[filmscanners] Re: Understanding dpi +

["Brad Davis" <DocBradd@mac.com>]

Art,

Thanks for the verification, and your comments re: "coming from a printing
(offset) background" make sense, the use of the term dot means something
quite different there.

The rest of your discussion relates to what I understand as the convolution
function relating any sensor attempting to acquire information from an
analogue signal (film, the magnetic domains on tape, whatever).  Not wanting
to go into that, I was attempting to simplify to verify my understanding of
scanners My experience is with very sensitive differential measurements (on
the order of on microvolt) from human subjects using an A/D, and doing the
necessary filtering digitally, ultimately going to an FFT.

Your discussion has brought up two questions.  The first is the apparent
grain in a photograph (say when displayed at "actual pixels in photoshop).
There are times when I have purposely used grainy film, and gotten quite
satisfactory results (i.e. grainy as if I were printing such a negative). I
had always assumed that I was seeing the "grain" - I know, dye clouds, and
grain or dye clouds, are really the result of clumping etc. - lets not go
there).  Am I seeing grain, or an artifact of the inherent grain and the
convolution with the sensor (as you clearly describe below)?

Second question is this.  I've seen scanners advertised with less resolution
(dpi or ppi) for use with 6X7 negs.  Simple calculation indicates that given
the lesser resolution (say 2800), the resulting size of the file is similar
to a higher resolution scan of 35 mm - say about 65 Meg + (it will be
somewhat larger, but the nominal dimension of 6000 ppi for the long
dimension on 35 MM is similar to the 7000 + ppi for a 6X7 neg.

That implies to me that such a scanner (again all other things being equal)
doesn't produce greatly  higher quality images than I do with my higher
resolution scanner and 35 MM. This assumes that my negatives are of the
highest quality - I hope that Contax and Zeiss qualify.  It seems to me that
the potential information at 2800 ppi on a 6X7 is only slightly greater than
4000ppi on a 35 MM image.  Of course there is the effect of the aspect ratio
difference which will provide some more information, but nothing like the
straight up comparison of the two formats when used in a traditional
darkroom (which becomes just a comparison of image area).

There are some other questions too, but I will let them be for now.

Thanks

Brad
>
> 1) sampling accuracy lessens by more than the basic assumption when one
> is sampling irregular shaped, positioned and sized objects like grain or
> dye clouds (oh please, let us not get into what's grain and what's dye
> clouds again!)  (this is something often discussed as Grain Aliasing).
> Basically, this is because the larger the area being sampled "under" any
> one "recording element" the more the information is "averaged" since the
> sensor is unable to resolve the differences of shape and color when they
> are all under that one sensor element. Each sensor element is a one
> pixel "eye".  It can only "see" the information under it as one color
> (made up of the average R G and B values) and will report it as a square
> regardless of the shapes under it. (assuming square sensor elements)
>
> I could show you this more clearly if I could do it graphically.
>
> 2) how much of the area of the image is actually being sampled under
> each element does matter as well. If I had an area one CM square, and
> each sensor element was exactly 1mm x 1mm with no border, a full 100
> samples of the area would give me 100 "average readings" covering the
> full area.
>
> If I have a one CM square, and each sensor element is only .1 mm x .1 mm
> square, and it takes the same 100 samples evenly spaced over the one CM
> square, I have only sampled 1/100th of the total area.  Although each
> sample itself is a more accurate rendition of the R G B  values directly
> under it (much less averaging) the process has completely ignored 99% of
> the total surface area from being measured, and the accuracy of the
> sampled area relative to the actual surface may be quite incorrect.
>
> If you use an image manipulation program, like Photoshop, and select a
> one pixel eyedropper resolution, and randomly select points within an
> area that may appear gray when looking at the image at screen
> resolution, each sample may prove very different than the "gray" you are
> seeing.  You may end up with red, or green, or blue or yellow or ...
> anything in between, and although accurate for this exact pixel
> location, it gives a very poor average of the gray you are seeing.
>
> So, yes, there are surrounding issues, and no, the answer is rarely as
> cut and dry as it may appear, but as Einstein alluded to: If you can't
> explain the concept to a child, you probably don't fully understand it
> yourself (believe me I don't, but I try...)
>
> I need some air ;-)
>
> Art
>


>
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