  The purpose of this article is to show that the standard method of
introducing the quantum description of the electromagnetic field -- by
canonical field quantization -- is not the only one. We have chosen instead as
the starting point the relativistic quantum mechanics of photons. Our present
understanding of the nature of photons significantly differs from what has been
known years ago when the concept of a photon has only been emerging. We show
how the description of photons treated as elementary particles merges smoothly
with the classical description of the electromagnetic field and leads finally
to the full theory of the quantized electromagnetic field.
