Originally posted by MossyRocks Unless you have a fixed maximum aperture zoom this is expected. On your lens
(I assume this is the one) the maximum aperture is f/3.5 which is likely at 18mm. As you zoom the length of the lens increases while the aperture opening remains the same making the total aperture value greater up to a value of f/5.6 at 135mm.
There are some zooms that have a fixed max aperture either because the opening varies as they zoom, or they are an internal zoom
like this lens.
If you are zoomed to 135mm and the aperture won't open up to f/5.6 then I would be concerned. If it only opens up to f/5.6 at 135mm then everything is normal and this is expected behavior.
I checked with my 18-135 mounted on my KP {below are approximate values}
At 18mm, it is a f/3.5-16 lens
At roughly 24mm, it becomes a f/4.0-16 lens
At roughly 35mm, it becomes a f/4.5 -16 lens
At roughly 75mm, it becomes a f/5.6-16 lens