Center Camera
Previzion will next display the screen above. Move the camera to the distance requested in the calibration, in this case 94cm. (The camera is probably already located here.)
Click Enter to continue.
Move Lens to Match Guideline
Next, move the zoom lens to match the zoom lens fraction specified (0.000). The Focal Length slider will turn green when the value is within 0.002 of the target value.
The values on the focal length slider update in real time, so you can zoom the lens back and forth to see them change. Previzion assumes that the minimum focal length and focus distance of the lens positions will correspond to 0.000 on the focal length and focus distance scales. If the scale is reversed, go to the Tracking tab, click Configure by the Lens tracking source, and check the boxes to invert the zoom or focus values until the sliders correlate properly.
Make sure the target is spinning, and click Enter after this is done, and the line is green to start capturing the first calibration sequence.
Calibration Capture
Previzion next captures 20 frames of the rotating calibration object, in about 1 second intervals. Each frame is processed to recognize the marker, and a blue cross is drawn on each calibration marker.
A row of green boxes to the right graphs the number of targets found on each capture.
Calibration Results
After capture is complete, a dialog box will come up displaying the results of the calibration on this lens position. Verify that the various numbers are correct.
Focal Length should be similar to the value marked on the lens barrel at smaller focal lengths. It can diverge considerably at higher zoom levels.
The Entry Pupil represents the forward ‘nodal point’ of the lens. For a zoom lens, it will typically begin near the front element of the lens, and progress backwards as the focal length of the lens is increased, typically becoming negative at some point.
The Center Offset is the distance from the center of the sensor that the optical axis is located. Lenses are not perfectly centered, and zoom lenses will move around considerably as they are zoomed in and out.
Distortion measures the amount of radial lens distortion. It will typically be a small negative number (about -0.1) for wide angle lenses, and rise to be a small positive number (about 0.2) as the focal length increases. Significant variations can be seen toward the end of the zoom range.
Click Keep to keep the value, or Discard to discard it and try again at this lens point. Most calibrations are fine the first time.