Hello,
Let's assume your zoom lens is from 70-200mm ya?
A constant f2.8 means you can use an constant aperture value of f2.8 throughout 70mm to 200mm. This doesn't mean that it's the only aperture value you can use. Depending on the lens, you can use it from f2.8 to f22.
For a f2.8-4 lens, it means that the minimum fstop you can use at 70mm is f2.8. However, at 200mm, the minimum will be f4 and not f2.8. So in summary,
for a f2.8 lens,
at 70mm : 2.8 - 22
at 200mm : 2.8 - 22
and
for a f2.8-4 lens,
at 70mm : f2.8 - f22
at 200mm : f4 - f22
Yeah?
If it's wrong, please be gentle with me, I'm a book. :sweatsm: