  Filtergrams obtained in Ca II H, Ca II K and H-alpha are often employed as
diagnostics of the solar chromosphere. However, the vastly disparate appearance
between the typical filtergrams in these different lines calls into question
the nature of what is actually being observed.
  We investigate the lack of obvious structures of magnetic origin such as
fibrils and mottles in on-disk Ca II H and K images by directly comparing a
temporal sequence of classical Ca II K filtergrams with a co-spatial and
co-temporal sequence of spectrally resolved Ca II 854.2 images obtained with
the Interferometric Bidimensional Spectrometer (IBIS), considering the effect
of both the spectral and spatial smearing. We find that the lack of fine
magnetic structuring in Ca II K filtergrams, even with the narrowest available
filters, is due to observational effects. Signatures of fibrils remain however
in the temporal evolution of the filtergrams, in particular with the evidence
of magnetic shadows around the network elements. The Ca II K filtergrams do not
appear, however, to properly reflect the high-frequency behavior of the
chromosphere. Using the same analysis, we find no significant chromospheric
signature in the Hinode/SOT Ca II H quiet-Sun filtergrams.
  The picture provided by H-alpha and Ca II 854.2, which show significant
portions of the chromosphere dominated by magnetic structuring, appears to
reflect the true and essential nature of the solar chromosphere. Data which do
not resolve, spatially or spectrally, this aspect may misrepresent the behavior
the chromosphere.
