Many DSLRs and Mirrorless cameras have what’s called an APS-C sensor inside. The idea of an APS-C sensor is comparable to the 35mm full frame (35mm film) format and 645 (6×4.5 medium format) format digital sensors in that they’re all based on a film size. But what isn’t spoken about very much is APS-C sensor sizes and just how many other APS film standards there are–or were. Before digital photography came along, APS film was an option for photographers who wanted to work with a smaller format than 35mm. There were many different types of APS films for different reasons and needs.