HP 4850 out of focus in slide scanning

Hallo!

I have a HP 4850 and I have bad slide/negative scans. I think it is a focus
problem.
I found this post; he explain how to adjust focus, but I do not understand
how!! I am italian, but I ask you: is it wroten in a correct English?
Do you understand?

"Some month ago I bought a HP Scanjet 4890 with the intention to scan my
slides. The resolution was not what I expected.
I measured the resolution using the "slanted edge" method ( see some other
threads in this group ). It looked more like the resolution of a 1200 dpi
scanner. Got no help from HP support. The reaction to my measurements was
basically "that's the way it is"
After some testing I made a guess that it was an "out of focus" problem and
that the focus position was some mm below the glass plate. So I made the
decision: "make it or brake it". Opened up the scanner, removed the cylinder
with the lenses.
Shorted the cylinder with 1 mm in the end that was closest to the detector.
That way I had the possibility to adjust focus
to a "more far" position. Well as it turned out 1 mm was too much but 0.2 mm
was about right. That moved the focus about 3 mm to a position about 1 mm
above the glass plate. Just right for my framed slides. It is slightly out
of focus when scanning images, but I have not any need for max resolution
with images. One other thing I found is that the focus position is moving
with temperature, so make any testing with a warmed up scanner. I guess that
the HP Scanjet 4850 have the same problem."

Thank you a lot
E.