Black shading maximizes image quality by ensuring that pixel sensitivity remains consistent throughout an image. The technique has been used for a variety of high-end applications in digital capture over the years. In this article, we'll discuss when this is beneficial and how it is applied.
Noise in any digital image is the result of both "fixed pattern" and random noise. The former is caused by persistent variations in light sensitivity between pixels, whereas the latter is caused by thermal fluctuation, photon arrival statistics, and other non-repeatable sources. Everything else being equal, fixed pattern noise is therefore the same for every image, whereas random noise is not.
Note: The above examples are extreme cases from early DSLR and compact camera models, shown at 200%. Actual results will be far more subtle, and will depend on specific shooting conditions and camera settings. Fixed pattern noise also varies more in appearance than does random noise; above image is just one example.
Black shading works by measuring the pattern of fixed noise, storing it in memory, and then subtracting it out of all subsequent frames leaving only random noise behind. The pattern stored in memory is called a Calibration Map in the RED ¨ menus, and is effectively a map of the black level for every pixel hence the name black shading.
WHEN TO APPLY
In order to establish a noise floor, every RED camera ships already calibrated. Upon receipt of a new RED camera though, we recommend customers perform an additional calibration to ensure optimal camera performance.
The following are the most important considerations, along with general guidelines of when to calibrate a sensor:
- 1) After an extreme change in temperature (+/– 30°F or +/–15°C) from the current calibration map.
- 2) After an extreme change in exposure time (+/– 1/2 sec) from the current calibration map.
- 3) If either the T or E in the CAL: T/E indicator is not green.
- 4) If the noise profile shows patterns or lines.
- 5) After each firmware upgrade.
When shooting 3D, we recommend running black shade calibrations more regularly since the images are literally being laid over one another and the cameras will have different thermal characteristics based on their orientation in the rig.
Optimal results will be achieved using conditions and camera settings that are representative of what they'll be during the shoot. For maximal consistency, some also black shade prior to using extremely fast shutter speeds, such as in excess of 1/1000th a second, or prior to starting a new project. Also ensure that fan speed settings and internal camera temperature remain similar. The currently-used black shading conditions can be viewed by going to Settings > Maintenance > Calibrate > Sensor > Calibration Capture in the advanced menu. Other than time spent, there is no disadvantage to performing additional black shadings.
In any case, the above temperature and shutter speed recommendations shouldn't be taken strictly. For example, when shooting using a mix of 1/4, 1/2 and 1 second exposure times, black shading is only necessary using one of these exposure times they are close enough. However, when choosing from amongst a range of exposure times or temperatures, and only one black shading is practical, better results are typically achieved by basing this black shading on the upper end of the range.
HOW TO APPLYTo learn more about the black shading process including how to perform a black shade calibration and the differences between manual and auto calibration, please watch the following RED TECH video.
Saving more than one commonly-used black calibration setting can help save time. For example, some store a separate calibration map for typical outdoor time lapse conditions, along with a calibration map for indoor high-speed captures. These presets can then be toggled using the Calibration Management tab located at Settings > Maintenance > Calibrate > Sensor.
After a few calibrations, one often has a better feel for how often black shading is necessary, in addition to how many presets you require. The benefits are more visible at higher ISO speeds, when strong shadow recovery is applied in post-production, and with white balances at low Kelvin.