, etc. Using the Zone system, which I do, a given "Zone" from negative to negative should produce similar results and density on the film. Of course film testing and development testing must be done prior to this. For example, the 8 bit value measured, of say 120 will always result in the same "Zone", and consequently, the same approximate film density, if it were measured on the densitometer. Regardless if you way overexposed or underexposed the film, there will be density, and they will fall into a certain zone, regardless if you intended them to or not! This idea assumes all other variables are accounted for, proper exposure, proper development, and proper scanner fixed calibration for all negatives, as it should be. My settings, would probably not work for someone elses negatives, as some people interpret zones slightly different. Using the aforemented math, it works for me just fine now. If your scanner can not be set to raw or manual control (or the software), this will not work!

  10. #20

    Join Date
    Dec 2009
    Location
    Pacifica, CA
    Posts
    1,710

    Re: 8 bit linear value conversion to Log Density? FILM

    Great! I'm fortunate to use a real densitometer, and so I can verify (calibrate) what the scanner returns vs densities. Vue-Scan includes a densitometer tool that I occasionally experiment with but haven't calibrated yet.

    Of course once you start watching your densities, you will start to see differences between what you expected and what you got.

    Errors in exposure computation could cause all your Zones to fall on different densities than you expected (with no adverse impact on the "print-ability"). And flare totally plays havoc with where shadow Zone densities fall based on where you placed them (hopefully it improves them anyway).

Similar Threads

  1. Jobo edge density film problem
    By John Powers in forum Darkroom: Film, Processing & Printing
    Replies: 18
    Last Post: 19-Apr-2014, 12:34
  2. Scanner Optical Density vs X-Ray Film Optical Density
    By NancyRoberts in forum Digital Hardware
    Replies: 4
    Last Post: 21-Sep-2011, 15:29
  3. PMK - Film Density Discrepancy?
    By William D. Lester in forum Darkroom: Film, Processing & Printing
    Replies: 2
    Last Post: 15-Aug-2007, 05:04
  4. Measuring Film Density
    By ageorge in forum Darkroom: Film, Processing & Printing
    Replies: 16
    Last Post: 8-Dec-2006, 08:22
  5. Calumet 6 x 12 roll film back - linear fogging
    By Malcolm Sharp in forum Cameras & Camera Accessories
    Replies: 6
    Last Post: 16-Sep-2005, 04:32

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •