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.

Digital Sensitometry for Black and White ~ Automation ~ RAW and In-Camera jpg image processing

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

It's taken more than a few blog posts to get here.  At long last I come to the  question I had that launched this little digital sensitométrie adventure.

Original question: Is it possible to generate a decent black and white image in-camera?

Using the Zone System as a framework of understanding, from here on out the answer to this question should be quick and relatively easy.   So I'll dispense with my usual background/setup hoo-haa and get down to it.

RAW automation ~ 

Adobe Standard camera profile

I have to admit that manually controlling RawTherapee can be a bit scary.  There are so many options, so many tools, and so many operations to choose from.  Which is why it might be useful to look at an automation in the form of a camera profile to see how far down the Zone System road I can get on RAW file import.

Using the baseline Zone 5 as 0EV luminance reference, I took a look at Adobe Camera Standard .dcp. In RawTherapee I enabled Tone Curve, Base Table, and Look and looked at just the luminance channel.

Advantages -

• Easy to load a .dcp file into RawTherapee or Photoshop (where I'm sure it comes with the system as well as in Adobe's ACR tool)
• Pre-defines a usable tone curve
• Uses 0EV as Zone 5 so no "unusual" exposure compensation normally required
• Tames +1EV to +4EV light area transitions 
• Expresses more than 11EV dynamic range
  
• Input correction curves easily constructed to Zone System references
• Working from RAW allows greater flexibility in extreme image processing
  

Disadvantages -

• Black and white conversion required
• -0.3EV underexposure might help meet the idealized step wedge reference (see how the Adobe Standard curve is to the left of the reference)
  
• Zone 9 at +4EV remains the pure white saturation point (not Zone 10, +5EV as described by the original Zone System)
  

In-camera automation ~ using the jpg engine ~ 

Sony Creative Style Light

Flying by the seat of my pants and using years of experience printing black and white images I found that Sony's in-camera jpgs generated using Creative Style Light looked pretty good when I downloaded images and simply desaturated them off-camera.  

So it didn't surprise me when I measured the luminance curves using settings of Contrast -2, and Saturation -1 that the idealized Zone System step wedge is matched by desaturated Creative Style Light.

Advantages -

• Exactly matches idealized Zone System step wedge
  
• Expresses 11EV dynamic range
• Uses 0EV as Zone 5 so no "unusual" exposure compensation normally required
• RAW+jpg function available 
• Could be a good solution for posting converted black and white images when outside of Big Computer range (ie: while on the road) where I could have color (if I like Light colors) and monochrome jpgs
  

Disadvantages -

• Black and white conversion required
• Zone 9 at +4EV remains the pure white saturation point (not Zone 10, +5EV as described by the original Zone System)

Sony Picture Profile ~ Cine 4, Black and White

Sony camera features have evolved to include configurable video tone and color controls. The tool can be used for stills photography as well.

I found that Gamma Cine 4 raises the midtones. Setting Color Mode to Black and White performs a full black and white conversion in-camera.

Advantages -

• Delivers in-camera generated jpg that addresses the original question
• Configurable for a variety of parameters that can match idealized Zone System step wedge where users can configure ->
  
• Knee to define highlight roll-off characteristics
• Black Gamma to define the point of pure black
• Color Depth to build in-camera B&W filters thru RGBCMY channel controls - which I find to be an excellent feature
  
• Detail to control image sharpness
  
• Expresses 11EV dynamic range
• Uses 0EV as Zone 5 so no "unusual" exposure compensation normally required, though...
  
• RAW+jpg function available  
• Could be a good solution for posting black and white images when outside of Big Computer range (ie: while on the road) 

Disadvantages -

• Limited legacy camera support ~ Picture Profiles are available on certain Sony cameras beginning with A6300, A7S, A7II, and A7RII
• In practice, underexposure of -0.5EV or -0.3EV might be useful (see how the curve is to the left of the idealized step wedge)
  
• Camera exposure bracketing functions are not available
• Shooting while camera in Silent Mode not available
  
• Best used configured before a shoot - menu diving to change settings might be annoying during actual photography
  
• Tones in Pure White (which might relate to a video standard)
• Zone 9 at +4EV remains the pure white saturation point (not Zone 10, +5EV as described by the original Zone System)

Creative Style Black and White, contrast = 0

Turning to Sony's Creative Style Black and White, I first looked at how this style behaves at its default settings.

