Quantum limits of measurability of the electromagnetic field

The method of integration over field configurations is applied to the problem of finding quantum restrictions on measurability of electromagnetic field strength. It is shown that there exist two different definitions of the field strength which coincide in classical theory but are distinct in quantum theory. They also correspond to different measurement schemes. In one case no absolute quantum limit exists but the corresponding measurement does not supply reliable information about the internal state of the field (but only about the external classical influence on the field). In the second case the measurement provides information about the internal state of the field but is restricted by quantum limit. Classical works by Landau and Peierls and Bohr and Rosenfeld are discussed in the light of the results obtained.