Our central nervous system is based on the generation and propagation of electrical signals along the neuronal pathways. These variations of the membrane potential are arranged by the concerted action of ion channels in the neuronal membrane. Therefore, the exact measurement of the electric field in the central nervous system is the focus of intensive investigation. While electrophysiological methods provide exact measurements on the single-cell or single-molecule level with high temporal resolution, they are limited in their spatial resolution ranging from a few single cells to a single molecule. To thoroughly understand how the voltage-dependent ion channels sense the membrane potential and are precisely gated by it, the electric field within the protein has to be investigated. Likewise, the propagation of electrical impulses in a network of neurons involves a large number of cells, which have to be monitored simultaneously. For these endeavors, optical methods have proven to be useful due to their scalability, temporal and spatial resolution. Here, we will summarize the properties of the optical probes that we used to determine the electrical field strength within voltage-sensitive ion channels and discuss the hybrid approach to detect membrane potential changes in genetically specified neurons in terms of design, limitations and future developments.