That doesn't seem right. Out of focus blur is a function of focal length and aperture. The sensor size only dictates the crop. In this case, the lens widest s

That doesn't seem right. Out of focus blur is a function of focal length and aperture. The sensor size only dictates the crop. In this case, the lens widest setting with a 1.4 extender is 98mm @ f/4. On a full frame sensor, something 10ft behind a in-focus 40ft away thing would still be very recognisable. Since a crop sensor is just a crop, I don't think it would have a different effect. The 1200mm focal length equivalent only applies to the fov radius, not to the blurriness. What's your definition of "pure blur" then?