Adjusting focus on a HP Scanjet 4890

[john rehn]

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.

Jfyi if u happen to have a HP scanner.

 

[Don]

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.

Thanks for sharing that! Always interesting to read stuff like that.
Well, for me anyway...

There was a short thread a while back about removing the glass
completely when scanning film on a flatbed. That causes some
complications (dust, for one) but it also has many advantages (no
glass to obstruct the light, for example).

I haven't done that because I have a film scanner, but you may want to
chase that thread up. The reason it caught my attention was because I
was wrestling with glossy photographs at the time which caused
Newton's rings by "sticking" to the glass.

Don.