Crop factor
Digital single-lens reflex cameras (DSLRs) use image sensors that come in a variety of sizes; the size of the sensor determines the field of view that can be captured by a given lens. When the sensor is smaller, the field of view becomes more restricted. The term "crop factor" refers to the process of changing the real focal length of a lens into its effective focal length. The crop factor for Micro Four Thirds cameras is 2x, but the crop factor for the most majority of digital single-lens reflex cameras (DX and APS-C) is either 1.5x or 1.6x. Full-frame digital single-lens reflex cameras do not require focal length conversion, hence their crop factor is always 1.
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