How to go from negative to print
Adrian Seet
I have been reading the postings on this newsgroup about how to make it
easier for the lab technician to print my photos so that if something
doesn't look right, I know that it's my mistake and not the printer messing
it up.
I read some postings where some people have recommended shooting a gray card
or a color chart in the same light as the subject to serve as a reference
for the lab tech. I thought that was a cheap (only waste 1 frame) and simple
way to provide a color reference. I went to speak to the tech at the local
photo lab (Ritz) about this idea and they basically said it won't help. I
don't really have first hand knowledge at all about how printing is done so
I couldn't really go any further with this. They made it sound like it's all
black magic. They tweak the color balance until something (such as skin tone)
looks right and that's it. If that's the case, aren't they up the creek if
there is no easy reference for them? (I once had an orange sunset printed
as a blue sunset!)
Could someone explain how color negatives are printed and how/why shooting a
gray card or a color chart would help?
Also, why is it that the color chart/gray card be shot under the same light?
I would have thought that once you get the right color balance for the
reference frame, the same color balance can be applied to the entire roll.
Thanks
adrian
Ken Hart
"Adrian Seet" <as...@iname.com> wrote in message
news:9tuhg9$632$1...@usenet.Stanford.EDU...
The grey card provides a reference point. Theoretically, your grey card
wherever you are is exactly the same as my grey card (wherever I am!). So if
I balance my printer so that your grey card prints correctly, then the rest
of the roll, taken under the same lighting conditions, and using the same
color balance, would print exactly the same.
Unfortunately, the printer (the machine, not the person), will not use the
same color balance for every frame. The printer (machine) may not have such
a setting, or the operator may not know how to activate such a setting.
Typically, the machine looks for a specific mixture of the various colors of
the spectrum, and a specific density (light/dark). If it doesn't see the
desired mixture, it attempts to correct for it. The machine operator is
trying to make the maximum number of prints per shift, so he just lets the
machine do it's thing, and waits for the occaisional customer demand for a
remake. If the lab is doing any quality control, the QC person can just
"tell" the machine that the picture needs to be made lighter/darker/a
certain color, and reprint it.
--
Ken Hart
kwh...@aec.nu
Pierre Renault
> [...]some people have recommended shooting a gray card
<...>
> the tech at the local photo lab (Ritz) [...] said it won't help.
I have no idea what sort of lab Ritz is but the tech in question was wrong on
one level, and right on another. Read on.
>Could someone explain how color negatives are printed and how/why
>shooting a gray card or a color chart would help?
There are two ways to print colour: printers and enlargers. A printer is a
fairly large machine that prints on long rolls of paper and usually have some
sort of automated colour and exposure correction built in. An enlarger is
basically like a slide projector with the ability to change the colour of the
light passing through the negative.
Setting up a printer is essentially the same as printing with an enlarger.
Test prints are made at various exposures (let's say 1, 2, 4, 8, 16 seconds),
developed, and dried. The test prints are evaluated by eye for gross
corrections (exposure and colour). A second test print is made, and so on
until an acceptable print (depending what "acceptable" is) is achieved. At
this point, the work on the enlarger is essentially finished, unless the
printer decides to burn or dodge (lightening or darkening certain areas of the
print) or some other refinement. Most colour printing is performed on a
printer (its much much less time consuming, usually consumes less paper, and
is simpler for printing large quantities of negatives).
Setting up the printer is usually performed with a standard negative (called a
"shirley" after the first woman who posed for a Kodak test neg). Shirleys are
shot on regular film (usually professional film) and developped in a known-to-
be-correct process. Shirleys usually have one or two skin tones (from real,
live persons) and various shades of grey and certain "pure" colours (red,
green, photo blue, yellow, magenta, or cyan). The pure colours are usually
used to detect certain specific processing or environmental problems (too
complicated to explain here) and to check for something called "colour
crossover", a problem when the dark shades go one way (too blue, for example)
and the light shades go another (lets say: too red).
Colour negatives are very low contrast when compared with black and white or
with colour slide films. The paper used to print from colour negs is,
consequently, quite contrasty (overall contrast is the difference in exposure
to go from total white to total black) so any processing error that causes one
colour to have an incorrect contrast (the cause of colour crossover) shows up
quite quickly. Having various shades of grey on a shirley lets the printer
technician determine if there is any crossover by eyeballing the print.
Colour negative films have more than one single emulsion (the light-sensitive
part of the film), some more than fifteen. As it passes through the various
emulsion layers, part of the light that hits your colour negative film gets
absorbed by that emulsion, leaving less for the next layer. This causes a
problem when constructing colour film because it means that the various layers
have to have different levels of sensitivity to light, the most sensitive
being at the back, the least sensitive at the front. While coupling individual
colour dyes to specific layers is not really such a complicated problem, how
to "configure" the various layers is.
Generally speaking, the less sensitive an emulsion (layer) is to light, the
more it is contrasty. The various layers, therefore, don't have quite the same
contrast characteristics. To complicated matters even further, the contrast is
not even throughout the range of greys. Obviously, the solution is to reverse
match the colour paper (which is also multi-layer) to the colour neg: a colour
on the neg that is very contrasty is matched to its low-contrast opposite and
vice-versa.