Advantages -

• Delivers in-camera generated jpg that addresses the original question
• Uses 0EV as Zone 5 so no "unusual" exposure compensation normally required
• Exposure bracketing functions available 
• RAW+jpg function available 
  

Disadvantages -

• Zone 5 to Zone 0 are darker than the idealized step wedge standard (which is here I saw problems in my own work)
  
• Expresses 10EV dynamic range
• Zone 9 at +4EV remains the pure white saturation point (not Zone 10, +5EV as described by the original Zone System)

Creative Style Black and White, contrast =-3

On a lark I thought I'd try Sony's Creative Style Black and White with the contrast turned down to the minimum -3.  Surprise! This simple setting seems to do the trick.  The dark regions under Zone 5 shift to the left to move up the exposure scale which correctly "opens up" the shadows.

Advantages -

• Delivers in-camera generated jpg that addresses the original question
• Expresses 11EV dynamic range 
• Uses 0EV as Zone 5 so no "unusual" exposure compensation normally required
• Exposure bracketing functions available 
• RAW+jpg function available
• Could be a good solution for posting black and white images when outside of Big Computer range (ie: while on the road) 
  

Disadvantages -

• Zone 9 at +4EV remains the pure white saturation point (not Zone 10, +5EV as described by the original Zone System)

I'm no longer flying by the seat of my pants when it comes to black and white photography and I believe I've adequately answered the original question.

--------- 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
• Accurate - Yes
  
• Repeatable - Yes
• Controllable - Yes
• Clearly understandable - Yes, but readers can judge otherwise if they like
  
• Reveals what Sony's in-camera jpg processor is doing - Yes
  
• Helps me choose a set of in-camera jpg engine operations and/or settings that match my RAW output - Yes, with certain perhaps narrow/small limitations
  
• Huge Bonus: I now have a linear Zone System digital match to the old idealized film-based Zone System for both RAW and jpg files.
  

Note: What I'm sharing in this series is limited to my experience with Sony full frame and APS-C mirrorless cameras and the sensors they come with.  In truth I have zero idea how other digital systems map tones, though I'm developing my own suspicions.  I hope the descriptions of what I'm doing here helps interested parties sort this out for their situations.  If not, let me know and I might lend a hand.

Digital Sensitometry for Black and White ~ RAW processing ~ Details and Observations

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

This series of blog entries are a result of someone on Flickr pointing out that my in-camera processed BW wasn't as good as RAW work processed on a Big Computer.

In processing RAW images I've had a good feel for what makes a decent image.  Raise the mid-tones and use the Lightness slider to control the overall distribution of tones.  What I'm about here is something ever so slightly different.  Greater understanding.  Greater control.

This started off as an exercise to better understand the difference between in-camera and off-camera results.  For this I needed a framework of understanding to work within.  There are several valid ways to approach this and they involve measuring things.

I chose the Zone System because I can measure and see things in a format I am familiar with.  Importantly, I can accurately measure luminance using Open Source Software image processing software that I regularly use (ie: daily) and am familiar with (RawTherapee, the Gimp).

To this point in trying to get my arms around what's going on I've gotten somewhat side-tracked into the details of how Sony RAW files are loaded into RawTherapee and the settings/functions which express the widest possible dynamic range.  I'll try and wrap up this series soon by looking at in-camera jpg processing.  I promise.

Before I move on, I'd like to share a few potentially useful details.

Perhaps it's already understood, but I'd like to stress that these efforts are simply a preamble to making beautiful images.  There is nothing magic, though it might at first feel scary to take full control of the image processing sequence.

