Lens Calibration Overview
Lightcraft’s Lens Calibration System is the most accurate and field portable way of mapping the lens’ focus, focal length, lens distortion and entry pupil (nodal point). For most lenses, it is hard to use the numbers on the side of the lens, like focal length, for instance, in camera tracking applications.
Many of the parameters of the lens changes over the course of its zoom/focus ring travel. The reason for this is because photo and cinema lenses are comprised of many elements and each glass element has its own set of parameters. Previzion, in it’s lens mapping process, measures these parameters as a whole over the course of the zoom and focus travel.
We use an optical marker pattern mounted on fiberglass aluminum honeycomb board to calibrate lenses on a serial number basis. In other words, even lenses that are “matched” may have variations that need to be mapped. Our lens calibration process takes that into account and calculates it throughout all steps of a lens.
Some lenses “breathe.” What that means is that by simply changing the focus from near to far there is a “zooming’ effect as the the focal length changes. In some extreme examples, a lens may say on its side 85mm, but when in fact it comes out to be 80mm to 100mm depending on the focus.
In VFX applications you need to take into account lens distortion at some point in your pipeline. Since Previzion renders everything in real-time, the lens calibration system calculates any barrel distorts or pincushions that a given lens may produce at the time of calibration.
For being able to map the limits and values of a lens’ focus and zoom ring, Previzion requires an encoded lens controller like a Preston FI+Z II/3, CMotion, LDS Lens, etc. It is important that the lens controller is encoded, because otherwise, there is no direct feedback from the lens’ positions.
For this, you will need:
- Previzion Computer
- Previzion Video Converters
- Power source
- HDSDI monitor
- Production camera
- Lens(es) being used
- supported Lens controller (see below)
- Lightcraft Aitrack
- Lightcraft Data Cable Tether
- Appropriate lens cable (goes into Aitrack via 4-pin Lemo)
- Lens Calibration Hardware (see next section)
Lens controllers are made up of encoders/motors that, when initiated, turn the focus and zoom ring from end to end and normalize this value between 0.000 and 1.000. This is the value for which Previzion uses to determine the position of the lens. Tracking the lens so to speak.
Previzion currently supports:
- Preston FI+Z II and 3
- CMotion Camin 2M, 4M and 6M
- CMotion Compact
- RT Motion (legacy support only)
- Canon ENG lenses
- Fujinon ENG Lenses
- Fujinon Cine Lenses
- LDS encoded lenses (Cooke, Angeneaux and Zeiss) paired with Arri Alexa Plus
Camera Settings For Lens Calibration
When setting up the camera for lens calibration, it is important to know a few things about the technical specifications of the camera itself.
You should know or have someone who knows how to change camera settings if necessary. Also, you should be familiar with or have someone who is familiar with the lens controller and lens controller operations.
Take a moment and verify all your settings before starting the lens calibration procedures. This process can take up several hours and, should there be a setting that is wrong from the beginning, the resulting calibration file may not be good for use.
Requirements for setting up lens mapping:
- Camera Support – the camera can be mounted on wheels, a tripod or dolly. With the new Static Lens Mapping procedures, there aren’t as many camera moves when there are any.
- Lens – you should have access to all lenses that need to be mapped in this process.
- Lens Controller – have power and the appropriate data cable for your lens controller.
- Camera Height – the camera does not need to be at the same height as the board. It often helps to have the camera at comfortable height, but know that the calibration board can be mounted at adjustable heights on its stand.
- Clearance of board – make sure that the room or location picked for the lens calibration has enough room to clear the motion of the 14-foot board. If space is an issue, you may use just the center section of the board, but this limits the lens calibration accuracy to mostly normal to telephoto lenses.
- Frame Rate – Lens calibrations cannot be done in interlaced frame rates. The camera should be set to a p or PsF frame rate.
- Image Adjustments – turn off all image adjustments such as sharpness controls, edge enhancements, image mirroring, etc.
- Power – if possible, keep the camera running on a reliable power source, preferably AC power or enough batteries to run for a few hours.
- Focus assist/Menu on Output – some cameras can send the on-screen menu and any focus assist magnification out the HDSDI Monitor Output. Make sure these are disabled.
- Lights – this process requires a considerable amount of light to achieve great depth of field. Plan on having studio lights. We recommend at least using cool lights (KinoFlo).
- ISO/EI/Gain – for the lens calibration process, you may boost the gain or raise the ISO rating for the sensor to register more light.
- Iris/F-Stop – change the f-stop (iris) to its smallest aperture. This value should be around f.22 or less. This will diminish depth of field and allow for better capture of the marker patterns.
It’s helpful to check that all your components connect together and are in the correct firmware version for the lens calibration process.
Previzion Settings for Lens Calibration
The lens mapping process in Previzion is a bit different from other processes in Previzion. Make sure to check the following parameters:
- Trackers – Other than your lens tracker, Previzion does not need to have SFserver open or any of its trackers enabled or connected. Set Sync to “Generated,” Location to “None” and Stereo to “None.” In Tracking Tab, the Lens Tracker is the only one that should be enabled and checked to be in proper running order.
- Scene – To avoid scene render load conflicts, make sure you are in the Previzion default scene. In Scene Tab, make sure to “Clear” scene or press CTRL + N to clear the scene.
- Video Input – the video input HDSDI 1 should be enabled and set to match the camera’s frame rate.
- Camera Sensor – this is very !IMPORTANT! and should be triple checked. Make note of the setting in the camera for resolution and the type of sensor. Check with the manufacturer’s documentation to obtain the proper sensor size (in millimeters) if in doubt. Previzion comes loaded with a variety of documented sensor widths and should cover most users.
- Other Effects – Keying, Color adjustments, Mattes, Markers, etc. all get overwritten during lens calibration.
Once all the settings and parameters above mentioned are checked and re-checked, proceed to the actual lens calibration process.