MATH IS CORRECT, if you understand.

At first glance i thought the math of this calculator was off. Why was it giving smaller DOF measurements for cameras with smaller sensors? It should be the opposite. But then i realized the difference between focal length and angle of view. If you take two cameras with the same 50mm lens, one camera w/ a full frame 35mm sensor and the other with a 1.5x crop sensor, the DOF of the 1.5x crop camera will actually be slightly smaller. The reason is that even though the camera has a smaller sensor (and therefore greater DOF) that lens is now behaving like a 75mm lens. So it is providing the angle of view and the DOF of a 75mm lens on a full frame body. If you want to compare the same angle of view between the two cameras then you need to compensate for the crop factor. In other words a full frame body w/ a 75mm lens will have shallower DOF then a 1.5x crop body w/ a 50mm lens and both cameras will have the same angle of view.

emac2082 about Simple DoF Calculator