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[filmscanners] RE: Understanding dpi



filmscanners_owner@halftone.co.uk wrote:
> 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.

You are technically correct.  Issues surrounding "halftone" are not directly
connected to scanner resolutions which are in terms of samples per inch
(spi) and only indirectly related to scanning resolutions in that one may
need to account for it in terms of screen frequencies to be applied to the
scanner resolution in figuring out what scanner resolution to use when
outputing to presses and devices that use RIPs or even to inkjet printers.
I only mentioned it in relation to scanning resolutions in order to
distinguish between the relation between such things as samples and pixels
and dots and halftone cells.  References to dpi often fails to distinguish
between if the reference is to actual dots (that are contained in "halftone
cells" or halftone cells (freguently termed "halftone dots") which may
contain multiple dots.  Furthermore, there is no necessary one-to-one
relationship between the number of samples or the number of pixels and the
number of actual dots or cells.


>
> 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.

You are correct on both accounts in your concrete example ( although that
may not be the case when reading the resolution specs for other scanners
which hype an interpolated resolution).

> 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.

In essence, this is correct
>
> 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.


My problem with this statement is that here you are confounding "samples"
with "dots" again [e.g., "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)"].  This gives rise to what is meant by
"dots" in this use; are they actual dots, halftone cells, or pixel
equivalents?  If they are actual dots without regard to representing the 3
colors, then the resoluton will be significantly less given that resolution
units do represent the 3 colors; if they are cells, then they do represent
the 3 colors and do represent the resolution.  On the other hand, if they
are pixel equivalents, they again represent the 3 colors; but each pixel may
comprise more than one halftone cell and be constituted by multiple actual
dots depending on the nature of the actual pixel and the colors contained in
that pixel.

>
> 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...

First, I am not sure that the original basic question was a comparision
between a 2000 spi and a 4000 spi scanner in terms of the latter allowing
one to enlarge the image size by 2x while maintaining an effective
resolution equivalent to what one would get from the original scan by the
2000 spi scanner.  If it was the basic question, I concur with you except in
the language used (e.g., spi, ppi, and dpi).  However, I read more into the
original question than just this. My understanding was that the comparision
was in terms of differences in quality in general, which includes bit depth
among other factors in addition to resolution per se.

Second, I think that one needs to be precise, clear, etc. when making
statements as opposed to oversimplifying or furnishing recipes.  While
questions may be simple that merely require a very simplified answer, I
believe it inadvisable to do this since one is (a) not merely responding to
the questioner alone but to many other readers of the response who may go
away with a misunderstanding and use that misunderstanding as a basis for
giving advice to or communicating with others who are novices as well, (b)
furnishing oversimplified information to both the original enquirer and
others whithout noting that the response is an over simplification that may
be applicable to limited cases or to the spicific question being asked and
may not be applicable to anything else which can result in mis-educating or
misleading novices, and (c) contributing to the misunderstanding of commonly
used technical terms and language which can cause those who are novices and
unfamiliar with the technical language to take actions, make decisions, or
spend money in ways that are ill advised.

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