I was wondering, at what focal length with the 70-200mm f/4L will I'll be able to recreate the 'light blur' size of the 85mm f/1.8? (Both wide open so bokeh should be rather similar?).I came across this equation online that I wanted to confirm.


It said that if I wanted to recreate the same size 'light blur' I would have to have an equivalent 'blur disk' size which is calculated by Focal Length / Aperture.


So if I wanted to mimic the 85mm @f/1.8 - 85/1.8=47.22


I would have to have roughly 189mm @f/4 - 189/4=47.25


The camera to subject distance was obvious, but does this calculation assume that the subject to background distance remains the same?