in case it is of importance
according to the in depth review
the 16 - 85 has a maximum aperture of
3.5 from 16 to 20ish
4.0 from 20ish to 30ish
4.5 from 30ish to 60ish
5.6 from 60ish to 85
"Optically, the lens is of the variable aperture type. At 16mm the lens has a F3.5 maximum aperture. This drops gradually as the lens is zoomed toward 85mm, where the maximum aperture will have narrowed to F5.6. This is a typical design for all but the most expensive zoom lenses and helps keep the size and weight in check. You do get a darker viewfinder at the long end and one cannot achieve as shallow adepth of field as with, say, an F2.8 lens.
The maximum available aperture changes with the focal length as shown in the diagram below, which also compares the 18-85mm to other lenses, including the faster Sigma 17-70mm "Contemporary":
Read more at:
HD Pentax-DA 16-85mm F3.5-5.6 Review - Construction and Handling | PentaxForums.com Reviews