macro magnification - 1" rule?

[n1as]

With an APS-C sensor, an item x inches long will fill the field of view at 1:x magnification.

Is this true? Have I done the math right?

If so, then it makes for an easy rule of thumb for figuring how mag is needed. If the flower is 2" across and you want it to fill the field of view, you need 1:2. If you want to fill half of the field of view, you'll need 1:4 magnification.

Yes?

[gasrocks]

1:1 means the area covering the frame is the same size as the sensor. FF cameras have (roughly) a 1" X 1.5" sensor. Then, you are correct, it is just math to guess what magnification you will need.