Wratten filterchart

GeoCue Support » Drone, Mobile and Handheld Mapping » TrueView Tools, Tips and Workflows » Converting Focal Length from Pixels to Millimeters to use in Bentley Context Capture

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.

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:

Wratten filteroptometry

White pine trees captured by infrared converted Canon PowerShots with Wratten 25A filter (top row) or Wratten 15 filter (bottom row). False color infrared (NRG) images are made by swapping color channels. NDVI images highlight the difference between the red and infrared channels.

Wratten filterfor slit lamp

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:

This square of Wratten 15 gelatin filter is large enough to convert four Canon PowerShots or other point and shoot cameras to plant analysis cameras. The small PowerShots typically used for aerial photography have an internal infrared block filter that is 8 mm by 9 mm. By opening the camera and replacing that filter with a rectangle cut from this filter, the camera will record near infrared light in the blue channel and mostly red light in the red channel. Those two channels can then be used to produce Normalized Difference Vegetation Index (NDVI), and along with the green channel can produce false color IR images. Unlike the Wratten 25A filter, the Wratten 15 does not block green light, so the green channel is more useful for false color IR images and other visualizations. This 2 cm x 2 cm square can also be used in front of the lens of a camera which has had its IR block filter removed. There is more information about this filter at Public Lab.

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.