A nAPS-C sensor is approximately 1.6x smaller than a full-frame sensor, so it’s exposed to a smaller area of the image projected by the lens. This ‘crop factor’ needs to be taken into account when choosing and using lenses. If you mount a lens with a focal length of 100mm on to a camera with an APS-C sensor, for example, you get a view that’s equivalent to 160mm on a full-frame camera. In order to get the equivalent 100mm full-frame view, you’d need to use a focal length of around 60mm on an APS-C camera (60mm x 1.6 = 98mm).
The APS-C extra reach has advantages for long-lens photography, such as wildlife and sports. It can often be difficult to get physically close to some animals and athletes, but the crop factor of APS-C means that distant subjects look bigger in the frame.