QC Series – Checking Exposure: Measuring Light in Silverstack

In the world of filmmaking, ensuring the technical accuracy and artistic quality of every frame is essential for effective storytelling. There are many steps necessary to accomplish this, but in this article, we will focus on checking exposure. Exposure plays a significant role in how your audience perceives your story by setting the mood and enhancing the visual impact. 

Throughout this article, we will first explore the significance of exposure in quality control (QC), briefly delve into the importance of RAW, LOG, and look recording in this context, and finally, provide a detailed examination of Silverstack’s software features designed to streamline QC regarding exposure. At the end of the article, you’ll find an overview of the various light meter tools available in Silverstack.

Why is Exposure so Important?

As aforementioned, exposure affects the technical quality, visual aesthetics, storytelling, and emotional impact of a movie. It is both a creative and technical consideration that plays a crucial role in delivering a captivating and visually pleasing cinematic experience.

The actual mechanism of controlling exposure involves regulating the light reaching the camera’s sensor or film stock. This is accomplished by adjusting three key camera components: Aperture, shutter speed, and ISO. The aperture (possibly assisted by an ND filter) dictates the light reaching the sensor, the shutter speed determines how long photons are collected in the sensor’s photodiode (the exposure time), and the ISO setting amplifies the voltage resulting from this process. The combination of these settings generates a corresponding video signal. Skillful management of these components is crucial for capturing images with the desired dynamic range, brightness, contrast, and clarity.

Optimal exposure settings ensure that the camera captures the full range of brightness levels in a scene. Underexposed images appear too dark and lack detail in shadows, while overexposed ones lose detail in highlights and appear too bright. A well-exposed image provides a balanced picture with clear, discernible details that can be manipulated according to the artistic intention during post-production color grading.

Exposure issues during filming can arise due to various factors such as camera settings, time constraints, lighting challenges on set, or even metadata or software pipeline settings. Therefore, it is crucial to monitor exposure during QC and promptly communicate any issues to the production team.

Before we shed some light on how to check the exposure with Silverstack during QC, let’s have a quick look at the three encoding types in which our original camera negative (OCN) is usually available.

From RAW to LOG and Look

As filmmakers, we usually strive to record the best possible quality, the greatest possible dynamic range, and a great color gamut without any loss of quality. This is why one of the frequently discussed questions is: RAW, LOG, or Look, and if so, which one?

Not all cameras offer all possibilities and for some productions, a RAW workflow may be far too expensive.

While we don’t have the capacity to go too in-depth with RAW, LOG, and Look encodings, understanding the essential distinctions is crucial for QC in post-production.

RAW:

RAW recording provides precise control and maximum post-processing potential in post-production. Considered as uncompromised sensor data, RAW captures information directly from the camera sensor, omitting two key image processing steps: the optoelectronic transfer function (OETF) for gamma or contrast, and color gamut transcoding for color representation. To generate a video image from RAW data, the de-bayering process is delegated from the camera to subsequent software tools. RAW recordings are often linked to large file sizes, requiring more disk space and longer copy times during data wrangling.

In Silverstack, the video image is generated based on camera metadata and the configuration in the RAW Settings Panel, utilizing the manufacturer’s RAW Software Development Kit (SDK) for de-bayering. While RAW offers flexibility, meticulous processing is crucial to avoid errors in the final output.

LOG : 

A LOG video signal provides a defined, neutral profile. Starting with a flat, desaturated appearance, LOG signals offer flexibility in post-production without the complexity and file size associated with RAW footage processing. LOG formats aim to preserve a wide dynamic range within the OCN, achieved by decoding the signal with a logarithmic-shaped curve. Most LOG formats record in a wide color gamut for vibrant color information, resulting in desaturated images that allow for grading flexibility in post-production.

However, since a LOG signal doesn’t align with our natural perception, a Look is applied for on-set monitoring and dailies. This helps preview and judge the image more naturally or support the film’s storytelling, providing a more intuitive representation during the production process.

Look:

In contrast to RAW and LOG, a “Look” in video applies a predetermined color and contrast profile, establishing an immediate visual style during filming. This aesthetic can be applied in-camera, within Silverstack, or during post-production. A Look can either be used for preview or baked into the OCN. When baked in during recording, color correction options are limited, and it adheres to a “what you see is what you get” principle. This means that the selected exposure should closely match the final look.