Process Details ~

- RawTherapee is the most flexible software for processing RAW images that I've thus far found.  It allows us to take as much control of image processing as we can handle. Here are the first two details I used in making these studies.

• Demosaic algorithms: RAW -> Demosaicing ->
  
• AMaZe (for low ISO)
• LMMSE (for high ISO)
• Camera profile: Color -> Color Management -> Input Profile -> 
• No Profile ... or...
  
• Auto-Matched Camera Profile with the following _not_ selected
• Tone Curve
• Base
• Look

- For one-click image processing setup after image import into RawTherapee here is the full recipe I use

• Exposure -> Tone Curve 1 -> Luminance Curves -> Input Correction Curve 
• from exposure stepwedge Correction Curves choose one of the following...
  
• Linear Zone System
• Film Emulation
  
• Color -> Black and White -> Luminance Equalizer
• Color -> Color Management -> Input Profile -> No Profile (my current preference)
  
• Transform -> Profiled Lens Correction -> Automatically Selected
• Transform -> Correct -> 
• Geometric Distortion
• Chromatic Aberration
• RAW -> Demosaicing -> select one of the following
  
• AMaZe (for low ISO)
• LMMSE (for high ISO)
• RAW -> Capture Sharpening (for low ISO)
• Save Current Profile with an appropriate name, for later use.  I've built as many of these recipes as I feel I need.  For instance, there is a Profile for low ISO film emulation, another for low ISO linear Zone System, and so on.
  

- Looking under Exposure -> Tone Curve I see there are actually two Curves I can use.  For what I'm illustrating here I put the Input Correction Curve in Tone Curve 1.  Then I use Tone Curve 2 to control the end points of the tonal range.  Tone Curve 2 can also be used to move Zones from one EV to another and to manage image contrast.

- Concerning Camera Profiles, the two mentioned here, No Profile and Auto-Matched Camera Profile, are both useful starting points.  There is no data conditioning when using No Profile.  I see there is a little data conditioning with Auto-Matched Camera Profile. This gently changes the tonal values.  What I want is as linear, as consistent a starting point for image processing as possible.  That way when I make my measurements which ultimately are used to create the Input Correction Curve I can save the proper values in the saved Current Profile recipe.

- When I shot film I used a Pentax 1degree Spot Meter.  I could accurately measure scene values and place them on the Zone System tone curve where I wanted them.  The meter has long since been sold.

Many cameras these days come with a Spot Meter function built-in.  The circle of metering is commonly larger than with the old stand-alone Pentax, but I don't find it all that important.  If I were wedded to the old process, I'd likely look for another Spot Meter.  But...

- In-camera digital metering systems are very sophisticated compared to light meters I used in the film days.  The Sony metering system allows me to see when highlights are clipped and when shadows drop out.  In general I don't use this feature.  There is plenty of dynamic range for the situations I find myself photographing in.  

The metering system does a good job distributing tonal values with information in each EV step.  Further, dropping the Zone 5 EV to -1 does an excellent job of protecting the highlight regions.

This is why I've gotten lazy in the traditional sense.  I would rather take a photo than stand there metering everything to perfection.  This is why I set the EV and let the camera make the metering decisions and call it good enough.  If I'm really in doubt, I bracket the exposure.

- Which leads to a comment about why I feel comfortable using -1EV as the Zone 5 middle gray, and not 0EV.  Remembering that using 0EV on my Sony cameras crowds the light tones, and remembering that -1EV more evenly spreads the tones between light and dark across the linear-ized Zone System, there was a concern about shadow noise.

In the early days of digital I owned a number of Canon DSLRs.  They _all_ suffered from noisy shadows, even when Zone 5 was shot at EV0.  Sony, on the other hand, showed me what is possible with their now 10 year old A6000 APS-C.  Sony is of course known to build "quiet" sensors.  That is to say the dark regions contain less noise than sensors from other manufacturers.  As manufacturers improve their design and manufacturing processes this might change.

- In any event, if shadow noise is too great, RawTherapee has a local Selective Editing local adjustment where dark tone noise can be managed.

