Message-Id: <200205101507.g4AF7f715871@sitelicense.arizona.edu> Date: Fri, 10 May 2002 09:55:50 -0500 From: "John R. Stokes" <mailto:johnr@GRANDECOM.NET> Subject: Re: Help clarifying term To: mailto:IMAGELIB@listserv.arizona.edu
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Marta,
You may have encountered this phase in relation to digital cameras. As
opposed to "flatbed" scanners which specifies resolution in terms of
dpi, cameras use a sensor with a fixed aspect ratio. While they may be
used to capture a variety of material sizes, the size of the digital
image is fixed. For example, a digital camera with a 3K x 2K (3072 x
2048) pixels will produce an image of this size regardless of whether
the material being scanned is 3x5, 8x10, 11x14, 20x30 inches, etc. The
aspect ratio of the original determines the size of the largest digital
image that can be captured. The largest digital images that can be
captured is 1843x3072, 2048x2560, 2048xx2606, 2048x3072 pixels,
respectively, from the four originals. Note that, except for the 30x30
inch original which has the same aspect ratio as the camera sensor, the
digital image size is limited by one dimension of the sensor. The dpi of
each image is different, 614, 256, 186, and 102, respectively.
You may have run across this phrase in material from LC, JJT, Inc. or
myself. JJT, Inc. uses a digital camera with a rectangular array sensor
with piezo and stepper mechanisms to obtain to capture images at higher
resolutions. While theses camera, designed by the Lenz brother's,
provide several resolution options up to 9400x 10000 pixels, each option
is a fixed aspect ratio which is likely to be different from the
material being scanned. To capture a image at a given dpi, a resolution
option is selected that equals or exceeds this dpi in both directions.
For example, if the client specifies dpi for the scanning resolution of
400 dpi with respect to the originals and the originals vary in size up
to 11x17 inches, JJT would select the 7450x5700 pixel capture option,
position the camera to capture an image 18.6 inches wide (7450/400),
capture all the images with theses camera settings, and crop the excess
area from the digital image.
I do not believe dpi is an appropriate term to apply to digital images
captured from transmissive materials such as negatives and
transparences. Almost all these materials except 35mm negatives and
transparences contain more information than it is practical to capture.
Hence, on normally bases the resolution on the larges print one may want
to produce. A 5000 pixel image along the long dimension will produce a
very high quality 11x14 inch print, hence large format negatives and
transparences are scanned at this resolution. To assure originals of
various rations will fit within an image captured at this resolution,
the 5000 pixels along the long dimension is specified.
John R Stokes
-----Original Message-----
From: Marta Bernal Jimenez [mailto:mailto:mbernalj@INTER.EDU]
Sent: Thursday, May 09, 2002 11:53 AM
Subject: help clarifying term
Hi
I'm starting a digital project in which photographs and other
material of very different dimensions will be scanned. Looking through
some standards I've come to the phrase X amount of pixels "across the
long dimension" or "across the long side". I'm assuming this means that
the longer side (no matter if it is the high or length of the picture)
should be scanned at the X amount of pixels recommended. Am I right?
This is for the archival version of the material.
I will appreciate your comments
Marta I. Bernal
Interamerican University of Puerto Rico
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