Black Shading CalibrationFebruary 06, 2013
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: 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 “CalMap” in the RED menus (short for Calibration Map), and is effectively a map of the black level for every pixel—hence the name black shading.
Note: above example is an extreme case from an early compact camera using a 4 minute exposure time.
WHEN TO APPLY
With RED, black shading can often be ignored and left at the default factory setting. It’s only necessary when exposure conditions differ substantially, such as with extreme changes in temperature and exposure time, or after a firmware update. Conversely, even large changes in frame rate or resolution do not require black shading on their own. The following are the most important considerations, along with general guidelines:
Temperature: when ambient conditions change by more than roughly 30°F or 15°C.
Exposure Time: when going above or below about 1/2 a second.
Firmware: sometimes after upgrades, if instructed.
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 from “Settings > Maintenance > System Status” in the top 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 APPLY
The entire black shading process can require up to 20 minutes, so make sure to plan your shoot accordingly. This amount of time is necessary in part because of the uniquely high sensor read-out speed, but may also vary depending on camera model and firmware version. Once ready, use the following steps:
Prepare the Camera. First, insert a formatted blank media into the camera, which will be used to store temporary calibration data during the process. Second, ensure that the camera is plugged in or has sufficient battery charge to remain powered on throughout the process. Finally, make sure your camera’s shutter speed is set to what you will want for black shading.
Block All Light. Remove the lens and apply a sensor cap. If you’re outside, envelop the camera with an opaque cover. If you’re inside (much better), place the camera in a dark room.
Initiate Warm-Up. Turn the camera on and make sure that the camera has had enough time to reach its steady-state operating temperature. Waiting about 10 minutes is usually sufficient.
Select the Type. Access “Settings > Maintenance > Calibration” within the camera’s top menu. Then select either “Default” or “Current FPS/Exposure” as the black shading type. The former uses the same 1/48 second exposure time that was used for the factory calibration, whereas the latter uses the current shutter speed setting. The default setting is recommended if black shading is being performed primarily due to a temperature change.
Start the Process. Once you select OK within the confirmation popup, a series of status screens and progress bars will appear. No interaction is required. These will list the capturing, analyzing, erasing and programming steps in succession, and then each will be shown again in a second pass. This part can take up to about 10 minutes. When finished, a “Calibration Complete” message will appear. Select OK and you’re done.
If an error message appears and the process is unsuccessful, this is usually because not enough light has been blocked from hitting the sensor. Double-check that the sensor cap has been secured, and try moving the camera to a darker location. Also make sure the blank media has been fully inserted.
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 “Current CalMap” drop-down menu within “Settings > Maintenance > Calibration.”
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.