Add to all that (there is a point to all this), the fact that your brain tends
to adjust colour instinctively, so you don't notice, for instance, that
objects in the shade (on a sunny day) are not the same colour as they would be
in sunlight (they're bluer) or that the light from incandescent bulbs is much
less blue and green (or more orangey, if you like) than light from the sun or
an electronic flash. The various films available are more or less sensitive to
this effect (called "colour temperature"), the more colour-temeprature
sensitive films generally having a better ability to distinguish fine
variations in colour than the less sensitive films. So pictures taken on an
overcast day will not have quite the same colour (and the accompanying
contrast problems) as they would on a sunny day.
Obviously, all these layers being matched to one another is a sort of house of
cards because if a negative is improperly exposed (too light or too dark), the
contrast characteristics of the various colours change differently (that why
underexposed negs tend to look to green, overexposed are too pink).
Printers (the machines) usually have light meters built-in to evaluate the
negative being printed for colour and exposure. Many different methods are
used to determine how to adjust the exposure, some highly sophisticated
systems use computer-style databases of typical types of photos (seascape,
child sitting on Santa's knee, close-up of a caucausian face, etc) and try to
guess what the picture is and adjust accordingly. Most systems simply scramble
the neg, usually giving more weight in its decision-making algorithm to the
center of the photo than to the corners.
If the machine thinks that the negative will print too blue (a photo taken in
the shade, for instance), it will adjust accordingly. An experienced print
technician will usually adjust the printer according to his quick visual
evaluation of the neg. If the neg has a large green blob (a photo of Santa for
example, green is opposite to red), he or she will dial in more red to be
printed. If the negative looks underexposed or overexposed, he or she will
usually print using special corrections settings (called "channels", for some
reason) which try to adjust for the variation in colour contrast described
earlier. Most contemporary printers will print the settings used to print the
negative on the back of the correpsonding photo with indelible ink which lets
the person evaluating the finished print know if a correction was applied,
whether a channel was used, or whatever, in case the print has to be remade.
The roll of paper is then processed, dried, and cut. Most of the negatives get
printed (more or less) properly on the first go. The prints that are off-
colour or improperly exposed are sent back for reprinting. The process
sometimes require a third or fourth try.
Different brands and types of film have different characteristics, so printers
usually have to be set up for various types of colour negative. Take a look at
different brands of processed negatives, side by side. You'll probably notice
that the salmon (orange) film base, called a "mask", is a slightly different
colour from film to film (don't ask me why there needs to be an orange mask,
this post is long enough as it is). The paper, being quite contrasty, markedly
reacts differently to the various brands and types of film. Usually the
various channels available on a printer are set in accordance with the usual
types of film the lab in question processes as well as for variations in
exposure.
> They made it sound like it's all black magic.
To most lab technicians, it is black magic. The reality is that most of these
problems have already been solved and the only person who has really knows
anything is the quality control technician (who controls the processing, the
machinery, etc.)
>Also, why is it that the color chart/gray card be shot under the
>same light? I would have thought that once you get the right color
>balance for the reference frame, the same color balance can be
>applied to the entire roll.
Now you know why the grey card has to be exposed under the same light as the
subject. If the entire film was shot under the same lighting conditions and in
a timely manner (oh yes, the amount of time between the picture being taken
and when the film is processed also affects the colour and contrasts), then
once the right colour balance is achieved, the same should apply for the rest
of the roll (assuming the frames are all properly exposed).
The grey cards used are of a standard colour and shade. They reflect 18% of
the light that strikes them. Why 18%? Because when measured with a
densitometer (a ridiculously precise special type of light meter used to
measure the light being reflected from a print or going through a negative or
slide), the same number (0.075) is obtained when reading 18% gray and its
corresponding shade on a negative or slide (which lets through 18% of the
light). A lab technician with a very good eye for colour (they are rare) can
pin the colour of a grey card down to 1 or 2 CC (a unit of colour correction)
from perfect, an accuracy of 4 or 5 times that of most people.
In a very very conscientious lab (another rarity), photos from grey cards are
read by densitometer (in theory, all three colours should read 075) and the
printing is adjusted accordingly. Some labs will even print the entire roll as
is, without letting the printer adjust the printing, whenever the first frames
of a roll contain a grey card *and* the client request this. This usually
involves the client agreeing to pay for whatever prints are printed, whether
the colour or exposure be correct or not (called a "print regardless" order).
Its quite possible that the lab you use (Ritz) doesn't offer this service and,
if so, shooting a grey card would *nearly* be pointless because they wouldn't
(not "couldn't") adjust the printer accordingly.
If there's a professional lab in your area (try the Yellow Pages), you might
request a contact sheet be made of your negatives. The negatives are pressed
onto the photo paper and held flat against it with a sheet of glass and the
entire film is exposed in one shot; all the negatives get exactly the same
amount and colour of light. You'll be able to see the variations in exposure
and colour I've been talking about. You'll probably notice right away that
objects in the shade or very far away are bluer than "normal". You'll probably
also be surprised to learn just how much variation there is in exposure (light
to dark) from negative to negative.
HTH
Pierre