difficulty drum scanning negatives

I sent some negatives and slides to drum scan to have the operator
claim that negatives show more grain in the final scan than slides. I
used 6x6 Fuji NPS 160, a film has low granularity rating. The other
film I used was E100G slide film. I find it hard to believe the
operator's claim. It seems that he is doing something wrong. What
could it be and how to get the best scan out of my negatives?
By the way, they use Crosfield drum scanners.

thanks
J

This sounds like grain aliasing. The mathematics of this are rather complex
because it involves the MTF of the scanner spot and lens, the line spacing
of the scanner, and the grain size distribution of the film being scanned.
It occurs when the grain size is small enought that it exceeds the Nyquist
limit of the sampling process. It results in the high frequency portions of
the grain being duplicated as lower frequency noise, and adds to the normal
low frequency component of the granularity. The result is an apparent
increase in granularity.

There are only two solutions to this that I know of. The first is to
introduce an anti-aliasing filter in the optical path of the scanner. This
almost has to be done by the manufacturer of the scanner, as it must be
carefully matched to the MTF of the spot and optics. The second solution is
to scan with a higher lpi and a smaller spot size (and better lens MTF). If
that can be done with the scanner that you are currently using, you're in
business. Otherwise, you will need to find a scanner that can handle film
with the small grain size that you have.

Don



"Jytzel" wrote in message
m...
I sent some negatives and slides to drum scan to have the operator
claim that negatives show more grain in the final scan than slides. I
used 6x6 Fuji NPS 160, a film has low granularity rating. The other
film I used was E100G slide film. I find it hard to believe the
operator's claim. It seems that he is doing something wrong. What
could it be and how to get the best scan out of my negatives?
By the way, they use Crosfield drum scanners.

thanks
J

In article , Don
writes
This sounds like grain aliasing. The mathematics of this are rather complex
because it involves the MTF of the scanner spot and lens, the line spacing
of the scanner, and the grain size distribution of the film being scanned.
It occurs when the grain size is small enought that it exceeds the Nyquist
limit of the sampling process. It results in the high frequency portions of
the grain being duplicated as lower frequency noise, and adds to the normal
low frequency component of the granularity. The result is an apparent
increase in granularity.

There are only two solutions to this that I know of. The first is to
introduce an anti-aliasing filter in the optical path of the scanner. This
almost has to be done by the manufacturer of the scanner, as it must be
carefully matched to the MTF of the spot and optics. The second solution is
to scan with a higher lpi and a smaller spot size (and better lens MTF). If
that can be done with the scanner that you are currently using, you're in
business. Otherwise, you will need to find a scanner that can handle film
with the small grain size that you have.

With a drum scanner the spot size (and it's shape) *is* the anti-alias
filter, and the only one that is needed. One of the most useful
features of most drum scanners is that the spot size can be adjusted
independently of the sampling density to obtain the optimum trade-off
between resolution and aliasing to suit the media being used, but there
is usually an automatic option which will achieve a compromise at least
as good as any CCD device.

I doubt that this is just aliasing though, especially if both were
scanned at 4000ppi or more. Remember that negative images are
compressed on film (the corollary being that negative film has more
exposure latitude and the ability to capture a wider tonal range).
Consequently, when producing a positive from the film image, whether by
scanning or by conventional chemical printing techniques, the image must
be contrast stretched. So, even if the grain on the film has the same
amplitude as the same as in slide film (a reasonable assumption for
similar speed films of the same generation from the same manufacturer,
the resulting image from the negative will always appear more grainy
than the image from the slide film.

There is a lot of truth in what the drum operator told Jytzel. Whether
its the truth, the whole truth and nothing but the truth is another
story. ;-) However, when viewed at 100% scaling, the size of the
original has little bearing on the results so I would expect to see more
grain on the 6x6cm negative image than from the 35mm slide under those
conditions.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

Kennedy McEwen wrote in message ...
In article , Don
writes
This sounds like grain aliasing. The mathematics of this are rather complex
because it involves the MTF of the scanner spot and lens, the line spacing
of the scanner, and the grain size distribution of the film being scanned.
It occurs when the grain size is small enought that it exceeds the Nyquist
limit of the sampling process. It results in the high frequency portions of
the grain being duplicated as lower frequency noise, and adds to the normal
low frequency component of the granularity. The result is an apparent
increase in granularity.

