In article <5d2ku0$q9h@bmerhc5e.bnr.ca>, jgao@bnr.ca says...

>Most modern zoom lenses have variable apertures. For example: 28 - 200 mm
>f3.5 - 4.5 etc. The only thing I am sure is that it has f3.5 for 28mm,
>f4.5 for 200mm. But what about for the focus length in between. It's
>unlikely to have a interpolated aperture for every single focus length.
>But then how do I know at which focus length the aperture abruptly changes >from f3.5 to f4.5?

The maximum aperture does shift smoothly as the lens is zoomed...
In practice, this is often not as bad as it seems, since greater
illumination roll-off at the shorter end of the zoom range
compensates somewhat for the speed change. Many variable-aperture
zooms (when using color negative materials, or even sometimes B & W)
can be used without exposure compensation (manual exposure mode) if
metering is done at medium to longer FL's in the zoom's range. Of
course, if auto modes are used (or TTL flash), the compensations
will be made automatically.
Hope This Helps