- Once an image is selected and opened using one of the film emulation or linear Zone System curves, the true fun begins: interpreting the image.  

I commonly use standard image processing tools ->

• Exposure -> Lightness, which respects the curve end-points
• Exposure -> Contrast
• Exposure -> Tone Equalizer with sliders controlling...
• Blacks
• Shadows
• Mid-Tones
• Highlights
• White 
• Exposure -> Vignette Filter -> Strength: -0.30 to -0.90 (to taste)
   

Observations ~

- In this exercise I've built two different styles of Input Correction Curves, one for Film Emulation and another for correct to the Zone System definition linear Zone System curve.

The Film Emulation Input Correction Curve was built to illustrate how RAW files could be processed to match film.  Looking at the old film luminance curves I know what the Output Curve from digital RAW processing should look like and make it so.  Such is the flexibility of digital image processing.

As I've noted elsewhere, the original Zone System definition specified 1EV steps from Zone 0 to Zone 10.  The problem with film were the toe and highlight regions.  The toe rolled on and the highlights rolled off.  Some film photographers did a lot of image processing manipulations to ensure their images put valued information on the linear portion of the curve.  This commonly narrowed dynamic range and the Zone tones were spread over a shorter exposure range.

In digital work I've found the 1EV per Zone challenge much easier to manage.  Using a synthesized step wedge where tonal values exactly match the 1EV per Zone definition, I'm able to build fully correct Input Correction Curves and retain 11EV to 12EV dynamic range.

- The reason I select Color -> Black and White -> Luminance Equalizer in black and white conversion is to get as much tonal separation as possible.  Since the Zone System strives for as much tonal separation as possible, I feel that luminance adds just a bit more of a good thing. This is an area of improvement digital provides over what silver halide film delivers (which is mainly simple de-saturation). 

- If Zone 5 has been exposed at 0EV, as the vast majority of my work has been, truth be known, there ways to still take advantage of good tonal separation. That is, accepting Zone 9 as pure white in place of Zone 10 can still yield excellent results.  As I mentioned earlier the lights will be compressed above Zone 5 by one EV.  However, if I don't like the steep slope from Zone 8 to Zone 9, I can always violate the Zone System definition of Zones 5, 6, 7, and 8 by gently raising their values, and then use the Exposure -> Lightness slider to adjust the overall tonality of the image into something rather pleasing.

- I realize that I have to be discerning when looking for useful information. YouTube videos on the digital application of the Zone System don't take into account that +4EV, Zone 9 in the traditional sense, is the digital top end of the scale, not Zone 10 as they're describing it.  I've found the EV shift to be important, particularly in normal to high contrast situations, and I've learned to take what I've seen on YouTube with a grain of salt.

- I also have to be discerning and intellectually critical when strolling around the web looking at "testing" assessments.  One popular tester's assessment of digital adaptation of the Zone System, in the end, has been less than helpful.  Their conclusion that there's only 7EV of usable information in digital Zone System application does not match my experience.  I'm drawing a very different set of conclusions, and understand I could've been more cautious when I thought this particular tester was providing a properly researched conclusion.  For several years I felt that a digital Zone System was not possible based on his "testing."

- It sometimes pays to be skeptical, to have an attitude of curiosity toward details, to talk with others, and to not take everything/anything on the 'net at face value.  This is why, if I've done a decent enough job here, anyone who's interested in such things can do the very things I have for themselves to see what is true and what is not.  

Testing is not in and of itself the end.  Testing can lead to understanding, which might lead to better control.  In the end, making good photographs is the really only important goal.

Digital Sensitometry for Black and White ~ High(ish) ISO dynamic range

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

Scrolling through various cameras tested on Photons to Photos I couldn't help be notice that the dynamic range decreased as ISO increased.  To see for myself how this worked in relation to B&W, EV steps, and the Zone System I grabbed an old Sony NEX-7 and a newer(ish) Sony A6300 and set them both to ISO 1600.

Here's what I see.

The Sony NEX-7 looks pretty good down to -1EV.  Then the curve takes a sharp hook to the left.  I take this to mean the noise floor of this sensor at this ISO has been reached.

