  The H$_2$ 1-0~S(1)/Br$\gamma$ ratio (R(Br$\gamma$)) is used in many studies
of the molecular content in planetary nebulae (PNe). As these lines are
produced in different regions, the slit configuration used in spectroscopic
observations may have an important effect on their ratio. In this work,
observations and numerical simulations are used to demonstrate and quantify
such effect in PNe. The study aims to assist the interpretation of observations
and their comparison to models. The analysis shows that observed R(Br$\gamma$)
ratios reach only values up to 0.3 when the slit encompasses the entire nebula.
Values higher than that are only obtained when the slit covers a limited region
around the H$_2$ peak emission and the Br$\gamma$ emission is then minimised.
The numerical simulations presented show that, when the effect of the slit
configuration is taken into account, photoionization models can reproduce the
whole range of observed R(Br$\gamma$) in PNe, as well as the behaviour
described above. The argument that shocks are needed to explain the higher
values of R(Br$\gamma$) is thus not valid. Therefore, this ratio is not a good
indicator of the H$_2$ excitation mechanism as suggested in the literature.
