  We calculate theoretical isochrones in a consistent way for five filters (K,
K', Ks, F205W, and F222M) in the atmospheric window between 1.9 and 2.5 micron
using the Padova stellar evolutionary models by Girardi et al. Even when
displayed in the same Vega magnitude system, the near-infrared colors of the
same isochrone can differ by up to 0.18 mag at its bright end, depending on the
filter. We present magnitude transformations between K-band filters as a
function of color from H and K band filters. Isochrones with extinction at K of
up to 6 mag are also presented. We find that care is needed when comparing
extinction values that are estimated using different filter sets in the K band,
in particular when comparing those of atmospheric and space filter sets:
extinction values for space filters can be in error by up to 0.3 mag. To reduce
this error, we introduce an "effective extinction slope" for each filter set
and isochrone model, which describes the extinction behavior of isochrones in
the color-magnitude diagram more correctly than the actual extinction law. Our
calculation also suggests that the extinction law implied by the observations
of Rieke et al. for wavelengths between the H and K bands is better described
by a power-law function with an exponent of 1.61 instead of 1.55, which is
commonly used with an assumption that the transmission functions of H and K
filters are Dirac delta functions.
