Generally, weakly exposed crystals have smaller silver clusters. Silver clusters of smaller sizes have a higher Fermi level, and therefore more crystals are developed as the developer's reduction potential is increased. However, again, the developer potential must be well below the conduction band of silver halide crystal. Thus there is a limit in increasing the photographic speed of the system by boosting the developer potential; if the solution's reduction potential is set high enough to exploit smaller silver cluster, at some point the solution begins to reduce silver halide crystals regardless of exposure. This is called fog, which is metallic silver made from non-imagewise (exposure-nonspecific) reduction of silver halide crystals. It was also found that, when developer solution is optimally formulated, the maximum photographic speed is rather insensitive to the choice of developing agent (James 1945), and there exists a limit for the size of silver cluster that can be developed.