Focal length specified using --init_focal_length does not change during bundle adjustment

[nirmalsnair]

For images without exif data, I've given the focal length estimate using the --init_focal_length option. But it remains constant throughout the bundle adjustment process.

Contents of options.txt: --match_table matches.init.txt --output bundle.out --output_dir bundle --init_focal_length 1078.836 --constrain_focal --constrain_focal_weight 0.0001 --run_bundle