There are only two solutions to this that I know of. The first is to
introduce an anti-aliasing filter in the optical path of the scanner. This
almost has to be done by the manufacturer of the scanner, as it must be
carefully matched to the MTF of the spot and optics. The second solution is
to scan with a higher lpi and a smaller spot size (and better lens MTF). If
that can be done with the scanner that you are currently using, you're in
business. Otherwise, you will need to find a scanner that can handle film
with the small grain size that you have.

With a drum scanner the spot size (and it's shape) *is* the anti-alias
filter, and the only one that is needed. One of the most useful
features of most drum scanners is that the spot size can be adjusted
independently of the sampling density to obtain the optimum trade-off
between resolution and aliasing to suit the media being used, but there
is usually an automatic option which will achieve a compromise at least
as good as any CCD device.

I doubt that this is just aliasing though, especially if both were
scanned at 4000ppi or more. Remember that negative images are
compressed on film (the corollary being that negative film has more
exposure latitude and the ability to capture a wider tonal range).
Consequently, when producing a positive from the film image, whether by
scanning or by conventional chemical printing techniques, the image must
be contrast stretched. So, even if the grain on the film has the same
amplitude as the same as in slide film (a reasonable assumption for
similar speed films of the same generation from the same manufacturer,
the resulting image from the negative will always appear more grainy
than the image from the slide film.

There is a lot of truth in what the drum operator told Jytzel. Whether
its the truth, the whole truth and nothing but the truth is another
story. ;-) However, when viewed at 100% scaling, the size of the
original has little bearing on the results so I would expect to see more
grain on the 6x6cm negative image than from the 35mm slide under those
conditions.

thanks Kennedy,

Now I need some definitions of some terms: "spot size", "sampling
density", and "grain aliasing". And how can I tell if it's real
amplified grain or "grain-alaising"? Is there any solution to this
problem or should I give up using negatives altogether?

J.

In article , Jytzel
writes

Now I need some definitions of some terms: "spot size", "sampling
density", and "grain aliasing".

Spot size: the size of the scanning spot which each sample in the scan
is averaged over. Usually this is of the order of a few microns in
diameter at the film plane, and anything from 3 to 10um are commonplace.
If the spot is a uniform circle, then the photomultiplier in the scanner
produces a signal which is proportional to the average illumination over
the area of the spot. More commonly, the spot has a gaussian profile or
something similar, so the average is weighted accordingly.

Sampling density: the density that the samples are taken, which is what
you would usually refer to as the pixels per inch on the image. Many
novices assume that the spot size and the sample pitch (ie. the inverse
of the sample density) should be the same for optimum image resolution,
but fairly simple diagrams demonstrate that this is not the case.

A spot can resolve image detail which is finer than the diameter of the
spot, or the side of the spot if it is square or rectangular (as in the
elements of a CCD scanner). However a sampled system cannot
unambiguously resolve detail which has a spatial frequency greater than
half the sampling density. Anything which the spot can resolve but the
sampling system cannot is aliased, and appears at a greater scale (if
the frequency extends over many samples, this can be a much greater
scale) than in the original.

Quite often, a scan is obtained at which all of the image detail is
fully resolved by both the spot and the sampling system, however the
latter is inadequate to resolve the grain from which the image is
composed, but the spot can resolve it. As a result the grain is
aliased. This is especially true of well defined grain with sharp
structures - the edges of the grains produce the spurious high spatial
frequencies which are aliased. However, since the grain is random and
smaller than the spot size, each aliased grain only extends over a
single pixel in the image - but this can be many times larger than the
actual grain on the original. Consequently the scanned image can appear
much more grainy than a chemically produced equivalent.

