  The near-infrared emission lines of Fe$^{+}$ at 1.257, 1.321, and 1.644
$\mu$m share the same upper level; their ratios can then be exploited to derive
the extinction to a line emitting region once the relevant spontaneous emission
coefficients are known. This is commonly done, normally from low-resolution
spectra, in observations of shocked gas from jets driven by Young Stellar
Objects. In this paper we review this method, provide the relevant equations,
and test it by analyzing high-resolution ($R \sim 50000$) near-infrared spectra
oftwo young stars, namely the Herbig Be star HD 200775 and the Be star V1478
Cyg, which exhibit intense emission lines. The spectra were obtained with the
new GIANO echelle spectrograph at the Telescopio Nazionale Galileo. Notably,
the high-resolution spectra allowed checking the effects of overlapping
telluric absorption lines. A set of various determinations of the Einstein
coefficients are compared to show how much the available computations affect
extinction derivation. The most recently obtained values are probably good
enough to allow reddening determination within 1 visual mag of accuracy.
Furthermore, we show that [FeII] line ratios from low-resolution pure
emission-line spectra in general are likely to be in error due to the
impossibility to properly account for telluric absorption lines. If
low-resolution spectra are used for reddening determinations, we advice that
the ratio 1.644/1.257, rather than 1.644/1.321, should be used, being less
affected by the effects of telluric absorption lines.
