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[filmscanners] RE: scanning at less than optical res



Art,

Bob clarified what he was referring to in a later post, which you may have
seen.  The gist of it was that the post on this list was a repost of a
response he made on another list where the original poster said that they
were essentially cropping a smaller portion of an image from a larger one
during the scan.  His reference to the large format scan was to a scan of
the total image versus reducing the scan to the cropped portion only.

> My UMAX Astra 1200S (Flatbed) definitely, at least in the direction of
> the scan "head", doesn't scan full optical if you choose a lower than
> optic resolution.  I can both see and hear the stepper motor change
> number or size of the discreet steps it uses.

My understanding is that the horizontal line of sensors define the optical
resolution and this definition is uneffected by the step motor operation
which controls the verticle movement of the horizintal line of sensors,
which are fixed.  Thus the size of the steps taken may change and probably
do, but the effective optical resolution remains the same and the reduction
in the size of the sample is a software instituted operation performed on
the full optical scan.

filmscanners_owner@halftone.co.uk wrote:
> Maybe it should have been called "larger file scanning" or Larger
>   format output scanning?
>
> It would be nice to know which scanners did what when lower resolution
> modes are selected.
>
> My UMAX Astra 1200S (Flatbed) definitely, at least in the direction of
> the scan "head", doesn't scan full optical if you choose a lower than
> optic resolution.  I can both see and hear the stepper motor change
> number or size of the discreet steps it uses.  I don't know what it
> does
> in the other direction, but based upon the speed difference, I'm
> guessing it is not interpolation from a full optical scan, but instead
> just ignoring the intermediate ones.
>
> Art
>
> Laurie Solomon wrote:
>
>> Yes, I did understand that; but I did not think that too many
>> scanners out there used this approach except to produce low
>> resolution preview scans. But I could be wrong.  What I did not get
>> and may have been a terminological confusion was his reference to
>> "large format scanning" with respect to the discussion.  I suppose
>> he really meant to say - now that I re-examine it - "high resolution
>> scanning" as contrasted to "large format scanning."
>>
>>
>> ----- Original Message -----
>> From: "Arthur Entlich" <artistic@telus.net>
>> To: <laurie@advancenet.net>
>> Sent: Friday, December 10, 2004 5:58 AM
>> Subject: [filmscanners] Re: scanning at less than optical res
>>
>>
>> I believe what Bob is stating is that some scanners literally skip
>> lines
>> or sensors and just record the spaced information, rather than taking
>> the full resolution and then averaging the pixels out via a series of
>> algorithms.  This, of course, would introduce a great many sampling
>> errors, since it is a much coarser sample of the actual data.
>>
>> Some scanners do seem to do just this, as the resulting scan takes as
>> little as one third the time to be scanned and produced as a raster
>> image when the resolution is quartered.
>>
>> Art
>>
>> LAURIE SOLOMON wrote:
>>
>>
>>>> If you scan at 1200dpi, the scanner usually either samples all the
>>>> 4800 possible data points per inch and throws three out of every
>>>> four away, or only samples every fourth possible point. So you are
>>>> only getting one quarter of the possible data from the film. So
>>>> why scan at large format if you are throwing three quarters of the
>>>> film data away?
>>>
>>>
>>> Bob, I beliee you are correct; but I do not understand your
>>> question. What do you mean by "scan at large format" in this case?
>>> I must have missed something in the discussion.  The first
>>> method,which you note, involves the actual sampling of original
>>> data using sampling algorithms and does result in a loss of
>>> ortiginal data; but the second method, which you speak of with
>>> respect to Vuescan and is available in almost all other scnning
>>> applications, involves resampling of the original sample data using
>>> formulas for combining and recombining data on the basis of all
>>> existing data and the formulas.  Both methods, however, would
>>> involve the scanner reading during the scan all 4800 points; so
>>> both would involve a "scan at large format" - using your terms - or
>>> whatever optical format is used by the scanner. After the scan,
>>> everything else by way of sampling or resampling is either digital
>>> conversion via hardware or software generated.
>>>
>>> -----Original Message-----
>>> From: filmscanners_owner@halftone.co.uk
>>> [mailto:filmscanners_owner@halftone.co.uk]On Behalf Of Bob Frost
>>> Sent: Thursday, December 09, 2004 4:10 AM
>>> To: laurie@advancenet.net
>>> Subject: [filmscanners] scanning at less than optical res
>>>
>>>
>>> Is this correct?
>>>
>>>
>>> "As I understand things, a scanner with an
>>> optical resolution of 4800dpi can take a sample reading every
>>> 1/4800 of an inch. If you scan at the optical resolution, that is
>>> what is does and you get 4800 readings per inch along that axis
>>> (usually a different resolution on the other axis).
>>>
>>> If you scan at 1200dpi, the scanner usually either samples all the
>>> 4800 possible data points per inch and throws three out of every
>>> four away, or only samples every fourth possible point. So you are
>>> only getting one quarter of the possible data from the film. So why
>>> scan at large format if you are throwing three quarters of the film
>>> data away?
>>>
>>> With Vuescan software, you can set it to scan all 4800 data points
>>> per inch, but then to take the average of every four data points
>>> and reduce them to one, so that the file you get out is the
>>> equivalent of a 1200dpi scan, but all the data points have
>>> contributed to the final result."
>>>
>>> Bob Frost.
>>>
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>>
>>
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