From the Sony A6300 I can see the application of advances in sensor performance.  There is a difference between the 1600 and 100 ISO X positions, but the slopes are fairly equivalent.

Interestingly, I see more dynamic range in my A6300 than reported on Photons to Photos.  There are 12EV from pure white to pure black.

So... why is that?

Well, it has to do with how I measured luminance at each EV step.  Photons to Photos is measuring  pixel to pixel luminance differences and making a different calculation than I.  In my case I use a 20pixel brush to measure luminosity, which averages out noise.

How can I justify this?  Easily, it turns out.  

When we view a photograph, even with a lot of grain (film) or noise (digital), our eyes average out luminance differences and we can see an overall tone.  Sure, if we stick our noses up to the photo we'll see grain or noise, but step back to a "normal" viewing distance and we see things a little differently.

Why is this important to me?

I'm often in dark museums where my little cameras really crank the ISO up.  I've noticed in practice that the dynamic range "felt" broader than reported.  Now I understand a little better why.  Which is good.  I really don't want to spend more money on a camera that might suit just a single purpose (ie: one with reportedly excellent high ISO performance).

Lastly for this blog entry, each high ISO input curve can be tailored to map directly into the linear 1EV step per division Zone System.  The curve shapes are a little different than those at low ISO's, but not terribly much.

Digital Sensitometry for Black and White ~ Zone System ~ RAW processing for linear curves

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

In a prior blog post I laid out a process for developing a set of luminosity curves that emulated rather closely silver halide film. When working more carefully with the Zone System and trying to apply it more accurately to digital work there are a couple challenges I needed to consider.  

One is a legacy challenge that comes from the film era that involves both the "toe" (roll-on region) and "shoulder" (roll-off region) of a film sensitivity curve.  To solve this, photographers like Bruce Barnbaum would place important shadow detail at Zone 4, instead of Zone 3 (see the original Zone System description).  They would then develop for the highlights, and would do these two things to keep 1EV (1 stop) steps even across the linear portion of  the curve.  This would help keep information from attenuating at the top and bottom of the EV range.

A digital challenge is introduced by the nature of sensors and the way that data is written.  I've come to learn about about it in doing this little research project.  What would be Zone 9 at +4EV with some tonal value in the old Zone System is saturated pure white in digital.  The Zone System description puts pure white at Zone 10 or +5EV.  As confirmation and in practice, whites in my digital work have too often felt attenuated and seldom have the same tonal separation as my film work.  Now I'm beginning to understand why.

What I'd like to do with this blog entry is to try and solve the film and digital challenges when working with images from my Sony APS-C and Full Frame cameras.

Here is the idealized step wedge for digital.  I made this using RawTherapee.  Using the fact that 0EV is defined as Zone 5, 76(hex)/118(dec), middle, or 18% gray, I took that tone and raised/lowered the exposure value in 1EV steps to build a step wedge from -7EV to +5EV.  Here is the linear step wedge result. Note the tonal values at each EV step.  These will act as my reference points.

Bottom Row represents 0EV set as Zone 5
Top Row represents -1EV set as Zone 5

Once I had the values at each Zone/EV step from the idealized 1EV step simulation I could then try and match the output of my Sony sensors to the linear ideal.  Using the Zone 5 definition, here is the step wedge I built centered around 0EV, middle gray.

 

This _exactly_ matches the idealized curve which solves the film-era "toe"/"shoulder" problem. For those situations where detail in +4EV is not required, this does the trick.  But... it does not solve the +4EV/Zone 9 digital saturation challenge.  To do that, I recalibrated -1EV upward to Zone 5, moved Zone 10 to +4EV, and remeasured the output luminosity curve.  Here is that result.

 

 

As can be seen, the output curve is shifted to the right for better highlight control, while retaining the slope of the linear ideal.  The overall curve is now better balanced between the highlight and shadow regions as well.  Usable dynamic range appears to be around 11EV with this early Sony A7 Full Frame sensor.  

