Re: Is the EPSON 4990 really 16 bit?

On Fri, 27 Jan 2006 07:50:16 -0500, rafe b <rafebATspeakeasy.net>
wrote:

>>Finally, open each. Don't bother looking at the picture, just
>>take a look at the histogram. There will be (should be) no
>>comparison between the two. Last, reduce the 16 bit image to
>>8 bits, and compare once more... the difference will be even
>>more apparent.
>
>Right. Now take that image with the "gappy"
>histogram and add the slightest touch of
>gaussian noise. Voila. Gaps gone.

And so has the quality! (BTW, another "fix" is to change image size,
even by a pixel, because interpolation will also eliminate gaps.)

But the proper way to correct such problems is not by *corrupting* the
image further but at source i.e. scanning at higher bit depth.

>Besides: you haven't shown that an image
>with a "gappy" histogram looks better or
>worse than one with a "smooth" histogram.

That doesn't really prove anything because it depends on image
content.

Therefore, there are two options: Either, only shoot/process images
whose content does not expose those gaps (not a realistic option) or
make sure "gaps" are not present in the first place (use 16-bit) and
tshoot/process anything you like with one less thing to worry about.

>Where limited bit-depth will bite you is
>in regions of near-monochrome, for example
>in clear blue skies.

Exactly! In other words: Image content.

>My take on all this: scan at 16 bits,
>do your major color moves/corrections,
>save at 8-bit.

On that we agree 100%!

>OR, simply do you major
>color moves in the scanner driver, and
>go 8-bit the rest of the way.

The only caveat is to make sure the driver works on 16-bit originals.

>Or if you don't mind the consequences
>(bigger files, more processing time)
>by all means go 16-bit.

It's certainly a good idea if for no other reasons but to be able to
archive such image and be "future proof". Not only will the lossless
digital format freeze any further film deterioration but saving at
16-bit will enable the image to be reprocessed later once high
definition range monitors become common place.

Don.
.

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