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     áòèé÷ :: Filmscanners
Filmscanners mailing list archive (filmscanners@halftone.co.uk)

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[filmscanners] Re: Understanding dpi




HI,

I thought I understood this, but somehow there is meaning getting lost - for
me at least.  The problem seems to be in the issues around "half-tone",
which isn't a part of my understanding of scanner resolution.

To make this concrete, I would like to make reference to my Polaroid SS4000
scanner.  Its listed optical resolution is 4000 dpi.  My bel9ief is that
this is not interpolated, but as I said, optical.  That implies, to me, that
the scanner is sampling my negative every 4000th of an inch.  Further, this
dimension works in both of the two orthogonal directions.  Since we are
talking about area here, that implies that there 4 times as many samples in
a given area than with 2000 dpi.

To make this even more concrete, assume that a 35 MM negative is 1 in. X 1.5
in. ( I know, it isn't quite that, but this is an approximation).  Then the
number of samples taken on a negative is 4000 dpi X 1 in. multiplied by 4000
dpi X 1.5 in.  This gives 4000 dots X 6000 dots (inches cancel out) which
equals 24 X 10^6 or 24,000,000 dots (there is no reference to the
representation of 3 colors here).  Similarly, if the resolution is 2000 dpi,
then the same calculation (2000 dpi X 1 in. X 2000 dpi X 1.5 in.) is
6,000,000 dots (again, no representation of tricolor).  Thus, a 4,000 dpi
scanner has a linear resolution that is twice that of 2000 dpi, and the
total resolution for my Polaroid scanner is 4 times that of a 2000 dpi
scanner.  That last sentence is not quite right, we get into issues of the
definition of resolution.  I would like to ignore that for this topic, and
leave aside the issues of MTF.  KISS.

To me, that implies that, everything else being equal, I can blow up an
image to a size that is twice the linear dimension of a print made from a
2000 dpi scan, or equivalently, 4 times the area (example would be from 8X10
to 16X20).  In practical terms, this is probably not quite true as it would
assume a perfect negative.  I am aware that my Polaroid picks up grain
nicely, and that if I want 16X20 quality, I had better be using one of my
better lenses (all Zeiss), at one of the lenses better apertures.  This also
assumes equal viewing distance, an unwarranted assumption, one usually views
a larger print from a greater distance, which changes some of issues here.

If this understanding is incorrect in any material way - material to the
basic question of a simple comparison of 2000 dpi vs. 4000 dpi, no other
issues considered - I would like to be enlightened - but then wouldn't we
all...


Thanks,


Brad

--Brad Davis, Ph.D.
5924 W. Second St.
Rio Linda, CA 95673
Ph (916) 991 5011

'A cucumber is bitter.' Throw it away. 'There are briars in the road.' Turn
aside from them. This is enough. Do not add, 'And why were such things made
in the world?'
                    Marcus Aurelius
                    --Meditations




> At 01:26 PM 4/25/2004 -0500, you wrote:
>> If I understand what you are saying, I think that I cannot agree with your
>> explanation. Your analogy appears to be confounding halftone dots with
>> halftone cells.  Moreover, it is not necessarily the case that either
>> translate one-to-one into pixels or into samples. Also I believe that if
>> your analogy was correct, the 2000 dpi would represent a halftone cell
>> consisting of two dots while the 4000dpi would be a halftone cell with 4
>> dots, such that there would be 2000 cells per inch (or 4000 halftone dots
>> per inch) versus 4000 cells per inch (or 16000 halftone cells per inch).
>>
>> First, technically there is a difference between dpi which is usually used
>> in reference to resolution in terms of halftone dots or cells per line per
>> inch ( or lpi -lines per inch in printing press terms) on a printed page
>> versus ppi which is used with regards to pixels which typically refer to
>> resolutions in terms of picture elements in a monitor display versus spi or
>> samples per inch which refers to resolutions in terms of the number of
>> samples captured by a capture device such as a scanner or digital camera
>> from the original subject.  Often and usually wrongly, these measures and
>> terms are used interchangabley as if they were identical or equivalent to
>> one another.
>
> I agree with the above in a casual sense, although you seem to want to make
> a worst-case scenario with regard to different working resolutions on the
> monitor and whatnot. I assumed a state of "other things being equal" in
> order to give Bill what I consider to be a practical (i.e. useful) answer
> to his query.
>
>> Second, the key factor in determining the quality and equivalance of scanner
>> resolutions is the difference between the native optical resolution of the
>> scanner (whatever terminology is used to define the units per inch) and
>> interpolated resolutions or software generated resolutions of the scanner
>> (whatever terminology is used to define the units per inch).  The former
>> comprises the actual scanner resolution as opposed to some mathematically
>> generated derivative of the actual resolution.
>
> I'm afraid you're slipping away from "plain English" here. Given Bill's
> basic quandary, what purpose could this sort of techspeak have?
>
>> Third, with respect to output resolutions and the original question, the
>> quality of a scanner and its output is as much determined by the bit depth
>> of the scanner ( i.e., the dynamic range of tone that the scanner can
>> capture and recognize and the capacity of the scanner to recognize tonal
>> distinctions within that dynamic range and discern or differentiate those
>> distinctions from noise) and the quality of the scanners design, sensors,
>> and hardware componets as by the optical resolution if it capable of
>> capturing and out putting at.
>
> Yes. But. With Bill's question in mind ("I'm a bit perplexed at what the
> dpi means on a film scanner. Trying to compare apples to apples, will a
> 4000 dpi Brand X film scanner in theory produce a better quality image
> outputted than a 2000 dpi Brand X scanner, given that the output resolution
> is the same, say 1600 x 2400 pixels? Or does it simply mean the 4000 dpi
> scanner will output a much larger image than the 2000 dpi model? Thanks for
> clearing this up, Bill") my response assumed our samples derived from the
> same scanner or very similar technology only were rendered at different
> resolutions. Pointedly, my response wanted only to explain (as effectively
> as possible) what the real-world difference between these two scan
> resolutions would likely be for our prospective editor (Bill) of said
> images once he began to wrestle with them inside of Photoshop, say.
>
> From my perspective Bill asked a simple question and so I dutifully gave
> him a simple answer. The rest may be researched at one's convenience, of
> course, but I think at present Bill and other with similar questions just
> need the basics in order to get on with it.
>
> I enjoy your attention to detail and willingness to take pains, but there's
> a time and place for everything, I believe.
>
> Tris
>
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