There are quite a few variables that can cause blurry photos. Camera shake probably being the most common cause.
We can help eliminate this issue by having a shutter speed that at least matches the length of the lens.
On full frame camera, for example, a 50mm lens would need 1/50th shutter speed as a minimum. An 85mm lens would need at least 1/85th and a 200mm lens would need at least 1/200th shutter speed.
A crop sensor camera is usually 1.5x or 1.6x that of a full frame, making the 50mm lens have the same crop as a 75mm lens (1.5x). An 85mm would become 127.5mm (1.5x) and 200mm would become 300mm (1.5x). The shutter speeds would also match the new lens crop.