From: boutzev@lcaohp2.epfl.ch (Lubomir Boutzev)
Newsgroups: rec.photo
Subject: Analytical Multigrade exposure parts 1 & 2.
Keywords: photo , multigrade , b&w
Date: 6 Apr 92 08:49:05 GMT
Reply-To: boutzev@lcaoap1.epfl.ch
Organization: AI crazed sickos.


HI Folks,
Here I come once again with one of my boring posts about the non artistic
side of B&W photography.

PART 1

This time I'll like to entertain you with the EXPOSURE PROBLEMS & SOLUTIONS
for the ILFORD MULTIGRADE papers.
If you don't have any problems with them and your pictures are perfect from the
first and only try then stop reading.
If you are still printing several grade/exposure tests before the final result then 
get yourself some aspirine and read my junk. This could save you some cash on
paper waste.

ORGANISATION
This is the first of some postings about the analytical way of determining the 
exposure/grade binomial for multigrade paper printing.
It was tested with ILFORD III RC papers but with some data changes it could be 
applied to other types/brands.
It would work even for normal fixed grade papers too.

INTRODUCTION TO DENSITOMETRY
The key point in analytical exposure control (analytical=calculated , experimental=guess-
work + trial&error), is the densitometry.
The densitometry itself is a very simple and intuitive thing. It expresses the 
darkening of an emulsion as a function of the exposure.
The darkening (DENSITY == D) is expressed as follows:

D=Log( Iinc/Itrans)
where	
	Iinc is the incident light power density             [Lux]
                Itrans is the transmitted light power density     [Lux]

This means that if a negative is dark enourg to let only 1/10 of the light go trough it,
it has a density D=Log(10/1) = 1

In a similar way a negative of density 2 lets only 1/100 part of the incident light flow trough
it's dark part.

The exposure is expressed in [Lux]*[Second] or  [(watts)/(square meter)]*[time] ,
ant the resulting is the [Joules/(square meter)].  To resume:

exposure: e=Iinc*te
where:
          e is the exposure;
          Iinc is the incident light power density [Lux]
          te   is the exposure time                         [s]

The photosensitive emulsion needs to absorb a well defined energy/surface in order to
get exposed.

The emulsions are caracterized by their dpeed indication. There are several possible 
such definitions, as for example DIN , ASA , GOST , e.t.c..
The ASA (now ISO) speed definition is the most straight-forward and intuitive one.
In fact it boils down to this:

ASA = (0.8)/(emin)
where:
          emin is the exposure required to produce a minimal density of 0.1,
          i.e. Iinc/Itrans = 10**0.1 = 1.26

This is the norm definition because the 0.1 density is believed to be the least perceptible
by the muman eye and the 0.8 factor comes from some other considerations
I could fire out at the right time.

In other words, a film with an ISO speed index of 100 will require at least:

            emin =   (0.8)/(100) = 0.008 [Lux]*[s]  to start "just darkening" .

THIS definition holds of course for VERY PRECISELY CONTROLLED and defined
processing conditions. 

The Densitometry itself is the plotting of the Density (D) versus the Log of the 
exposure:
(I'll try some ASCII drawing , let's see!)



	D	|
		|
		|
		|
		|                                                                                             *
Dmax 	           __	|                                                                                      *
		|                                                                                *
		|                                                                           *
		|                                                                       *
		|                                                                    *
		|                                                                 *
		|                                                              *
		|                                                           *
		|                                                        *
		|                                                     *
		|                                                  *
		|                                              *
		|                                           *
		|                                       *
		|                                   *                                                          
Dmin=0.1	           __	|                            *I                                                                |
		|__*____*_______________________________________________
		     |			                         Log(emax)     |  Log(e)
                           Log(emin)


(ok , not bad for a first try !)

This curve IS NOT FULLY LINEAR  ! It has a quite linear portion somwhere in it's
middle BUT tends to saturate at the beginning  (Hi Mr. Schwarzshild !) and at the end.


The beginning of this lienar zone is (Log(emin) , Dmin) ; and the end is 
(Log(emax) , Dmax)  .

What happens if you expose outside the linear zone ? Well nothing but harmonic 
distorsion. The same that hurts your ears when listening to some cheapo stereo
at peak volume. But this time it will hurt your visual perception.

