Okay, just in case anyone else looks at this topic, it turns out that my working was correct and i was just using mislabled FOV reference images to test, which made me think otherwise!

The formula would be:

Code:

Scaled size = ( tan ( FOV_scaling_TO / 2) * size_of_original_image_dimension ) / tan ( FOV_scaling_FROM / 2 )

Or more simply, if you just want a scaling factor, treat the size of the original dimensions as 1, so:

Code:

Scaling factor = tan( FOV_scaling_TO / 2) / tan( FOV_scaling_FROM / 2 )