On the other hand, when the Look is used for preview on set or in post-production, internal camera recording is done in RAW or LOG. The Look is then applied to display the flat log image as a saturated and high-contrast video image in Rec.709 or Rec.2020 standard, giving an impression of the final product after color correction. This conversion LUT is often referred to as a Log -> Rec.709 LUT Preset and may involve both technical and artistic color and contrast manipulations.

While this approach helps establish a specific visual style from the beginning, incorrect LUT / Look preset settings can pose QC challenges, leading to brightness or color discrepancies across various monitors and deliveries.

The visual comparison of the above image clearly shows the sometimes enormous difference in color and contrast of minimally different settings or metadata. A statement about whether the exposure is “correct” is hardly possible without precise information about the recorded signal and the subsequent gamma and color transformation settings.

Choosing among these formats depends on creative goals, post-production capabilities, and project-specific requirements, all of which should be discussed and defined during pre-production.

This workflow is paramount for optimizing captured light and defining the desired look. Notably, the Look source & decoding settings are pivotal in resolving quality issues related to exposure or Look, through accurate adjustments within Silverstack.

Understanding the Visuals and Setting Expectations

The process of ensuring the right exposure goes beyond mere camera parameter settings and light measurement using tools like waveforms or false color. Filmmakers, and DITs responsible for QC must deal with the specifics of their observations and understand the image signal and its metadata. This chapter guides you in answering the fundamental question: What am I looking at, and does it align with my expectations?

1. Switching between Original and Graded

Mistakenly interpreting LOG images as Look images, or vice versa, can result in inaccurate exposure adjustments, false alerts in the QC report, and ultimately affect the final visual outcome. 

In the playback view, accurately distinguishing between the original (displaying footage as the OCN, often in LOG) and graded (displaying footage with the defined Look, typically in Rec.709) signals is crucial for precise exposure assessment.

To ensure clarity in understanding what you are evaluating: toggle between these views, either in the playback HUD, the top bar, or using the shortcut cmd + L to alternate between original and graded views. In the end, this feature should help to identify what you are looking at.

2. Table View: Checking recording and processing metadata

The initial step in QC, especially when it comes to exposure and color issues, involves examining clip metadata. Metadata, encompassing vital details like clip name, camera settings, encoding, and color settings, not only aids in asset identification and management within Silverstack but also ensures alignment with defined camera and workflow parameters.

The customizable table view layouts facilitate the comparison of numerous assets (current settings according to the Grading Panel, RAW Panel, and changes made in the General Info Panel), allowing you to swiftly identify discrepancies in the settings. For example, you can compare metadata from previous footage with current production footage against the defined workflow. 

For exposure QC, it provides insights into various recording and processing parameters, such as format, recording color space, and the activation status of custom looks in LUT nodes. Additionally, it allows you to check the Look source

Via Look source, you can see the currently selected grading mode for each clip. Set the Look source for each individual clip with the following settings:

• None: Plain OCN Looks without metadata, mostly a LOG image, depending on the recording metadata as shown in column Rec.Color space
• From File: As specified in the OCN metadata, in this case, the Default ALF4 LUT
• Preset: A user-defined LUT of the numerous standard LUTs from various manufacturers that Silverstack comes with out of the box.
• User-selected LUT: A custom LUT file selected by the user. Such LUTs are often used as preview LUTs when a certain look has been defined for a scene or the entire film.
• Custom Look: An individual color and LUT setting as specified in the grading node by the user. In this case, the user selected the preset ARRI LUT Log-C -> Rec.2020

To facilitate identification of the Look source, the table view proves particularly helpful as we will see in the next image.

Let’s take another look at the split-screen from the beginning, and see how OCN can become five look variants (see ‘Image A’ above).

Firstly, we’ll examine an OCN recorded in RAW. Subsequently, de-bayering takes place in Silverstack, utilizing the manufacturer’s SDKs and adhering to the Silverstack RAW settings. The resulting video image is then output based on one of the five available Look sources.

In the case of an OCN recorded in LOG, it’s also feasible to configure the five distinct Look sources within the Color UI. Not visible in this screenshot is the option for a user-selected LUT.

Use the table view to check exposure or color processing relevant metadata. In the above screenshot (Image B), you can see the different possible Look sources, as described before. Click on the image to expand it for easier viewing.

You can also build your own custom layout for the table view and save it as a preset in case you want to check specific settings more often during QC.

3. The Grading Panel: Checking the Color Processing Pipeline

The Grading Panel in Silverstack is essential for monitoring and checking the Look pipeline, especially for custom Looks. Check here if the correct Look and / or LUT is activated. 

