Part 2 of 3
A scanner with sufficient resolution to directly scan a 6x6 cm negative at
1400 PPI costs over $4,000, which is too expensive for most of us. So, you
must somehow adapt a 600 DPI flatbed scanner for medium format work if you
want to scan medium format material yourself.
I use an inelegant but effective method for scanning 35mm and medium-format
negatives with my 600 PPI flatbed scanner: I make an 8"x10" interpositive on
sheet film and scan the interpositive. This sounds like a lot of trouble,
but it really isn't. You don't have to worry at all about dust, as PhotoShop
makes spots and other defects on negatives a trivial problem. And, as long
as the DMax of the interpositive is under about 2.0, the contrast range and
density of the interpositive aren't critical: PhotoShop and the scanner
software will compensate for these variations in density and contrast.
I use Kodak TMax100 8"x10" sheet film to make my interpositives. It is
relatively inexpensive and has the advantage of being panchromatic. So, I
can make interpositives from color film without losing information in the red
portion of the spectrum, as I would if I used one of the orthochromatic
process films which many use for negative enlargement.
Another advantage of TMax is its availability. Film manufacturers are
discontinuing many process films in sheet film sizes; you can be fairly sure
that TMax will be around for many years to come.
There is one more advantage to having your own scanner that is seldom
mentioned in discussions on the merits of service bureau scans versus desktop
scanners. You can use a desktop scanner as a transmittance and a reflectance
densitometer. As a reference, you can scan a step tablet (transparency or
print) as a benchmark and make a curve comparing the optical densities of the
reference material with the PhotoShop electronic densities. Then, you can
scan the transparency or print you wish to evaluate and use the curve to
determine the density of any area in the image. A flatbed scanner with a
transparency adapter can be used in this manner for transmission densities
within a range of 0.10 to about 2.2 (In my experience, manufacturers' claims
of a dynamic range of 3.0 for low-end flatbed scanners is an exaggeration).
While the results are not as accurate as those obtained with a conventional
densitometer, they are adequate for most of our needs with alternative
printing methods.
MAKING DIGITAL NEGATIVES
Once you've finished working your image in PhotoShop, you must somehow
translate what's on your monitor to a piece of transparent material which
you'll use as a negative. Here is a list of output devices which you may
use, listed in ascending order of their usefulness for alternative process
work; I'll discuss them individually.
1. Desktop laser or inkjet printers
2. Home computer monitor
3. Film recorder
4. Imagesetter
OUTPUT DEVICES: DESKTOP LASER AND INKJET PRINTERS
Both laser and inkjet printers can use a clear transparency material rather
than paper as their substrate; the output on this material can be used as a
negative. However, there are two limiting factors with both of these
printers which make their use for high quality negatives impractical: their
resolution is too low to allow for a continuous tone image with no trace of
grain or halftone dots, and their DMax is too low for many alternative
processes. The blacks on transparency material often have a gray, somewhat
irregular blotchy appearance. Consequently, at least with the current level
of printer technology they cannot be seriously considered as useful tools to
generate high quality negatives for alternative processes. This situation
could change soon with new generations of printers, such as the new 1440 x
720 DPI Epson inkjet which has been discussed recently on this list.
OUTPUT DEVICES: HOME COMPUTER MONITORS
For negatives 4"x5" or smaller, your computer monitor can be used as a film
recorder. resulting negative for contact printing. The method works best
with a grayscale rather than a color monitor, because of the higher sharpness
of the grayscale monitor.
Several alternative printing processes are inherently unsharp, as the image
is composed of a tangle of paper fibers impregnated with an opaque insoluble
product rather than pigment particles suspended in a homogeneous substrate
such as gelatin. These processes include platinum/palladium, cyanotype,
kallitype, and gum. For these "fuzzy" printing processes, a 200 PPI negative
provides enough resolution to make a contact print which looks very sharp and
has no trace of its digital origin, provided that there is no clear or empty
space between the dots on the negative.
The 20" monitor on a Macintosh has 1062 x 850 pixels of resolution at 4"x5"
proportions, providing a bit more than 200 PPI of resolution with the
3.75"x4.75" image size on a sheet of 4"x5" film. A 15" monitor set at 800x600
pixel resolution, which is a standard Macintosh and Windows setting, gives an
image size of 3"x4" at 200 DPI. If you don't mind your prints being a little
less than really sharp, you might be able to get by with a 4"x5" negative
from a 15" monitor.