For some examples of this, see http://www.photoscientia.co.uk/Grain.htm

Part of the skill of the drum scan operator is adjusting the spot or
aperture size to optimally discriminate between the grain and the image
detail for particular film types, however some film types are difficult,
if not impossible to achieve satisfactory discrimination.

And how can I tell if it's real
amplified grain or "grain-alaising"?

Well, that's not so easy because once something, including grain, has
aliased there is no way to tell from the resultant image whether it is
an aliased artefact or not. In some cases, additional knowledge of the
scene may help - you know, for example, that the bricks on a wall do not
have that large pattern across them, or the colour fringing on that roof
isn't there in real life, but in general without anything to compare it
to, you just cannot say. Unfortunately grain in scanned images is just
like that - the only way to tell for sure if it is aliased is to compare
it to the original, unsampled, slide or negative - which usually means
comparing it to a conventional chemically and optically produced print
of the same size.

Is there any solution to this
problem or should I give up using negatives altogether?

There are several post scan filters around which purport to remove grain
from the image after it has been scanned. Examples are Neat Image and
Kodak's GEM. However, all ("all" being a relative term here!) that
these packages can do is analyse the image for fine random structure and
then remove as much of that as the user is prepared to tolerate. That
is fine if the grain is finer than the finest details in the image - the
two can be separated without loss of image detail. However, grain and
aliased grain cannot be finer than single pixel size thus, if your image
contains detail on the same scale (which it probably does, because that
is why you paid to have it scanned at such a fine resolution in the
first place) then you inevitably sacrifice image sharpness in the
process of removing or reducing the grain. How much you are prepared to
sacrifice is a compromise.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

Now I need some definitions of some terms: "spot size", "sampling
density", and "grain aliasing". And how can I tell if it's real
amplified grain or "grain-alaising"? Is there any solution to this
problem or should I give up using negatives altogether?


Spot size is the spot diameter. This is somewhat of a misnomer, since the
spots in commercial scanners are inevitably poorly formed. They are often
approximated as gaussian figures of revolution, but only for convenience -
they usually deviate from that in some important aspects. Ideally they
should be airy discs, but that is unachievable in the price range for
commercial labs or service bureaus. In most commercial scanners, 63% of the
spot energy is within a 3-8 micron diameter circle at the smallest
achievable spot size. When adjusted for larger spots, the spot shape
usually becomes less well defined.

Higher quality scanners with better spot shape control exist, but are
generally unavailable to the public. Scanning microdensitometers are an
example, though not necessarily optimum.

Sampling density is the spots (or scan lines) per millimeter. The scanning
density should be at least twice the resolution that you're trying to
maintain from the film. Current high resolution commercially available
color negative film can reach advertised resolutions of over 100 line
pairs/mm (high contrast), but with an affordable camera/lens combination
would rarely achieve over 70-80 or so on axis, less at the field edges.
Black & white films can be twice that. Consumer grade color films from the
1950s achieved maybe half that at best.

Aliasing (of any type, including of grain) was described by Nyquist in his
papers on information sampling. It arises when information is sampled less
frequently than the details that exist in the data, i.e. twice the highest
frequency in the data. For example, if you sampled a 60 hertz sine wave at
exactly 60 samples per second, each sample would occur at the same point on
the curve, and you would conclude that you had a DC signal, not a 60 hertz
AC signal. Without going into great depth here, sampling at anything below
twice the highest frequency contained in the data will cause the data to
later be reconstructed with the highest frequencies repoduced as erroneous
lower frequencies, with the resulting distortions. It can be avoided by
filtering out the high frequency data before sampling, an almost universal
practice in all sampling systems except photography, where it is usually
done crudely at best due to the difficulty and cost.

