^ The point about shutter speeds and evaluative/matrix/multipattern metering is that in so many cameras with such metering the Tv and Av values are stepless, and so a rudimentary testing device could potentially give a false result. Of course with older medium- and large format cameras it could be useful — but it will still depend on having a grasp of technical knowledge to know by how much is too much or too little. Modern cameras are best tested/calibrated by technicians, not gizzmos found on eBay.