[Klaus]

Not sure how a 70-200mm f4 FF equivalent is the closest thing...  :o

Unless of course soLong was talking about an four thirds or MFT platform, and in 4/3rds or MFT terms regarding aperture and focal length...

 

But knowing that he has a Sony FF and a Canon APS-C..

 

It will be probably not feasible in a normal design sense, because the back element of the zoom would need to be too big. One could think of a compressor lens group, like with the metabones speed booster, which makes lenses 0.71x wider.

Then one would design a 63-120mm f1.7 lens, and make the compressor group make it a 45-85mm f1.2 lens. One would have to design the 63-120mm groups far enough away from the lens mount though, to leave room for that compressor group. And one would need to oversize the lens if it had to be usable on FF too, else on FF the compressor group would make it vignette. All in all it would be a very big and expensive lens. 

The 4/3rds 35-100mm f2 basically is a 70-200mm f4 lens in its basic layout, with diverging elements included in it, compressing the whole to 35-100mm f2.

 

Wrong format - specs within the idea.

An equivalent FF lens - thus 'just' bigger elements would have double the size/weight roughly.

Make it a f/1.4 then we are talking about a factor of about 4.