If you are using a color monitor, you should consider sending a single-color
image (red, blue, or green) to the monitor via controls in PhotoShop. This
eliminates any loss of sharpness due to imperfect convergence of electron
guns in the CRT.
There are a few disadvantages to this procedure. As most monitor screens
aren't flat, straight lines near the edges of the image are a bit bowed.
This is a problem for architecture but shouldn't be noticeable for most
other subjects. The image on your screen should be slightly larger than the
image on the film, so that 100% of the area on the film is filled. Finally,
you must expose the film in a dark room to eliminate reflections and glare
off the screen.
Using a 20" grayscale monitor, I have made 4"x5" palladiotypes and cyanotypes
with this method, with excellent results; they are the equal of prints I make
with digital negatives from a high-end imagesetter at a service bureau. Even
with a loupe, there is nothing in the prints to give a hint of their digital
origin.
You can photograph a positive image directly off your computer screen and use
the negative to make your prints. Even if your goal is to make prints larger
than 4"x5", this is an inexpensive way to experiment with digital
photographic techniques and avoid the problems and expenses involved with
imagesetters and service bureaus.
OUTPUT DEVICES: FILM RECORDERS
A film recorder is simply a very high resolution B&W monitor with a built-in
camera to record the image off the screen. In contrast to a computer
monitor, which will have at most about 1100 pixels in a single horizontal
line, a film recorder has many thousands of pixels per line, resulting in
very high resolution images. With three images photographed sequentially
through red, blue, and green filters, they are used to make high quality
color film transparencies. The usual output from a high-end recorder is a
sheet of 4"x5" film, which is can then be used to make enlargements up to
16"x20" or more. These are expensive machines: the typical high-resolution
film recorder at a service bureau may cost $150,000.
The film recorder offers two advantages over other output devices. First, in
contrast to the imagesetter (see below), the film recorder does not require a
radical correction curve in order to get acceptable results for B&W printing;
this reflects the fact that its output was designed from the outset for
photographic film. Second, once you have a high resolution 4"x5" film
positive, you can make an enlaged negative of any contrast range and size you
want. This is an advantage for the photographer working with different
printing processes having different contrast requirements.
There are at least two disadvantages of film recorders for alternative
processes. First, their output is expensive: a service bureau typically
charges about $75.00 for a single 4"x5" film image. Second, unless you are
making small prints, you must use conventional techniques to enlarge the
4"x5" film positive to your final negative size. This negates many of the
advantages which digital techniques offer in the production of large
negatives for contact printing. Consequently, film recorders have seen
little use in the preparation of B&W negatives for contact printing.
Prices of film recorders have dropped sharply in recent months. For example,
Polaroid recently introduced an 8,000 line film recorder with a 4"x5" back
which sells for under $15,000. If the price/performance ratio on these
devices continues to improve in this manner, film recorders may become an
attractive alternative to imagesetters for B&W photography.
OUTPUT DEVICES: IMAGESETTERS
The imagesetter can be thought of as a very high resolution laser printer
that outputs to photosensitive film. In contrast to the 600 dots per inch
(DPI) of the typical laser printer, imagesetters have a maximal resolution of
up to about 5,000 DPI. The output of the laser is binary: it is either on or
off when it exposes the film. The film used in imagesetters is a
high-contrast, high-resolution Kodalith-like material with a very high DMax
(4.0 or greater). The image on the film is made up of closely spaced dots of
very high optical density.
The image on the film is constructed in one of two ways. Groups of dots may
be used to make halftone cells. In order to get 255 shades of gray, the
maximum number of halftone cells per inch that can be produced with a 5,000
DPI imagesetter is about 300. This frequency is traditionally stated in
lines per inch, or LPI. Platinum prints made from 300 LPI digital negatives
have a very smooth appearance, with virtually no visual clues as to their
digital origin. The dots are visible only if you examine the print under a
loupe.
Alternatively, the imagesetter can generate a seemingly random distribution
of dots, a method which has been termed stochastic screening. The "diffusion
dither bitmap" (DDBM) method described in Dan Burkholder's book is another
method for producing negatives with random dot distribution. Stochastic
screening eliminates the regular, mathematical array of dots that
characterizes the conventional halftone screen. But in my experience, even
with 5080 DPI imagesetters palladiotypes from both DDBM and stochastic
screening negatives have a rather grainy, gritty appearance which does not
occur in prints made from 300 LPI digital negatives.