With this curve I believe I have solved the film-era "toe"/"shoulder" and the digital Zone 9/+4EV saturation challenges.  I said the previoius film-emulation solution I worked on was "adequate", but I now have a better one.  

Voila! an accurate properly linear Zone System adaptation to Sony sensor digital.

--------- 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
• Accurate - Yes
  
• Repeatable - Yes
• Controllable - Yes
• Clearly understandable - Yes, but readers can judge otherwise if they like
  
• 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
• Bigger Bonus: I now have a linear digital match to the old idealize film-based Zone System 

Note: What I'm sharing in this series is limited to my experience with Sony full frame and APS-C mirrorless cameras and the sensors they come with.  In truth I have zero idea how other digital systems map tones, though I'm developing my own suspicions.  I hope the descriptions of what I'm doing here helps interested parties sort this out for their situations. 

Digital Sensitometry for Black and White ~ Zone System ~ RAW processing for film curves

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

Moving from synthesized digital Zone System step wedges into the real world, I gathered the photos necessary to see how Sony A7 100ISO RAW files behave.

For this blog entry I would like to do three things.  

First, I would like to look at characterizing the sensor by seeing how the Open Source Software distributes tones as a result of the DeMosaic stage and before applying any further processing. 

Second, using the data collected from the synthesized digital Zone System step wedges I created earlier, I would like to use RawTherapee Curves to match tones from Zone 3 through Zone 8 using 0EV as the Zone 5 reference.

Third, I would like to see what effect using -1EV as the Zone 5 reference does to the step wedge.

Importantly I will show my work by including screen shots of the input and output luminosity curves.  Hopefully the data will be more easily interpretable than reading hexidecimal Zone values.

Please note: The following is not really Rocket Science and I'm sure many other interested parties have already crossed this very same bridge years ahead of me. So if you find any errors, please feel free to help me better understand.

Materials -

• Sony A7
• 100ISO
• Manual mode (to accurately control exposure)
• Center spot meter (to avoid vignette impact on exposure)
  
• Light solid color surface
  

Data Collection -

• For each test pass, select the output file type, in this case...
  
• RAW - for processing on a Big Machine off-camera 
• Photograph a solid color surface making exposures...
• From -8EV...
• Through to +5EV
• On a Big Computer...
• Using Open Source Software RawTherapee
  
• Process each exposure to the demosaicing stage, avoiding any further image adjustments
  
• Then measure the tonal values using RawTherapee (or any appropriate image processing software) by
  
• Mousing over the center of the image
• Reading the tonal values (found under the histogram in the upper left)
• Make notes of the tonal values and corresponding EV the image were shot at
• Build a step wedge of EV, tonal values, and Zone indications
• Repeat this sequence after modifying curves
  

Results -

Here is a look at the tonal distribution after RAW file import and demosaicing, but before any further processing.

At this sensor characterization step it is easy to see the output curve is what I'll call hallowed out.  The shadow tone, toe region of the output curve is an interesting ramp, but the highlight shoulder region takes a rather sharp turn at the top.  It's easy to see that the 0EV value, specified as 18% middle gray, does not yet meet the Zone System definition.  Further image processing is required as this value needs to be raised to 76(hex)/118(dec) and Zones 3 thru 8 need also to match the specified tonal values.

Doing that now, here are the results.

0EV/Zone 5 now meets the center of the scale 18% middle gray as specified by the Zone System, and Zones 3 thru 8 tonal values align nicely with expectations.

In general, it should be obvious that the Sony A7 full frame sensor has good dynamic range, exceeding 11EV or 11 f-stops of potentially usable information.  This confirms Photon to Photos dynamic range measurement for this camera.

Looking at the symmetry of tones across the step wedge, the highlight roll-off shoulder region is much narrower than the dark tone roll-on, or toe, which is really quite long.

Stepping back a moment and thinking things through, there is nothing "magical" about using 0EV as Zone 5/18% middle gray.  It's what the industry does, so I've tended to _assume_ that is just how things are.  So what would happen if I liberalized my thinking slightly and select -1EV as the Zone 5 reference as well as correctly matching Zones 3 through 8 to a synthesized and idealized Zone System step wedge?

