An important feature of a camera is the sensor size (or format): this indicates the dimensions of the image sensor and its form factor. Typically, this parameter is expressed in inches (and fraction of inches). However, the actual dimensions of a sensor are different from the fraction value, which often causes confusion among users. This practice dates back to the 50’s at the time of TV camera tubes and is still the standard these days. The common 1" circular video camera tubes have a rectangular photo sensitive area about 16 mm diagonal, so a digital sensor with a 16 mm diagonal size is a 1"equivalent. Furthermore, it is always wise to check the sensor specifications, since even two sensors with the same format may have slightly different dimensions and aspect ratios.
Spatial resolution is the number of active elements (pixels) contained in the sensor area: the higher the resolution, the smaller the pixel size and, consequently, the smaller the detail that can be detected on the image. It is important to highlight that sensors can have the same dimensions but different resolution, since the pixel size can vary. Although for a given sensor format smaller pixels lead to higher resolution, smaller pixels are not always ideal since they are less sensitive to light and generate higher noise; also, the lens resolution and pixel size must always be properly matched to ensure optimal system performances.