Calculate fstop at any given focal length?

[NothingRare]

I have a 85-300mm SUN zoom lens (circa 1982), without any markings for aperture range. (Such as f/5-7.2 or whatever)


The widest f/stop is 5, so how do I calculate f/stop at ___mm focal length?

Or is it one of those fixed f-stop lenses that remain at the same f/number at any focal length?

Thanks

[Mark1616]

As you go from the 85mm setting to 300mm the widest aperture will change from f5 to 7.2 at certain points. I'm not familiar with the lens so have no idea at what points it changes. Putting it on a camera should show where the aperture is going from f5 to 5.6 to 6.3 etc.