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[filmscanners] Re: Understanding dpi



At 01:37 PM 3/25/2004 -0600, you wrote:
>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

Bill, try to think of it this way. If a 2000-dpi scanner renders a given
negative as an image composed of a single dot (".") then a 4000-dpi scanner
would render the same negative as an image composed of four dots, two over
two (difficult to display on an ASCII list, but, if the formatting
cooperates, something like this):

         ..
         ..

So, if you blew up both images in Photoshop, say, to where you were able to
work with individual dots/pixels on the screen, the image from the 2000-dpi
scanner would afford you just one dot (screen pixel) of scanned information
to edit (manipulate) from any given area of the scanned negative whereas
the image from the 4000-dpi scanner would afford you four dots (pixels) of
information to edit (two dots across and two dots down).

This difference (fourfold) often represents a significant difference for
"critical" editing work with regard to printing pictures (the kind we
actually hang on our walls) at larger sizes (I'm not talking 4x6 stuff
here). For purposes of display of the final image online somewhere, this
difference, when boiled down to real-world constraints, may or may not be
relevant (visible). It just depends.

An example of where this added (4x) dot/pixel information comes in handy:
let's say you've sharpened (USM has been a recent topic of interest) an
image with someone's face the center of attention, and you're left with
some unattractive artifacts around the nose. It would be easier to go into
(blow up) the 4000-dpi image and "smooth out" (or if you will, more
attractively _blend_) those nose artifacts than it would be to do the same
thing with the 2000-dpi image for the reason that with the 4000-dpi image
you would have four times the information (four dots/pixels) to work with
as opposed to just one. It's impossible "blend" one dot/pixel. All one can
do is change any given unit to something else, yes? On the other hand, with
_four_ dots/pixels to work with the possibilities for change are relatively
much more various.

Hope that helps.

And yes, I often do go into my work and manipulate intransigent dots/pixels
one by torturous one. (I may not be especially talented but I always get an
A for effort. <g>)

Tris

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