Let's have a look.

The overall step wedge has shifted to the left and the highlight "shoulder" region has expanded.  

This is rather exciting.

Remember the AMPEX photo reference manuals?  They published film tone curves which I find useful, even now.  They help with understanding how film behaved.  These digital exposure correction curves are well within the "ball park" what those old references shared.  

It's fun to realize that I now have a much truer "film-like" tonal curve for digital than I've ever had before.  Additionally, if for any reason I don't like the shape of the output curve, modifying the input correction curve is incredibly easy to do.

Regarding Zone System digital image processing, I think I have an adequate answer. 

---------- 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
• Accurate - Yes
  
• Repeatable - Yes
• Controllable - Yes
• Clearly understandable - I believe yes. 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
• Bonus: I now have a digital match to the old film-based Zone System 

Note: What I'm sharing in this series is limited to my experience with Sony full frame and APS-C mirrorless cameras and the sensors they come with.  In truth I have zero idea how other digital systems map tones.  I hope the description of what I'm doing here helps interested parties sort this out for their situations. 

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
  

Digital Sensitometry for Black and White ~ preamble

Someone noted on my Flickr site that recent in-camera generated jpg black and white images aren't as good as my RawTherapee luminosity curve, mid-tone raised, and channel mixer processed photographs.  

I had to agree.  

The Sony in-camera jpg processor for black and white does things to the tonal range that have been difficult for me to understand.  Contrast quickly shortens the top end of the range and whites are easily blown out.  All too often, the dark tones drag down the overall "look" and "feel" of my images.  Nothing seemed to make sense and nothing was coming our "right."

Frustratingly I've been flying by the seat of my pants and getting mixed results and mixed messages off the 'net on how to proceed.  It's time for a "system reset." 

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.

Being of a certain age, I worked through Minor White's "Zone System Manual - How to previsualize your pictures."  I also briefly read-through Phil Davis' finely detailed approach described in his "Beyond the Zone System".  In general, the Zone System provides a framework of understanding/describing/controlling gray tones.  

Fundamentally the Zone System defines Zone 0 as pure black.  Zone 10 represents pure white.  Zone 5, which is logically half way between the two represents middle gray.  This is often thought of as 18% gray, and it's easy to find, even today, cards of this very specific shade of gray sold to photographers to help in metering a scene.  Further, Zone 2 is described as the very dark gray that still reveals details and Zone 8 is the very lightest tone with details.

The Zone System was incredibly useful back in the day and I shot using that system for several decades.

Could I adapt this system to digital?

So I sat back and had a Deep Think.

Here's the method I came up with (Note: It's not really Rocket Science and I'm sure many other interested parties have already crossed this very same bridge years ahead of me) -

• For each test pass, select the output file type
• RAW - for processing on a Big Machine off-camera
  
• JPG - for observing what the jpg engine is doing when considering ->
  
• Sony "Creative Style" "Black and White"
• Sony "Creative Style "Light" (for a very specific reason)
• Sony "Picture Profile" "Cine4" (for another very specific reason)
   
• Photograph a white surface making individual images from
• -8EV or -7EV through to +4EV or +5EV
• On a Big Computer, measure the tonal values using RawTherapee (or any appropriate image processing software) by
  
• Mousing over the center of the image
• Read the tonal values (found under the histogram in the upper left)
• Make a note of the tonal value and corresponding EV the image was shot at
• Build a chart of EV and tonal values
  

Here's an example.  It is of the very thing that's been driving me nuts.  That is, Sony "Creative Style" "Black and White" at its default settings (Contrast = 0, Sharpness = 0).  I'll talk about what's entirely wrong with this in another blog post.  The problem could be very obvious to old film shooters who are used to thinking in terms of the Zone System, but for now, this simply illustrates the output of the method I just described.

In the next post, I will set the Digital Zone System baseline by working from RAW files.

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.