Digital Sensitometry for Black and White ~ Setting a baseline

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

 In the prior post I shared...

"What I want is a measurable, repeatable, _controllable_, and clearly understandable framework for black and white digital image processing. Something that reveals what Sony's in-camera jpg processor is doing. Something that, in a wonderful world, could help me choose a set of operations and settings that match my RAW output.."

Where to start?

The best place, I believe, is to set a reference point for the B&W imaging model.  There are plenty of frameworks of understanding, certainly.  One simply has to choose one and to stay long enough to see if it'll work or not, and to then evaluate the results.  Using what I know as a path forward, I'm trying to transfer what knowledge I have of the old film-based Zone System into the digital imaging realm.

For this blog entry I synthetically built two Zone System charts.  No cameras were harmed in their creation.  They both illustrate Zone 0 thru Zone 10, pure black to pure white.  

One chart was built assuming whole EV value steps represent a Zone change.  In this case Zone 5 would be EV0 and Zone 6 would be EV 1, and so on.  Using RawTherapee, I took Zone 5 as #76(hex)/118(dec) as the reference point, and raised/lowered the EV value in the exposure panel.

Here it is:

Look at the bottom row
where 0EV is set to Zone 5

In the second chart was built based on the fact the above EV chart tops out at Zone 9, not Zone 10, as intended.  For this second chart I choose -1EV, and using standard curve, carefully raised the -1EV value until it read 76(hex)/118(dec).  The resultant curve was saved and applied to every EV value from -5 through to +5.

Here it is:

Look at the top row
where -1EV is set to Zone 5

Comments -

It should be clear that if I want to realize a digital equivalent to the old film-based Zone System that simply relying on whole f-stop exposure values (EV) centered on 0EV is utterly and completely insufficient.  Digital systems top out at +4EV/Zone 9 where that value is completely saturated and contains zero useful information.  That's a whole f-stop short of the Zone 10 definition.

This has serious implications for how digital systems process black and white images.  There are hints, too, of how carefully highlights would need to be treated in color, too.

The fact of +4EV/Zone 9 saturating led me to drop down the tonal scale to work from -1EV as a reference point.  What I needed to do was raise the -1EV value to the Zone 5 standard of #76(hex)/118(dec) middle gray.  Using a standard curve from the Curves function, I could then save and use that curve to measure other EV. 

What I see is that every EV/Zone now contains information where we would expect them to.  Further, the relationship between the EV/Zones seems more or less appropriate.  There is just a hint of color at Zones 1 and 9, exactly as the Zone System describes.

In the next post I will have a look at a real world Sony RAW implementation of these two approaches (0EV as Zone 5 vs -1EV as Zone 5).

Note: What I'm sharing in this series is limited to my experience with Sony.  In truth I have zero idea how other digital systems map tones.  But if anyone would like to stop by and let me grab a few photos off their non-Sony systems, I'll buy you a beer.   

---------- Where am I on my checklist of perceived "needs"? ---------------

Is what I'm doing here in trying to transfer old film-based Zone System knowledge into something practicable for digital...

• Measurable - Yes
• Repeatable - Yes
• Controllable - Remains to be seen
• Clearly understandable - I believe so, if not, please let me know
• Reveals what Sony's in-camera jpg processor is doing - Not yet 
• Helps me choose a set of in-camera jpg engine operations and/or settings that match my RAW output - Remains to be seen