As a debugging feature during QC, you can activate and deactivate every single layer of the Grading Panel, in case you want to figure out where a gamma or color shift comes from.

When making changes in the Grading Panel, the table view will immediately reflect these adjustments, displaying the current settings for easy comparison and analysis.

For more in-depth information on the Grading Panel and its functionalities, please refer to the Knowledge Base^[1].

The Grading Panel in Silverstack unlocks creative possibilities and allows you to check for the current Look settings. In this case, the image in the above screenshot was recorded with S-Gamut3, Cine, and SLog. If you take a closer look, you can see that this matches with the general info metadata.

4. RAW Settings Panel: Custom Settings and Decoding Color Space

For RAW footage, ensuring accurate RAW decoding settings, especially Look source and decoding colorspace, is crucial. It’s vital to match these settings with the defined workflow settings and therefore adjust them if necessary.

Silverstack Lab utilizes RAW development parameters from the OCN metadata for de-bayering. The “custom settings” checkbox in the RAW Settings Panel allows changes to the original metadata, providing flexibility. It’s not active by default and can be toggled for before-and-after comparisons.

RAW settings are clip-specific but can be copied to other clips. Activating custom settings enables adjustments for exposure ISO, white balance, and tint settings.

As you might change settings during processing the table view offers clip and current table columns to differentiate between metadata and custom settings.

Also, you can adjust the Look source & decoding setting for each RAW clip individually.

For further information about the RAW Settings Panel, visit our Knowledge Base^[2].

5. General Info and Header Info Panel: In-depth metadata analysis

The general and header info panel located in the right sidebar provides valuable insights into clip information extracted from the OCN metadata, ensuring clarity and precision in your analysis. This also allows for manual metadata changes, indicated by a little pen icon. Use this to make changes to timecode, exposure, production info, and camera details. Your changes will be updated in the table view to reflect your new settings. 

The detailed technical metadata in the header tab becomes very helpful if something seems wrong and all the exposure control options described above have not provided any answers. 

Visit our Knowledge Base^[3] for more details about the header and info panel.

In conclusion, a thorough examination of clip metadata in Silverstack serves as the cornerstone for effective QC. With a close look at this metadata, the Grading Panel, and the toggle between graded and original, filmmakers can confidently answer the crucial questions about what they are looking at, crosscheck whether the visuals are LOG or Look, and validate their alignment with the intended workflow settings and can thus make a qualified statement about the quality of the exposure.

Now, our journey into exposure QC comes to an end, and we’ll transition our focus to different light meters within Silverstack, unraveling the tools that empower you to precisely measure and check exposure levels.

Light Meters in Silverstack

When you record footage in challenging lighting conditions, it’s vital to prevent over or underexposure and to maintain a certain brightness for reference objects. Silverstack is equipped with a variety of built-in light meters to help you with this. Let’s take a look at the different analytics tools, what each one is good for, how it works, and how it can be customized for use in Silverstack

Clipping Highlighting

Clipping highlighting is a fundamental tool for evaluating overall footage quality by pinpointing over or underexposed areas. This feature is especially crucial during QC playback, enabling a detailed check for potential loss of image detail by highlighting problematic white or black areas.

The screenshot exemplifies this with white clipping marked in pink and black clipping in green, ensuring quick identification of exposure issues.

The feature can be effortlessly enabled or disabled using the Visual Controls HUD. Customization options include adjusting sensitivity thresholds for black and white levels via sliders. In SST settings > playback, users can further personalize their experience by customizing colors and selecting the clipping and range unit, offering options like % video level or a specified code value range between 8 and 16 bits.

Exposure Range

Exposure range enables meticulous footage assessment. It allows you to check the brightness levels of specific elements, such as blue- or green screens, faces, or surfaces. When employed during QC, exposure range allows users to guarantee consistency throughout the scene 

In terms of functionality, users can define a luminance level and highlight a range around it. This tool accentuates the defined luminance range, offering a clear visualization and assisting in the assessment of specific image content.

Activation and customization are seamless processes, easily managed through the Visual Controls HUD. The sliders provided enable users to set the luminance level and the signal range around it. Further customization options, available in SST settings > playback, include adjusting colors and selecting the clipping and range unit (% video level or a specified code value range between 8 and 16 bits).

False Color

False color, a widely adopted exposure control technique within the industry, seamlessly integrates into Silverstack, offering a valuable tool for assessing exposure levels swiftly and effectively.

