[filmscanners] Re: HP PhotsSmart - questions

["Arthur Entlich" <artistik@shaw.ca>]

Laurie Solomon wrote:

> Maybe my math is bad; but if it has a native resolution of 2400 ppi/dpi
> scanning 1" film, then my math says it will have a native resolution
> scanning a 5 inch subject which is much lower than 300 ppi/dpi
> independent of the light path factors(e.g., around 75 ppi/dpi). For the
> size print that the original poster mentioned which was smaller ( but I
> forget the exact size but I think it may have been either a 3.5 x 5 or a
> 4 x 6), the native optical resolution would be in the range of about 150
> ppi/dpi to 300 ppi/dpi.
>

Your math is bad.  Let's assume the CCD line sensor is 1/2" across (this
dimension actually isn't relevant for this discussion, but I wanted to
give it a size), and it indeed has 2400 usable discrete sensor
locations.  Through the optical lens (forget about measurable ppi
ratings, we are speaking of "native", regardless of what the optics or
electronics do to it) an image of a band of the film width is projected
onto this CCD sensor.  If the film width is exactly 1", then 2400
sampling locations exist, so it records 2400 ppi in a 1" wide swath.

Now, imagine through changes of the optical path, the same system, but
this time a five inch wide image is also projected on this same 1/2"
sensor with 2400 discrete sensor locations.  The output is still 2400
pixels, but this time, across 5" rather than 1".  So, 2400 pixels wide,
divided now by 5 (inches) equals 480 ppi.  However, I imagine what
happens with this beast is that the image is "condensed" and projected
on to less than the full sensor width, and only about 1500 of the 2400
sensors are used, perhaps toward the middle sweet spot of the lens to
improve what is probably not a great design to begin with.

> But this is based on the assumption that a scanner can have variable
> native optical resolutions; however, to the best of my knowledge and
> understanding, scanners have a single native optical resolution.  The
> effective optical resolution is a by-product of the number of inches
> that one divides into the native optical resolution.  Thus, an
> enlargement of the image without any interpolative resampling will
> result in a lower effective resolution while the reduction of the image
> size without such sampling will result in a higher effective resolution.
>
> Nevertheless, it is still unclear to me if you are saying that the
> native OPTICAL resolution of this scanner is variable or not; and if
> not, if the native OPTICAL resolution of this scanner is 2400 ppi/dpi or
> something else that would produce an effective native resolution of 2400
> ppi/dpi when scanning a 1 inch horizontal length as opposed to some
> other horizontal length.
>
>

I basically agree with your description of optical native resolution.
As I believe you are stating, however, it is dependent upon the size of
the original. The number of discrete sensor locations within the line
sensor are finite and absolute (let's ignore double sampling or ganged
CCDs).  Assuming they are all used, no more than that number can be
resolved.  So, a 8" wide flatbed scanner which has a native resolution
of 600 dpi, has a line sensor with 600 x 8 discrete sensor locations
(per color or line) or 4800 sensors.  The same scanner might, with the
right optical path, scan a 1" wide 35mm negatives at 4800 dpi, using the
same sensor.  They often cannot do so, because of the nature of the
optical path which doesn't allow for the lens to resolve the outer areas
of the lens that well, so a sweet spot is used, only projecting to a
portion of the line CCD.

Art

>
> ----Original Message----
> From: filmscanners_owner@halftone.co.uk
> [mailto:filmscanners_owner@halftone.co.uk] On Behalf Of Arthur Entlich
> Sent: Monday, August 08, 2005 4:45 AM
> To: laurie@advancenet.net
> Subject: [filmscanners] Re: HP PhotsSmart - questions
>
>
>>The native optical resolution of this scanner varies
>>dependent upon the size of the image being scanned.  In the
>>case of 35mm film, which is just under 1" wide, the scanner
>>sensor/CCD scans at 2400 ppi/dpi.
>>However, when switched to reflective mode, the scanner can
>>scan up to 5 x 7" prints (I previously incorrectly noted
>>4x6").  In this mode the maximum is 300 ppi/dpi (although the
>>math implies it could scan up to about 450 ppi/dpi) but who
>>knows what kind of optical light path bending they had to do to
>>accomplish that.
>>
>>Art
>>
>>Laurie Solomon wrote:
>>
>>
>>>I looked at the web site you gave the link for; it was not clear from
>>>its contents as to what the unit's native optical resolution is.  If
>>>the native optical resolution is 150 dpi and the other resolutions
>>>are all interpolations, that might account for the reason that the
>>>150 is sharper than the 300 dpi.  Moreover, the screen resolution
>>>might also enter into the equation since the screen rendering of the
>>>image will be such as to make the 300 dpi scan be rendered on the
>>>monitor at twice the size as the 150 dpi scan which can result it
>>>some apparent fuzziness with the smaller rendering appearing sharper
>>>even at lower resolutions.
>>>
>>>The standard rule of thumb sage advice is to scan at the scanners
>>>optical resolution and not at an interpolated resolution to get the
>>>maximum sharpness and the minimum flaws, artifacts, and noise.
>>>
>>>But you have me a little confused.  You speak of scanning a 3x5
>>>print; but then you say you also had this negative roll scanned at
>>>Target. Are we talking about positive paper prints or film
>>>negatives?  They are two very different things.
>>>
>>>Unless you will be enlarging a hard copy print to a print size larger
>>>then the original or a portion of a cropped print to the size of the
>>>entire original print or larger, a 300 dpi is sufficient since hard
>>>copy prints typically do not yield resolutions greater then 300 dpi
>>>since the information is not there in the original to support a
>>>higher resolution with actual original data.  To scan 35mm film, one
>>>will normally scan it at a resolution of around 4000 dpi since the
>>>frames will typically be enlarged to at least 3.5 X so as to produce
>>>a 3.5 X 5 image at around 300 dpi.  A 1200 dpi scan of a 35mm film
>>>frame is a relatively low resolution to be scanning 35mm at and
>>>would require interpolation in the event that one wanted to enlarge
>>>the image in its entirety or in part.  Thus, Target is really not
>>>doing any better than your machine would do on a 35mm film frame.
>>>Moreover, we do not know if the 1200 dpi that Target scans at is
>>>real optical
>>
>>resolution or interpolated resolution.
>>
>>>
>>>>-----Original Message-----
>>>>From: filmscanners_owner@halftone.co.uk
>>>>[mailto:filmscanners_owner@halftone.co.uk] On Behalf Of Rich Koziol
>>>>Sent: Saturday, August 06, 2005 1:01 PM
>>>>To: laurie@advancenet.net
>>>>Subject: [filmscanners] Re: HP PhotsSmart - questions
>>>>
>>>>On 6 Aug 2005 at 12:06, Laurie Solomon wrote:
>>>>
>>>>
>>>>
>>>>>As for the question of " why 150 dpi appears sharper than
>>>>
>>>>300 dpi when
>>>>
>>>>
>>>>>scanning a 3 x 5 color print," you did not tell us if the
>>>>
>>>>result you
>>>>
>>>>
>>>>>speak of was on the monitor or on a hard copy print
>>>>
>>>>At this point I'm just looking at the results on a 19inch monitor.
>>>>Used the HP software to scan with.
>>>>
>>>>I also had this negative roll scanned at Target, for comparison.
>>>>Target offers 1200dpi scans for about $4/roll.  They just started
>>>>this service and are still somewhat sloppy with film handling.
>>>>
>>>>Rich
>>>>
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>>>
>>
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>
>
>

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