From: donl@glass.esd.sgi.com (donl mathis)
Subject: Re: Question for photo techies
Reply-To: donl@glass.esd.sgi.com (donl mathis)
Organization: Silicon Graphics, Inc., Mountain View, CA, USA
Distribution: rec.photo
Date: Sat, 14 Nov 1992 02:15:03 GMT


In article <1992Nov13.233001.17136@digi.lonestar.org>,
gpalo@digi.lonestar.org (Gerry Palo) writes:

|> Would automatic printers be better able to adjust for correct 
|> color balance if, in addition to scanning the  negative and 
|> taking manual input from the human operator, they knew the 
|> actual settings and conditions under which the picture was taken?
|> 
|> Suppose we could have a 35mm or other film with a magnetic strip 
|> on which an auto-exposure camera could record the exposure 
|> information - light value, aperture, shutter speed, and perhaps 
|> even the presence of a filter (that would have to be electroni-
|> cally coupled too), whether there was an electronic flash, etc.
|> 
|> [...]
|>
|> Anyway, would making the information electronically available 
|> to the printer or enlarger be worth the trouble?
|>
|> My first thought of this comes due to unpleasant experiences 
|> with photo labs, who often don't get the color balance right, 
|> especially with mixed indoor/sunlight lighting conditions or 
|> changed lighting conditions on the same roll.  [...]

It seems best, before arriving at a solution, to define the problem
as precisely as possible.  It's well known that photo labs often don't
get the color right, and the print exposure can be wrong.  We have to
assume that the lab is taking care of their equipment and chemistry,
and are using decent materials.  Given the practical aspects of running
the lab, there can be no choice of contrast -- the paper and development
determine a mapping function from the film to the print, and there isn't
much you can do about it.

What we're left with, then, are two specific variables: print exposure,
and the color of the exposing light.  The film presents a range of
densities in each of three colors.  Sending light through the film
provides a range of exposures for the paper in each of three colors.
Changing the print exposure changes all three colors at once, and
changing the color balance of the light lets us control the exposure
of individual layers.  So all we really get to control are the three
exposures.  Period.

Each color layer of the paper emulsion has a specific range it can
deal with.  Going below the minimum exposure by any amount will 
produce the same results -- no exposure.  Going above the maximum
exposure by any amount will produce the same results -- maximum
dye density in that layer.  Between these two specific limits, we
have a workable range of exposures where we can get variations in
dye density that correspond to the exposure: more exposure, more
density; less exposure, less density.

Producing correct print exposure is a matter of arranging, for each
layer, that the lowest of the important exposure values don't fall
below the minimum, and the highest of the important exposure values
don't fall above the maximum.  The problem here is the definition of
"important".  It is a subjective quantity.  Can those shadows be
printed as black, or should they be a little lighter so you can see the
details in the tree branches?  Can this patch of snow be printed as
white, or should it be printed a light gray so you can see the texture
and shadows in it?  No machine, camera, meter, or etc. can make such
a decision and be definitely "right" -- to do that will take a human.

Machines can make a best guesses, however, and that's precisely what
they do.  The problem is simplified in many cases because the range
of densities on the film, which becomes an exposure range when you
shine light through it, is usually not *that* much greater than what
will fit on the paper.  So you lose a little bit of shadow, and a little
bit of highlight, and everything works out about right.  If you want
better, you have to put a human in the loop.  There's no way for a
machine to make a better decision than that; it can't "see" the image
and make the subjective call.

This exposure question pops up not once, but three times.  The goal of
setting all three exposures in conjunction is to arrive at a good
color balance in the print.  Guess what?  That's subjective, too.  When
I photograph under tungsten lights, the image recorded on film leans
very strongly toward yellow and red.  When I make a print, do I want
to completely compensate for that, so the image would look the same
as if it were illuminated by daylight, or do I want a little of that
indoor-yellowish light to remain, to make it more like real life?  Do
I want those blue early evening light to record more like daylight,
or remain bluish and convey a sense of coldness?  On it goes.  Somebody
(not some machine) has to decide how the print should look.