This feature replaces traditional luminance values with a spectrum of colors, providing an immediate visual representation of various exposure levels. It proves ideal for a quick assessment of whether or not images are appropriately exposed, thereby reducing the risk of underexposed or overexposed areas.

Functionally, false color substitutes luminance values with a range of colors, each corresponding to a specific exposure level. This technique offers a rapid overview of an image’s exposure, highlighting brightness gradations and distributions for simplified exposure control. However, it’s important to note that, while powerful for exposure control, false color may obscure other image issues due to color indexing and grayscale content. For example, QC issue M-400_096 visible production equipment, in reference to the boom^[4].

Activation of false color is straightforward, and easily managed through the Visual Controls HUD. For more details, see the Knowledge Base^[5].

Video Scopes – Waveform and Histogram

Silverstack incorporates essential video scopes, such as waveform and histogram, for in-depth image analysis. These scopes play a critical role in overseeing and fine-tuning exposure, color balance, and overall image quality. However, depending on your experience level, your job definition, and the content you’re working with, these scopes can sometimes feel overwhelming due to their depth and detail.

It is common practice to have the scopes shown during QC playback to check them in case something seems suspicious.

Waveform:

In the above image, pay attention to the differences in brightness on the y-axis, while the x-axis remains consistent and aligns with the positions of the gray patches.

The waveform tool in Silverstack is a crucial component for achieving precise exposure control, and ensuring your images maintain the desired brightness and contrast levels. This feature provides a graphic representation of luminance levels across the frame, offering enhanced exposure issue detection and a clear understanding of luminance distribution in the image.

Functionally, the waveform tool facilitates precise exposure control by visualizing luminance distribution across the entire image. The x-axis aligns with the image’s horizontal axis, while the y-axis displays the luminance signal. This allows for a detailed analysis of either only the luminance signal Y, individual RGB channels, or utilization of the RGB overlay and RGB parade functionalities.

Activation and customization are user-friendly processes, with the option to toggle between windowed and docked views by clicking “show scopes.” Additionally, users can customize the waveform units, choosing between percentages or nits in the scope settings.

You can find more details about our video scopes in the Knowledge Base^[6].

Histogram:

The histogram tool in Silverstack is a fundamental asset for comprehending overall exposure, tonal distribution, and contrast within your footage. Utilizing the histogram assists in making informed decisions regarding exposure adjustments to achieve your desired image characteristics.

In terms of functionality, the histogram provides a visual representation of tone distribution in your image, presenting options for the Y luminance signal only or as an RGB overlay or RGB parade. The x-axis illustrates luminance from dark to bright as a percentage, while the y-axis showcases the number of pixels at each corresponding luminance level.

Activation and customization are seamless processes. Clicking “show scopes” allows you to toggle between windowed and docked views. Furthermore, users can enhance their analysis by utilizing the result-curve feature in the scope settings, enabling the observation of color grading impacts on recorded code values.

You can find more details about our video scopes in the Knowledge Base^[7].

Clipping highlighting, exposure range, false color, and video scopes (waveform and histogram) are invaluable tools that empower professionals to ensure optimal exposure, color balance, and overall image quality, ultimately enhancing the QC process. 

What to do in case the Exposure is “Wrong”!

Have you checked all the settings in SST as recommended in this article?  Are you still finding the image exposure to be incorrect? 

As you already know, time is money on a film set. If you communicate your QC findings too late, the production may move on and it could lead to the possibility of a reshoot being compromised.

Therefore, it is important to gather information about clip metadata, and decoding settings, and then contact the responsible departments to find a solution for the issue.

Also, refer to the introduction article^[8] of this series for QC-related features in Silverstack to learn how you can track your findings and communicate them to the post-production pipeline using the reporting functions in SST.

In our next article, we will look into the quality issue of “aliasing” and which settings in Silverstack are related to this issue.

Footnotes and Knowledge Base Links

• [1] Grading Panel functionalities
• [2] Raw Settings Panel
• [3] Header and Info panel
• [4] Visible Production Equipment
• [5] Activation of Flase Color
• [6] Waveform, Video Scopes
• [7] Histogram, Video Scopes
• [8] QC Series Introduction Article

All posts in this series:

• Silverstack QC Series
• QC Series – Checking Exposure: Measuring Light in Silverstack
• QC Series – Aliasing and Moiré: Navigating Quality Challenges in Film and TV