Radio engineers call this effect hetrodyning. Physicists and other
engineers call it intermodulation. Photographers call it aliasing. It is
the source of the "jaggies" you see on straight edges in improperly
digitized imagery as well as other problems. Grain aliasing is also a form
of this, and is caused by using scan dot spacings too far apart for the
grain sizes, without using a proper low pass filter in the image stream,
e.g. a properly shaped scanning spot. A good commercial drum scanner
operator (or the scanner often does it automatically) tries to matrch the
spot size to the line spacing. Unfortunately, the more-or-less gaussian
spot shape is not a very good low-pass filter. When sized to adequately
reduce information that exceeds the Nyquist limit it also considerably
reduces the in-band information that produces the fine detail that you would
like to keep.

The only practical solution to this is to oversample the image, i.e use a
sample spacing and spot size that are much smaller than necessary, and then
down-sample the result using an algorithm which approximates an optimum
filter. While this sounds good, in practice it is hard to do with
commercial grade equipment and fine grain films. Films with an average
grain size of 4 microns will have a significant fraction of the grain at
less than half that size. A scanning density of 1000 lines/mm or so (25,000
lines per inch) with a spot size on the order of 1 micron would be required,
and the resulting file size would be huge, nearly 5 gigabytes for a 35 mm
negative scanned with 16 bit depth. This would have to be stored
uncompressed (or with lossless compression) untill after the downsampling
was done. Also, the computer doing the downsampling would have to cope with
a file size that large - pretty much beyond the ability of current 32 bit
chips in common workstation use. And the whole operation would be slooooow.

The upshot is, practically speaking, accept the grain as a fact of life,
whatever the source. You might want to try several service bureaus, as the
quality of the equipment and competence of the operators does vary.

Don

I had to adjust focus on my drum scanner, an Optronics Falcon, when scanning
negs to avoid high apearant grain. But once I made this slight focus
adjustment the results were excellent
http://www.jonlayephotography.com

"Jytzel" wrote in message
m...
I sent some negatives and slides to drum scan to have the operator
claim that negatives show more grain in the final scan than slides. I
used 6x6 Fuji NPS 160, a film has low granularity rating. The other
film I used was E100G slide film. I find it hard to believe the
operator's claim. It seems that he is doing something wrong. What
could it be and how to get the best scan out of my negatives?

I also find that negative materials scan grainier than slide films (although
I haven't tried either of those films). Try shooting some Reala
Konica-Minolta Impressa 50.

Here's a page with a lot of scan samples to get an idea of what to expect.

http://www.terrapinphoto.com/jmdavis/

David J. Littleboy
Tokyo, Japan

In article ,
(Jytzel) wrote:

I sent some negatives and slides to drum scan to have the operator
claim that negatives show more grain in the final scan than slides. I
used 6x6 Fuji NPS 160, a film has low granularity rating. The other
film I used was E100G slide film. I find it hard to believe the
operator's claim. It seems that he is doing something wrong. What
could it be and how to get the best scan out of my negatives?
By the way, they use Crosfield drum scanners.

thanks
J

I find NPS to be really a horrible film to scan for whatever reason,
E100 films are T grain color emulsion. NPH (400 asa) scans better
than NPS.
--
LF website http://members.bellatlantic.net/~gblank

For best results expand this window at least 6" at 1152 x 768 resolution

Gregory W Blank wrote:
In article ,
(Jytzel) wrote:


I sent some negatives and slides to drum scan to have the operator
claim that negatives show more grain in the final scan than slides. I
used 6x6 Fuji NPS 160, a film has low granularity rating. The other
film I used was E100G slide film. I find it hard to believe the
operator's claim. It seems that he is doing something wrong. What
could it be and how to get the best scan out of my negatives?
By the way, they use Crosfield drum scanners.

thanks
J


I find NPS to be really a horrible film to scan for whatever reason,
E100 films are T grain color emulsion. NPH (400 asa) scans better
than NPS.

What are the best films for scanning say one or two brands/types
in each of these categories:

B&W (what's best old tech, new tech, chromogenic)
Colour Consumer films ( what's best Slide or Negative?)
Colour Pro films (what's best Slide or Neagtive?)

Paul