[filmscanners] Re: HP PhotsSmart - questions

["" <wbgilloolyjr@charter.net>]

You are not wrong, but you have very much oversimplified things.

Basically, the optical resolution of a scanner is determined by how many
pixels the sensor (CCD,CMOS) captures and how wide a strip the scanner
scans.

So, a scanner with a 5,000 element (pixel) sensor that covered a strip
1" wide would be a 5,000ppi scanner (5,000pixels/1"=5,000pixels/inch).
If the same 5,000 element sensor covered an 8.5" wide (full-bed) strip,
you'd have an 588ppi scanner (5,000pixels/8.5"=588.25pixels/inch).

There is, of course a lens between the artwork/flatbed and the sensor
and the lens-sensor assembly is very much like a camera.  Just as you
can move closer or farther away from a subject refocusing the lens to
make sure everything is sharp a scanner can move the sensor-lens
assembly closer (for higher resolution) or farther away (to cover a
wider area).  It must refocus to achieve maximum sharpness.  Not all
scanners do this (in fact pretty few), but some can.

Any scanner can downsample to achieve lower resolution than optical,
thats why optical resolution is so important.  Refocusing to capture a
wider strip is NOT the same as downsampling.

My original description is the most accurate way of describing the
focusing procedure for any optical assembly (including your camera).
Your lens to film (or CCD/CMOS) distance that you set while "focusing"
is actually setting the magnification ratio.  Your distance from the
subject sets what's in focus.  This isn't obvious for large distances
(greater than about 2'), you can move your self then "focus" the lens.
But anyone that does a lot of macro photography knows that moving the
camera-lens assembly forward and back to achieve focusing after setting
the lens is much easier and more straightforward.  This is why a scanner
with variable optical resolution sets the lens to sensor distance first
(to achieve magnification ratio/strip width/optical resolution - they
can be used interchangeably here) then moves the lens-sensor assembly to
achieve focus.  Scanners work at macro distances as your reproduction
ratio is usually about 1:1.

Mr. Bill


Laurie Solomon wrote:
>>If the optical resolution is variable and YES there are
>>scanners that can do it
>
>
> I will take your word for it; but according to my understanding, the optical
> resolution has little to do with the distance between the lens and the
> sensor and more to do with the size and capacity of the sensor.  The
> effective resolution may change with the changes in the distances between
> the lens and the sensor; but the actual native hardware optical resolution
> remains the same.  But I could be wrong in my understanding; I am not an
> engineer.

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