Converting Focal Length from Pixels to Millimeters to use in Bentley Context Capture

The focal length field in ContextCapture requires a focal length in mm instead of pixels. To convert a known focal length in pixels to mm:

F(mm) = F(pixels) * SensorWidth(mm) / ImageWidth (pixel).

For an X4S, the image width in pixels is 5472. Context Capture indicates the sensor width is 13.2 mm, so the equation is simply:

F(mm) = F(pixels) * 13.2 / 5472;

 

ContextCapture also assumes the center of the sensor is ( (width-1)/2, (height-1)/2 ) or ( 2735.5, 1823.5).  Since PhotoScan considers (0,0) as the sensor center, just add 2735.5 to CX and 1823.5 to CY, as shown in the PhotoScan calibration report.

 

All other parameter mappings are the direct between PS & CC.

 

As for settings for Loki processing, just be sure when you’re setting up AT, the you specify you have “accurate” position data, and that your “Keep” your calibration settings.

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