code: def geometric_conj_af(a, f):
      """
      Conjugation relation for geometrical beams under paraxial conditions.
  
      Explanation
      ===========
  
      Takes the object distance (for geometric_conj_af) or the image distance
      (for geometric_conj_bf) to the optical element and the focal distance.
      Then it returns the other distance needed for conjugation.
  
      See Also
      ========
  
      geometric_conj_ab
  
      Examples
      ========
  
      >>> from sympy.physics.optics.gaussopt import geometric_conj_af, geometric_conj_bf
      >>> from sympy import symbols
      >>> a, b, f = symbols('a b f')
      >>> geometric_conj_af(a, f)
      a*f/(a - f)
      >>> geometric_conj_bf(b, f)
      b*f/(b - f)
      """
      a, f = map(sympify, (a, f))
      return -geometric_conj_ab(a, -f)
  
imports: from sympy.core.expr import Expr
  from sympy.core.numbers import I, pi
  from sympy.core.sympify import sympify
  from sympy.functions.elementary.complexes import im, re
  from sympy.functions.elementary.miscellaneous import sqrt
  from sympy.functions.elementary.trigonometric import atan2
  from sympy.matrices.dense import Matrix, MutableDenseMatrix
  from sympy.polys.rationaltools import together
  from sympy.utilities.misc import filldedent
label_desc: Use image node_id 4 for calling a global function with example usage: geometric_conj_af(a, f) and returns: unknown