A machine can make a best guess, though.  In general, it will assume
that the overall image is made of a balanced mixture of colors, and
when they're all shuffled together, they should make gray.  So it mixes
up all the colors, sets the filtration so that the resulting light is
what the paper expects when it makes gray, and that's it.  Sometimes
this works, and sometimes it doesn't.

We can define proper color balance any way we want.  We must keep in
mind that the photographic process is far from perfect: we might get
a perfect gray (according to our personal definition) out of a particular
film, processing, and print combination, but in that same print, the greens
will be a little funny and the skin tones won't be right.  We can set
the color so the skin tones are right, but now the gray isn't precisely
gray.  Somebody has to decide.

Once somebody makes these decisions, getting the machine to cooperate
is not such a big deal.  The color analyzer will make a reasonably good
guess about how to set the filtration and exposure, and the print will
look reasonably close to what was expected by whoever made the
decision.  To make the process easier, we can photograph a gray card,
and then tell the color analyzer "this is gray", and we should be able
to get a fair approximation of gray in the print.  Or the same for a
skin tone, or some other color.

If we leave the machine to make its own decisions, we'll get the best
guess of an inanimate object that can't see.

Now, to be more specific: recording the color temperature of the light
at the original scene might help to some extent, but different films
respond to the same kind of light in different ways.  For example,
there is a reasonably wide variation in the way different films respond
to flourescent lights, because flourescent lights present an extremely
odd sort of spectrum to the film.  Even in the same kind of film from
the same manufacturer, there will be variations.  People are bringing
Kodacolor 100 that is 2 years old and 2 months old into the photo lab,
and the machine can't possibly guess how long each one has been sitting
out in the sun, or in the trunk of the car.  Even different batches of
the same film, treated identically, can end up with different responses
to the same images.

I can't see any real value in record the subject brightness, aperture,
and shutter speed that were used when the image was made.  The net
result is density on the film; the exposure was good, or it wasn't.
When it's time to print, denser film will require more light shining
through it, and less dense film will require less light.  Not a big
problem; something the machine can do if we just tell it what we want.

The real problem, then, is that throughout the process, there is a wide
variety of variables (and will always be).  In the end, the printer has
to take a piece of film, make a guess about what it's supposed to look
like, and set the exposure.  If we want better results, we have to put
a human in the loop, looking at the image, and making decisions.  Some
humans will make better decisions than others; the photographer should
ultimately choose, but this is rarely the case.  And then we have the
problem of making sure the prints look like we told the machine they
should; this is not an entirely predictable science, and can involve
Trying Again, depending on how picky we will be.

|> One more relevant thought.  Most of the computer oriented 
|> thinking about cameras is looking to all digital cameras replacing 
|> silver and dye based photography very soon.  This may happen, 
|> but I think it will be a long time before digital photography can 
|> match conventional photography in quality, resolution, and above 
|> all the range of cameras, lenses and peripheral equipment that 
|> are already at hand. So how about more hybrid developments, such 
|> as the above, which would combine digital technology with 
|> conventional photography?  It is done, of course, with modern auto-
|> focus/exposure cameras and printers, but it seems that there are
|> more possibilities waiting to be discovered before the all digital
|> future arrives.

Well, consider camcorders.  Ever heard of "auto white balance"?  I
don't know much about camcorders, but I think what's happening is that
the machine assumes the brightest values are "white", and then sets the
three exposures appropriately, to put the right three signal levels on
the tape.  A digital camera should do much the same thing.  But keep in
mind, there is still a "tint" control on your television, and will
always be.  Variations happen.  They are unavoidable in the real
world.

I also wonder about how a magnetic strip would survive the trip through
the camera, the film spool, the developer and the whole film processing
line, and so forth, if it's attached to the film itself.  If it's *not*
attached, it will get lost.  :)  I think that if we're going to try to
incorporate more information about the original scene and exposure, for
use when it's time to print, that an all-digital camera is the only
practical way of doing it.  To lay the extra complexity on what has
become a very simple process (film and chemistry) would no doubt make
it as expensive as full digital would be anyway, so why not just byte
the bullet?

- donl mathis at Silicon Graphics Computer Systems, Mountain View, CA
donl@sgi.com