Observations on Scanning Transparent Media
by Oliver Seely
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The two images, left and right, from a color negative, were carried out at 1200 pixels per inch. The image on the left was captured by placing the color negative in the transparent media adapter(TMA) of an HP Scanjet 4570c scanner according to manufacturer's instructions. The image on the right used the neutral white light of the adapter, but the negative was placed under a glass plate with the emulsion side of the negative contacting the glass of the scanner. These two scans clearly show an image at the left somewhat out of focus compared to the one on the right. Directly below these two images are enlargements of the ones above giving a better presentation in which the image of the negative contacting the glass, although less blurry, is less than highest quality, likely due to a movement of the camera or other shortcoming during the original photograph.
Conjecture: The TMA raises the medium above the focal point of the
scanning optics, resulting in a blurrier image.
The image on the left is exposed B&W negative film scratched with a stylus on the emulsion side. The film was placed
in the TMA emulsion side down according to the manufacturer's instructions. The image on the
right is the same scratched negative placed emulsion side down on the glass of the scanner, then covered
with a piece of glass to keep it flat and the transparent media adapter light source placed above that. Note the
improved resolution of the image on the right.
Enlargements of the images above are shown at the left and right here.
Note the improved resolution of the image at the right. Recall that the emulsion was in contact with the
scanner glass.
The image at the left is a scratched slide placed in the TMA according to the manufacturer's instructions. The
television image is incidental. It was simply an old slide with some black on it which I needed for scratching.
The thing to look at
are the scratches, which in an ideal world would not be blurry or have fringes on either side. The image on the
right is of the same slide placed on the glass of the scanner and the light source of the TMA placed above it.
In this case the the film itself is not touching the scanner glass because of the thickness of the cardboard slide mount.
At left and right here are enlargements of small areas of the slides above, illustrating that the slide that was
placed on the glass of the scanner (right) shows slightly better scanning resolution.
On the left is the enlarged image of the scratched negative scanned with the emulsion touching the scanner glass.
On the right is the scratched slide placed against the scanner glass but raised by the thickness of the slide
mount. Both pictures have the same number of pixels per unit length across your screen. No desaturation of colors
was carried out on the enlargements
so as to enhance the impression of blurriness caused by the color fringes. It would seem, sadly, that
even a displacement by the thickness of the slide mount produces detectable defocusing of the image.
Department of Chemistry
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