A crop sensor is any image sensor smaller than the full-frame 36x24mm reference size derived from 35mm film. APS-C (approximately 23.5×15.6mm on Sony, Fujifilm, and Nikon; 22.3×14.9mm on Canon), Micro Four Thirds (17.3x13mm, used by Olympus/OM System and Panasonic), and 1-inch (13.2×8.8mm, used in some Sony RX-series and most premium compacts) are all crop sensors. The term “crop” describes how the smaller sensor captures only a portion of what a full-frame lens would project, equivalent to cropping the center of a full-frame image.
The most consequential effect of a smaller sensor is the crop factor: the multiplier that converts a lens’s stated focal length to its effective field of view on the crop body. APS-C cameras have a crop factor of 1.5x (1.6x on Canon), Micro Four Thirds is 2x, and 1-inch is roughly 2.7x. A 50mm lens behaves like a 75mm on APS-C Sony, an 80mm on Canon APS-C, a 100mm on Micro Four Thirds, and a 135mm on 1-inch. This is the practical reason crop-sensor shooters use wider physical focal lengths to achieve the same composition.
Depth of field behaves differently across sensor sizes for an equivalent framing. To achieve the same composition (subject the same size in the frame) and the same depth of field, a crop-sensor camera needs a wider aperture than a full-frame camera. A full-frame 50mm at f/2.8 has roughly the equivalent depth of field of a 35mm at f/1.8 on APS-C, or a 25mm at f/1.4 on Micro Four Thirds. This is why full-frame remains popular for portraits and shallow-DOF work: matching the look requires expensive ultra-wide-aperture lenses on smaller formats.
Crop sensors offer countervailing advantages. Lenses can be smaller and lighter because they need to project a smaller image circle. Telephoto reach is greater for the same lens (a 300mm becomes a 450mm-equivalent on APS-C), which is why wildlife and sports shooters often prefer crop bodies for distant subjects. Burst rates and buffer depth can be higher because there is less data per frame to process. Body costs and lens system costs are typically lower across the catalog.
Image quality at high ISO traditionally favored full-frame because the larger sensor area captures more light per pixel at the same megapixel count. The gap has narrowed considerably with backside-illuminated sensors and on-chip ADCs. A 26-megapixel Fujifilm X-T5 or 26-megapixel Sony a6700 produces clean results to ISO 6400 that would have been unthinkable on APS-C a decade ago. Micro Four Thirds remains roughly one stop behind APS-C and two stops behind full-frame in clean-ISO performance, a gap that has been stable for several generations.
The choice between crop and full-frame is increasingly a system decision rather than an image-quality one. For travel, hiking, street, and wildlife work where size matters, crop sensors win on portability per equivalent focal length. For shallow-DOF portrait, wedding, and landscape work where aperture matters, full-frame retains an edge. The 1-inch sensor sits in a separate niche: compact-camera quality (premium phones, the Sony RX100 series, the Canon G7X) where pocketability trumps everything. Medium format (44x33mm and larger) sits at the other extreme, with crop factors below 1.0x because the sensor is larger than 35mm full-frame.