NAME:
  iof2mag
 PURPOSE:   (one line only)
  Compute the flux (mJy) of unit surface area (1 km$^2$) with unit reflectivity
 DESCRIPTION:
  Cloned from amag.f

  The magnitude of the object is computed using an absolute B magnitude
  of the sun of 5.51 (at 10 parsecs).  This corresponds to a B magnitude of
  -26.066 at 1 AU.  The formula for computing the magnitude of the object
  is:                         ( galb * rad^2 )
        Mb = -26.066 - 2.5 log( ------------ )
                              (   r^2 * d^2  )
  The units in this equation are not the same as those passed to the function.
  The units are: [rad] = cm, [r] = cm, [d] = AU.
  All full powers of ten are removed from inside the log to prevent
  under/overflow.

  here galb is taken to be the same as I/f, or bi-directional reflectance
    divided by pi.

 CATEGORY:
  Photometry
 CALLING SEQUENCE:
  result=iof2mag(iof,filter,radius,sun,earth)
 INPUTS:
  iof    - I/F value, averaged over body
  filter = scalar string of filter of the calculation
  radius = radius of object in kilometers, only used to calculate area
  sun    - distance from sun to object in AU
  earth  - distance from earth (observer) to object in AU
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  return value is the standard magnitude for the given I/F value
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  2012/01/29, Written by Marc W. Buie, Southwest Research Institute