[filmscanners] Re: Understanding dpi

["Tris Schuler" <tristanjohn@mindspring.com>]

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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