[filmscanners] Re: Help with Vuescan?

["Arthur Entlich" <artistic-1@shaw.ca>]

This is a good question, but one which has and will evoke a lot of opinions.

I do scan in 16 bit/color because it lessens errors.  Let me see if I
can explain this in lay terms without someone complaining about my
terminology or usage.

Imagine a gradient of reds going from the darkest (let's call it black)
to the lightest (let's call it white).  For sake of this example, in
between you have 16 steps of reds, which we call 0 for the darkest
(black) and 15 for the lightest (white).  Now, let's say you have an
image which has a red that by this scale of 0-15 is 8.2 another 8.4 and
another 8.51.  What happens to those reds?  Well, if there are only 16
steps, the 8.2 and 8.4 get averages and become 8 (the same color), but
the 8.51 becomes  step 9.  So, by limiting the number of steps we have
introduced errors.

As I'm sure you recognize, the finer those steps between black and
white, the more accurate to the original source the reds will be.  If
there are 128 steps, or 256 steps or 512 steps, and so one, we become
closer to the source color without having to average very far in either
direction.

SO, you're saying fine, but if the printer only can reproduce 16 steps
and it gets converted back to those wide "averaged" steps anyway, why
bother?

And you would be correct, assuming you scanned the image and then sent
it right to your printer.  But, most people do not.  They change curves,
and levels, and color balance, and change brightness and contrast and so
on before they finalize the image to send to the printer.  Now imagine
if in each of those processes mentioned in the prior sentence, these
averaging errors were created which threw any one pixel further and
further away from the original source color, in other words, you are
multiplying the error with each process.

So, what we want to do is minimize the errors made via this averaging up
or down by having the least amount of movement to get to the next color
level.  And, you will notice that even in some pre-Version 6 Photoshop
versions, that certain key processes like the ones I mentioned, work in
16 bit mode.

Once all that work is accomplished with minimal error compounding damage
to the file, then and only then, you do a final conversion to 8 bit for
printing. (which in principle is using 24 bit (256 levels per color),
although inkjet printers can even accomplish that due to the size of
each cell and the number of ink colors available), but the do a darn
good illusion of 24 bit anyway.


Art

Titus Tucan wrote:

> Dear Arthur I agree with you, if we want to print, or save an image as JPG,
> we have to convert it to 8 bit, but there is something I do not understand.
> If at the end we convert the image to 8 bit because printing devices cannot
> beneficiate from the 16 bit depth what's the use of scanning in 16 bit?
> Wouldn't an image scanned with 8 bit colour depth be the same with an image
> scanned at16 bit and converted in PS at 8 bit?
> Or in other words what are the advantages of scanning in 16 bit?
>
> Thanks a lot
>
> Titus
>
>
> ----- Original Message -----
> From: "Arthur Entlich" <artistic-1@shaw.ca>
> To: <titust@sympatico.ca>
> Sent: Friday, April 05, 2002 4:35 AM
> Subject: [filmscanners] Re: Help with Vuescan?
>
>
> In general, it is suggested that USM or sharpening be the last thing
> done on an image (other than perhaps some cloning to fix defects that
> appear as a result of the sharpening.)
>
> So, by the time this is to be applied, all the color balancing, level
> and curves adjustments, etc, should have been completed in 16 bit.  I
> see no advantage to keeping an image in 16 bit mode for printing
> purposes.  No printing technique has the color range to make use of 16
> bit color depth (I'm assuming you are using an RGB or other color image
> with 16 bits per color (48 bit) or is it a monochromic image at 16 bits
> total?)
>
> Also, if you are only trying to see if the image benefits from
> sharpening, why not convert it to 8 bit within Photoshop, and try
> sharpening it?  Save it as a separate image file, which does no damage
> to the original 16 bit image.  If it works well, complete whatever other
> manipulation you need to for the image in 16 bit mode, and make sure the
> sharpening is the last process, so changing to 8 bit has no influence on
> the final result.
>
> Art
>
> Alan Harper wrote:
>
>
>>I have a 16 bit scan of a photo that's on the large side (10,000 x
>>7,000 pixels), and I want to see if it benefits from sharpening.
>>Photoshop seems to be unable to sharpen a 16-bit image. I tried to
>>"scan from disk" in Vuescan and use the sharpening there (and also
>>tried grain reduction). Vuescan may have improved these aspects of
>>the photo, but the exposure and saturation went completely kerflooy
>>(a technical term meaning awful). Anyone have some guidelines for
>>settings for Vuescan that will apply sharpening and/or grain
>>reduction, but not change the exposure, color, etc at all?
>>
>>If not, I'll continue in Photoshop.
>>
>>Vuescan for Mac OS-X if that matters.
>>
>>Alan
>>
>>
>
>
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