Digital Zone System (Part Two) ~ Concise Guide to System Characterization

[Updated 7 October, 2024 ~ Important correction: Zone 5 at 0EV is 76hex/118decimal, not 7F/127 as I previously wrote.]

None of this Zone System stuff is magic.  Hopefully what I write about is easily understandable, easily duplicated and quickly adapted by interested parties. 

In this, Part Two, I will present a process for understanding camera/software interactions at the very front end of the RAW image processing pipeline.  It sets the foundation for something that leads directly to Part Three. There, I will present a simple process for generating an accurate Digital Zone System tone curve tailored to systems interactions between camera sensors and software.

Definitions ~

• Using Luminance curves only (other curves will distort the color space which can impact the final result)
• Zone 5 remains 76(hexidecimal)/118(decimal) or "18% middle gray"
• Digital Zone System Zones 0 through 8 are separated by 1EV (1 f-stop for us Old Farts) per Zone

Assumptions ~

• The old film-based Zone System is adaptable to digital B&W imaging
  
• The vast majority of digital B&W output looks different from film because of highlight compression
• "Camera Profile" normally tries to adjust 0EV to 76(hexidecimal)/118(decimal) on all three channels (RGB) or something thereabouts, depending on the "look."

Building a Sensor/Software Step-Wedge ~

To understand how B&W tones are distributed at the start point for image processing, here is a method that I've found instructive.

Image creation ~

• Photographing a continuous tone scene (ie: blank wall, or blank sheet of paper)
• Setting the focal point well before or well behind the subject (to even out any textural/tonal variations of the blank subject)
  
• Shooting RAW
• Set ISO to lowest non-synthetic value (some camera systems allow for low and/or high ISO settings that "massage" RAW information, which we want to avoid for this procedure)
  
• Set the meter to "Spot" (to avoid problems of uneven field illumination, ie: lens vignetting)
  
  NOTE: I've found it easiest to set a camera on a tripod for this work to introduce as few lighting variables as possible
  
  
• Make an exposure at 0EV
• Increase exposure 1EV
• Make an exposure
  
• repeat this step to +4EV
  
• Make an exposure at -1EV
• Decrease Exposure by -1EV
• Make an exposure
  
• repeat this step down to -7EV or -8EV
  

Image processing ~

• Load RAW files into image processing software
• Convert to B&W (I use Luminance B&W conversion for its human perception properties)
  
• For each image...
  
• Mouse over the center of the image (where the Spot meter read the scene) 
• Note the Luminance value
  

After carefully noting the values from -7EV/-8EV through +4EV, you should have something that looks like the following.

In the example above I used a Sony A6300, Sigma 24mm f/3.5 DG DN focused to infinity, and photographed a blank white board.  It's easy to see that 0EV is _not_ 76(hexidecimal)/118(decimal).  That, in fact, is one of the roles of "Camera Profiles" when it applies a Tone Curve.

I'm exerting control over tonal values as early as possible in the process.  RawTherapee allows users to re-define the "Camera Profile" step.  In Part Three I will share how I define a new "Camera Profile" tailored to Digital Zone System work. I'm not aware of any other image processing software that allows for this kind of early intervention and I'll try to remember to describe the advantages of this in a future post.

As far as I understand the current State of Things, all other software use pre-defined "Camera Profiles" to get a RAW file to the point of image processing as quickly as possible.  What I present here in Part Two should be usable in all cases.  If what you see more closely matches the Digital Zone System synthetic step-wedge "out of the box", you're likely "good to go", within certain limits, such as accepting Zone 9 as pure white and associated highlight attenuation.

An interesting aside: The old Film Zone System described an 11EV or 11 f-stops of dynamic range.  From the above we can see that in Sony APS-C and before any further intervention that I have 12EV or 12 f-stops of dynamic range.

All this aside, the important point of this Part Two exercise is to understand how close we are to the old Film Zone System 1EV per Zone step ideal.