Of all the currently available output devices, the imagesetter produces the
best results and is used by most workers using digital methods for
alternative processes. It has several advantages over other output devices.
Imagesetter negatives can be quite large, up to 20"x24" or more. The
negative is made of silver on a film base; consequently, its archival
properties are identical to those of traditional silver negatives. The
material can easily achieve the relatively high densities needed for many
alternative processes.
The price of digital negatives varies widely among different service bureaus.
The best prices I have found are at Command-P, where an 8"x10" negative
costs $20 and a 16"x20" is only $30. I usually put several images onto a
single 16"x20" negative (four 8"x10"s, sixteen 4"x5"s, etc.), so that the
cost per individual image is quite reasonable.
However, the imagesetter has one significant drawback which has made its
adaptation to B&W photographic printing quite difficult: its output is
designed for the needs of the printing industry, not the photographer. The
characteristic curve of a negative optimized for the printing press is very
different from that of a negative for B&W photographic materials.
In order to understand this, you must know how PhotoShop deals with density.
Built into PhotoShop is a digital electronic densitometer, which reads from
0 to 100%. Pure white in a PhotoShop positive screen image has a PhotoShop
density of 0%, while pure black has 100% density. If you place the computer
cursor on a PhotoShop image, the program automatically reports a density
value for the point under the cursor. The DMax of a good digital negative,
which corresponds to 0% density in the highlight areas of a positive image on
the screen, should equal or exceed the density required by your printing
material to print a pure white. In addition, the optical density of the
negative should decrease in a roughly linear manner between the extremes of
0% and 100% on the PhotoShop densitometer.
The big problem with imagesetters is that their output deviates greatly from
the optimal curves for B&W photographic printing materials. This is easier
to understand in graphic form. In following graph, I have plotted two
curves. The first is the optical density of an ideal negative for
platinum/palladium (Pt/Pd) printing versus the PhotoShop electronic density
of a positive image on the computer screen. As you can see, the ideal
negative is for the most part linear; I have made the curve a bit steeper in
the 0-10% and 90-100% areas to compensate for the rather flat shoulder and
toe of the Pt/Pd D/LogE curve. The second curve is the optical density of an
actual digital negative made with an imagesetter, again versus the PhotoShop
density of the image on the screen. The curve for the imagesetter negative
deviates far from the ideal.
The table and graph will print correctly on your screen or printer only if
you use a font which has equal width for all characters and spaces, such as
one of the Courier fonts in either Windows or the Mac OS.
O [-----------------------------------------------------]
p [x ]
t 2.00[o oooooo: Digital negative optimized for ]
i [ o Platinum/Palladium printing ]
c [x ]
a [ o xxxxxx: Uncorrected digital negative ]
l [ o from imagesetter ]
1.75[ o ]
[ x o ]
d [ o ]
e [ o ]
n [ o ]
s 1.50[ x o ]
i [ o ]
t [ o ]
y [ o ]
[ o ]
1.25[ x o ]
o [ o ]
f [ o ]
[ o ]
1.00[ x o ]
d [ o ]
i [ x o ]
g [ x o ]
i 0.75[ x o ]
t [ x o ]
a [ x o ]
l [ x o ]
[ x o ]
0.50[ x o ]
[ x o ]
n [ x o ]
e [ x o ]
g [ x o ]
a 0.25[ x o ]
t [ x o ]
i [ x o ]
v [ x o ]
e 0.00[ x o]
[-----------------------------------------------------]
^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
----PhotoShop Density of Original Positive Image-------
A palladiotype printed from an uncorrected imagesetter negative will have
little if any separation of values in darker portions of the image, with dark
gray to black tones in areas with an original PhotoShop density of more than
about 40%. In contrast, at the opposite end of the tonal spectrum, in the
range of 0% to 20%, there will be too much separation in tones because the
curve of the digital negative is so steep in that region.
I should point out that with all output devices for digital negatives, the
D/LogE curve of the negative deviates to some degree from the ideal for our
printing materials. However, the situation is far worse for imagesetters
than with other devices, to the point that imagesetter digital negatives are
unusable unless strong corrective measures are taken.
End of Part 2