Primary Calibration Points
A lens calibration is made up of a series of “Calibration Points”.
These points are created at certain parts of the focus (and zoom) and contain the lens information for that part. By building up a “map” of these points it is possible to interpolate between them and allow for a good understanding of the lens parameters at all points of the zoom and focus.
The exact number and position of these points depends on the type of lens but there are some general guidelines that should be followed.
This stage of the calibration process aims to create the initial calibration points, that will be used to bootstrap the lens file
Depending on the type of lens being calibrated the graph will indicate where calibration data should be collected.
Target calibration points for a zoom lens
Target calibration points for a Non-Zoom (Prime) lens
Move to one of the marked areas and the “April Tag” calibration will become available.
Position either the 35mm or 70mm mosaic chart in front of the camera. Try and avoid having the chart too small or too flat in the frame
Poor chart framing - The chart is relatively small in the frame and is very "flat" to the camera
Good chart framing - The chart is angled and clearly seen in the frame
Choose the "Start April Tag Calibration" button. Do not adjust the zoom or focus of the lens until this calibration point is saved
Press "Take Image" to capture an image of the chart
Continue to take images until the grid is largely yellow.
It's not required to fill every grid square but try and avoid large gaps
After collecting enough images, press the "Calculate" button to calculate the lens data. This will be shown in the panel and if happy press "Save Calibrated Point" to save the data and complete this calibration point
If the calculated values seem completely incorrect, the "Cancel" button will discard the values, allowing the procedure to be repeated.
If the calculated values are consistently wrong, consider checking that the encoder mapping for the focal length is correct and that the sensor size information for the camera is right
Continue this process for the remaining points indicated in the lens graph
At certain parts of the zoom and focus of a lens it can be hard to detect the chart properly. Closing the iris can help with chart detection as it will create a deeper depth of field.
However, particularly on long focal lengths, it won't be feasible to use the chart at all. In these cases, pressing "Mark as undetectable" will skip the chart calibration for this point and will use estimated values based solely off the encoder mapping.
It is strongly recommended to closely check the calibration points made this way in the "Refine" stage later
