A histogram is a visual representation of the tones in a photo. A histogram is a graph that shows the distribution of the range of tones from black to white. Histograms can be found in most cameras, and also in image editing software such as Lightroom. A histogram will tell you if a photo has been underexposed or overexposed, or if the exposure is OK. Every histogram is different, and the data in a histogram can help with image capture and processing.