In the next postings we'll leaarn hos to obtain usefull information from this type of
densitometric graphs in order to expose our paper and choose the correct
grade.


If you are interested in following , then you can obtain the Densitometry curves for the
paper you are interested in.
Ilford provides them at the price of a phone call (at keast here in Switaerland) .
Then examine the curves and their variation as a function of their grades.

But beware: Ilford's graphs are D versus RELATIVE exposure log's.
In other words, the relative exposure log is :

                              Log(e/emin) 
         where emin stands for the minimal exposure used in the ISO (or ASA) speed
definition of the paper itself.


OK folks, no more time for writing today , the remaining parts will follow soon.

L. Boutzev
boutzev@lcaohp1.epfl.ch

(WARNING : Material protected by Copyright! Use it for personal purposes or
give it away free !)




PART 2

Reminder
This is the second part of the description of an analytical way
of determining the exposure/grade binomial for the multigrade papers.
Once again I don't endorse any advertisment for Ilford whose name I
quote quite a few times around here.
It could work with other media but I'm using this one in my personal lab.
The main point was that I got sick and tired of those loosy trial&error methods
of printing and decided to go further and get my prints ok from not more than
1.2 tries (statistics).
This little trick has already paid back for the small additional do-it-yourself
equipment it requires (about 150 US$).

DECIPHERING THE DENSITOMETRY CURVES
It's supposed by now that you have obtained the technical data sheets for the 
B&W papers you normally se and you can consult them in real time while reading 
this.

These curves were introduced in PART 1  , consult it if required.

The curves tell you the darkening of the paper as a function of it's exposure and 
llimit the linear (or better non saturation) zone of exposure.

The basic trick consists in fitting your negative's exposure into this non saturation zone
delimited by (Log(emin) , Dmin) and (Log(emax) , Dmax).

There is a little trick you must take care as from now, at least fo rthe Ilford's data.

In fact they express the Log(e) value as Log(e/emin) , i.e. the relative exposure. 
Relative to what ? Easy. relative to the emin used to determine the paper's ISO or
ASA speed (see PART 1).

The curves are Log-Log and this trick does not change much the shape of the
curve itself.

The linear portion of grade 0 has a smaller slope than the corresponding part of 
grade 5 (Hi-contrast).

It's very important to determine the limits ( (Log(emin)  , Dmin) and (Log(emax) , Dmax))
for all the grades you have data about.

In the case of Ilford RC III the  Log(emin) is almost constant (in fact you can assume
it as such, the errors are very small), for all the grades from 0 to 5 .

The other limit point ((Log(emax) , Dmax)) depends of the grade.
The difference ((Log(emax) - Log(emin) ) gives the Log of the exposure range for 
a corresponding grade. This is a very important notion , please keep it in mind.


EXPOSURE POINTS
We know by now the important limits not to be exceeded during the exposure.

The limit (Log(emin)  , Dmin) will provide the highlights of the final positive print.
In fact this exposure corresponds to a point where the darkest parts of the negative
will provide still visible details in the whitest parts (highlights) of the positive.
If your highlights are underexposed (i.e. your darkest parts of your negative are
located left to the point (Log(emin)  , Dmin)) , then your positive print will
dramatically lack of details in the highlights. In other words you will have only
white portions where bright details were supposed to be.

Inversely if you overexpose the highlights you will get gray portions instead of pure
white where the true white is supposed to be. Your picture won't contain any white
at all.


The UPPER limit (Log(emax) , Dmax) will determine logically the darkest parts in your
positive.

Same reasoning as before. 

If you overexpose these zones (The brighest on your negative) , then you will get
saturated blacks masking out the details in the shadows of your positive.

Inversely if you underexpose these shadow points (the brighest in your negative) , 
your picture won't have any pure black shadows at all when it was suposed to have
them.

We have now the indications of the exposure limits of the paper's emulsion itself.

Numerical examples will be given for a concrete case.



We have now progressed in our analytical investigation of the exposure times 
for printing on B&W papers.

In the next post we will see how to determine the grade of the paper required for each
negative.

OK folks, no more time for writing today , the remaining parts will follow soon.

L. Boutzev
boutzev@lcaohp1.epfl.ch

(WARNING : Material protected by Copyright! Use it for personal purposes or
give it away free !)