Digital Zone System (Part One) ~ Concise Guide to Step Wedge simulation

[Updated 7 October, 2024 ~ Important correction: Zone 5 at 0EV is 76hex/118decimal, not 7F/127 as I previously wrote.  The step wedge illustration has been updated as well]

There are no secrets to any of this, but I realize I moved rather quickly through a lot of material in my rush to understanding digital sensitometery and developing a decent black and white Digital Zone System solution.  Working in three parts, I will now try to show how anyone can do what I've done and make sure they, too, can achieve accurate results.  

This, part one, presents a recipe for generating a synthesized step-wedge.  Part two will present a process for understanding camera/software interactions which will lead directly to part three where I present a simple process for generating an accurate Digital Zone System tone curve tailored to cameras and image processing software.

Definitions ~

• Working toward a solution for black and white imaging (I may deal with color in the future)
  
• Using Luminance curves only (other curves will distort the color space which can impact the final result)
• Zone 5 shall be 76(hexidecimal)/118(decimal) or "middle gray"
  

Assumptions ~

• Digital Zone System Zones 0 through 8 are separated by 1EV per Zone
  
• "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 Synthetic Step-Wedge ~

Using an image processing software ->

• Create an image filled with 76(hexidecimal)/118(decimal) tone value
• Increase Exposure 1EV
• Note the new tone value
• repeat this step until you've found pure white - FF(hexidecimal)/256(decimal)
• Restart this process using the image filled with 76(hexidecimal)/118(decimal) tone value
• Decrease Exposure 1EV
• Note the new tone value
• repeat this step until you've found pure black - 00(hexidecimal)/0(decimal)

Carefully noting the values from -7EV through +4EV, as in the following.

 

Zone 5 as 0EV and -1EV
set to
#76/118decimal luminance value

Noting the decimal equivalent value in the event the image processing software uses that system (such as in RawTherapee), where the Gimp reports values in hexadecimal.

• 02(hexadecimal)/02(decimal) -7EV
• 04(hexadecimal)/04(decimal) -6EV 
• 08(hexadecimal)/08(decimal) -5EV 
• 11(hexadecimal)/17(decimal) -4EV 
• 1B(hexadecimal)/27(decimal) -3EV 
• 28(hexadecimal)/40(decimal) -2EV 
• 3A(hexadecimal)/58(decimal) -1EV 
• 76(hexadecimal)/118(decimal) -0EV 
• AD(hexadecimal)/173(decimal) +1EV 
• CB(hexadecimal)/203(decimal) +2EV 
• F5(hexadecimal)/245(decimal) +3EV 
• FF(hexadecimal)/256(decimal) +4EV

 

Zone 5 as 0EV and -1EV
set to
#76/118decimal luminance value

We will work with these values in part three.  

For the moment, note the difference where +4EV is pure white in digital work, and remembering +5EV is pure white in film work.  We will also use this knowledge in part three.  

Also note that exact 1EV tonal separation exceeds the original film Zone System definition by providing potentially usable information down through -7EV, or Zone -2.  This will "work" if the camera/sensor/software systems allow.