The “Optical Axis” passes through the centre of the lens and the image sensor. It is the axis about which you can rotate the camera and you will still get the same image at all points on the rotation. (True only for the centre of the image because there will be rotational size variation for landscape vs. portrait view sizes).
The optical axis of the camera is generally a straight line passing through the lens to the sensor. It is a theoretical line. However, it is also coincident with the “optical path” through the lens. The optical path may vary after the lens due to use of a mirror in the DSLR camera. So, the two terms are not identical.
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