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[filmscanners] Sharpening and JPEG/TIFF (was: Color spaces for different purposes)



Don, your support of Ken is a bit misplaced. TIFF vs. JPEG is non
sequitur, Anthony is correct.  This is about the pixels in the image,
not about the file format in which it's saved.

When an unsharp mask (a.k.a. sharpening) is applied to an image, it is
enhancing the contrast of "edges" or areas of rapid level transition by
lightening the light side, and darkening the dark side. This is done to
the actual pixels in an image, independent of the image size resolution
setting. The software analyzes the image and applies the sharpening
effect within a certain pixel distance; this is the "Radius" setting in
Photoshop's Unsharp mask, a typical distance could be e.g. 0.7 pixels.

If you open your 27 MB file and the low res catalog scan, then apply the
same unsharp mask to both, the edges enhancement is applied to the same
distance in pixels around that edge in the image. These pixels, however,
represent quite different distance in the image. If you view both at
100% resolution, and both happened to have a narrow feature 3 pixels
wide in both, they both would be appear "sharpened" exactly the same.

Now, naturally, a feature 3 pixels wide in the low-res image would be
something close to 15 pixels wide in the high-res image. Therefore the
edge-enhancing effect would appear much more pronounced on the low-res
one. If you compared them side-by-side, your catalog scan might be all
visible in the window when viewed at 100%; while the high-res would have
to be zoomed out to 20% of actual, and the sharpening effect would be
miniaturized on screen and be far less noticable.

This is why you should never apply the unsharp masking on your high-res
scans until the final target use of the image is known, and, if
necessary, the image is resampled down for that use.  For example, if
you print a 360dpi image on a high quality inkjet printer on glossy
media, you would need just a little unsharp masking, whereas printing
the same image on offset press where the 4-color process screening will
make images appear much softer you would need to apply a much stronger
unsharp mask for the same final apparent crispness.

If this same image was used for web, you would first downsample it to
72dpi, then unsharp mask it for appropriate level of crispness at that
resolution.

Sami

Ken wrote:
>>> ... but could someone offer a technical explanation
>>> of why sharpening has so much more visible effect
>>> on jpegs as opposed to TIFFs?

At 10:22 AM 09/06/2002 +0200, Anthony wrote:
>> It doesn't.

On Sunday, June 9, 2002, at 07:46  AM, Don Marcotte wrote:
> I support Ken. I'm currently scanning a large number of rolls of
> negative
> film. They are just 10x.6.67 inch by 72 ppi images for screen display.
> I'm
> keeping them in an electronic catalog of my images. Unless something has
> changed in Photo Shop 7, which I recently acquired, sharpening is much
> more
> noticeable on these small JPEGs than on 27MB TIFFs that I use for
> printing
> or creating slides. I would like to emphasize the word "visible" in
> Ken's
> question.

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