One first needs to differentiate based on an 8 bit file of what type? A TIFF
or a JPEG? The reason is that there are two things in question here. One is
resolution and the other is color detail (meaning subtle differences in shade).
An 8 bit TIFF file, whether uncompressed or compressed contains 8 bits of
color info for every pixel that was represented in the original raw file. And
8 bits of color info for each RGB pixel is more color detail than can be
printed by any existing print process on any existing paper. So, the short
answer for a TIFF file is that there is no difference at all between an 8 bit
and 16 bit file used to make a print. That assumes, however, that any editing
of pixel brightness was done in 16 bit form before conversion to 8 bit. But,
for editing at the pixel brightness level, there is a definite difference
between 8 and 16 bit. Keep your intermediate editing files in 16 bit form and
your final output files in 8 bit form.
Now the answer for a JPEG... it depends. It depends on the degree of
compression. JPEG compression is what's called "lossy" compression. "Lossy"
means that the compression algorithm is allowed to discard some information
based on the level of compression... what me might call fidelity or the level
of correctness. I'll make up a hypothetical case here. Let's say that there
is a string of 567 blue pixels with brightness 254 followed by one blue pixel
with brightness 253 followed by another 338 blue pixels of brightness value
254. Do you think you can spot the difference if the single pixel of 253 is
replaced with another one with value 254? At a very high correctness level the
253 might be retained. At a lower level it might be discarded and replaced
with 254 meaning that the entire string of blue pixels can now be recorded more
simply as a contiguous string of 906 blue pixels with value 254. That's not
really the way that JPEG compression works but the analogy should
suffice. Lossy compression algorithms discard information to simplify and
reduce the amount of data required to reproduce the image. At low fidelity
levels you can clearly see the difference from an uncompressed version. At
high fidelity levels you may not be able to tell the difference at all even
though the file is much smaller. So the answer for the 8 bit JPEG is the same
as for the 8 bit TIFF provided that the fidelity level is sufficiently high
that you can't see the difference. But there is no universal fidelity level.
Images with more detail in them to begin with will require a higher fidelity
level than a photo of blue sky and clouds.
Finally, regardless of the camera used to record a 35mm image on film, the
scanned image probably contains no more than about 6-8 megapixels of useful
image data. So a decent quality DSLR of 6-8 megapixels with a good lens (both
resolution and contrast) should be able to reproduce that 72" print. The
trick is that there really isn't a whole lot of detail in the 72" print. It
looks pretty good because you stand far away to view it. Come close and
you'll see the pixellation or the grain. Leica lenses are good but they are
not magic.
Chuck Norcutt
> -------Original Message-------
> From: John Hudson <OM4T@xxxxxxxxxxx>
> Subject: [OM] Re: 16 bit tiff
> Sent: Oct 05 '08 01:13
>
>
> As Agent Gibbs has a habit of saying "English please" to Abbs and Tim the
> Geek, what would be the bottom line consequence of having an 11 x 14, a 16 x
> 20, or a way bigger print made from an 8 bit file as compared to that from
> a 16 bit file?
>
> Back in the late 1960s I saw a huge .......... better than 72"' on the long
> side ............ black and white print made from a 35mm negative out of an
> M3 that look pretty damm good. In something of the same vein what would be
> the visual comparison between making that same print from an 8 bit or a 16
> bit file?
>
> This curious mind , and maybe those of others, want to know :-)
>
> jh
>
> ----- Original Message -----
> From: "Moose" <olymoose@xxxxxxxxx>
> To: <olympus@xxxxxxxxxx>
> Sent: Saturday, October 04, 2008 4:49 PM
> Subject: [OM] Re: 16 bit tiff
>
>
> > John Hermanson wrote:
> >> I had been using Raw Shooter Essentials for a while but am now trying out
> >> Raw Therapee. Is there any benefit to saving a processed ORF as a 16 bit
> >> tiff instead of 8 bit? I don't see it, but 16 bit is twice the file
> >> size, 47 meg versus 23.5.
> >>
> >>
> > The sensor is an analog device, as are the first part of the sensor
> > electronics.
> >
> > The analog to digital converter divides up the analog voltages into
> > discrete digital steps.
> >
> > Most DSLRs use 12 bit A/D converters. some high end ones, including the
> > E-1 and, I assume, E-3, are 14 bit. Extending dynamic range, as some
> > newer sensor stems are now doing, requires more bits if midrange tonal
> > detail is not to be compromised.
> >
> > That translates to 4096 separate brightness step for each color at 12
> > bits and 16384 for 14 bit.
> >
> > If you convert that to an 8 bit TIFF, the luminance range is downsampled
> > to 256 steps, throwing away much of the image data you bought the camera
> > for. 16 bit's 65536 step range can accommodate all that comes out of
> > the camera, without loss.
> >
> > If an image is simply to be converted and viewed as a JPEG (compressed 8
> > bit) or printed on an 8 bit printer, 8 bit files are fine.
> >
> > As soon as one starts manipulating the file, even with simple tools like
> > Levels, Contrast, etc. the software needs to move values around in the
> > histogram. With only 256 levels to work with, that can lead to uneven
> > stepping effects, even to holes in the histogram (yup, I've seen 'em),
> > where working in 16 bit gives the algorithms plenty of steps for subtle
> > interpolation.
> >
> > In practice, some images seem to come through considerable manipulation
> > in 8 bit just fine, while others start to look "funny" in ways that I
> > find hard to describe. Even starting with an 8 bit image, as in images
> > from digicams with 8 bit JPEG output only or those downloaded from the
> > web, conversion to 16 bit before processing can make a difference.
> >
> > How do I know? I "always" convert 8 to 16 bit in PS as a first step in
> > editing images from my F30 or Moosterizing images from others. Except
> > sometimes I forget that first step - and start wondering why things
> > aren't working quite as I expect. A quick glance at the header leads to
> > a "DOH" moment and starting over.
> >
> > Why not 12 and 14 bit versions of TIFF? While certainly possible, such
> > files would be messier to process. Going from 8 to 16 bit simply uses
> > two bytes per color per pixel instead of one, whereas intermediate
> > numbers of bits would require splitting bytes between channels and/or
> > pixels. Given the way most programming languages/systems and file
> > systems work, that requires custom programming. Back in the dark ages, I
> > encoded and decoded 8 different yes/no bits of data into single bytes
> > for storage. With the low and dropping cost of storage, I don't see
> > anybody going to that kind of trouble today.
> >
> > There is another reason to use 16 bits to work on 12-14 bit source data
> > relating to the limited number of digital steps available to the lower
> > values from a linearly converted analog image, but I don't have the time
> > or brain power just now to try to explicate it. I seem to recall that
> > there's a good explanation on Luminous-Landscape.
> >
> > Bottom line - 8 bit throws away data you may or may not want later. If
> > you don't care, no problem. If you plan to retain the original ORFs and
> > software to convert them again later, if needed, that will work short
> > term. Long term, storage in a universal format like TIFF is a better
> > solution.
> >
> > Moose
> >
> > Moose
> >
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>
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