The Bessel filter transfer function depends on two factors of insertion loss and order parameter of a normalizing constant. While the transfer function of a rectangular filter depends also on the insertion parameter, depth parameter, a center frequency and bandwidth.
It is interesting to note that in the “help” of the Bessel filter he makes an important observation:
Important: Previous versions older than OptiSystem 7.0 used a different equation to estimate the 3 dB bandwidth. The following table provides the multiplication factor that has to be multiplied by the current bandwidth in order to obtain the same results of versions older than OptiSystem 7.0:
(attached image)