An
Aid to Assessing Filtration Changes
By
Barry Martin Area 22
When
you make your first print from a new batch of paper, a new or different type
of film, or even a new batch of chemicals, there are usually some changes to
the filter pack required. It can be sometimes quite difficult to assess
these changes, particularly if the picture contains large areas of bright
colour. It is quite often difficult to estimate whether the colour bias is
blue or cyan, red or magenta etc. I have found the following helpful in
being able to make decisions as to what filters to change in the filter pack
and by how much.
First
it is necessary to produce a test card which you would then photograph on
the film that you normally use. This card could be made using poster paints,
acrylic paints or any type of paint that is not glossy, a glossy paint could
produce unwanted reflections and make it more difficult to use. Try to give
a good range of greys made by mixing white with various amounts of neutral
black, some black paints are slightly tinted. The colours shown are only an
indication. It is best to include as many colours as possible, particularly
pinks, greens and browns. That will make it easier to see the colour
variations. Having produced the Test Card it should be photographed in good
bright daylight and the card should virtually fill the frame. It could be
advantageous to bracket exposure and select the negative which has a density
nearest to those you normally print from. Now you should take this negative
and make as good a print as you can (about 5" x 4" is a good size)
and one which shows the grey scale as near neutral as possible; this could
be quite time consuming but it is worth getting it right.
View
the picture in day light and compare it with the original test card for best
results. You will probably find that it is not possible to get a full pure
tonal gradation due to inadequacies in the emulsions, but you should get it
as best you can. This print will be your standard.
Now,
using the same batch of paper, chemicals etc. make a set of prints of the
same size and degree of enlargement with the following variations in the
filter pack and write the filter pack changes clearly on the back of the
prints.
Increase
filter pack by
10Y, 20Y,
30Y
Increase filter pack by
10M, 20M,
30M
Increase filter pack by
10Y+10M, 20Y+20M, 30Y+30M
Reduce filter pack by
10Y, 20Y,
30Y
Reduce filter pack by
10M, 20M,
30M
Reduce filter pack by
10Y+10M, 20Y+20M, 30Y+30M
This
will produce a set of 19 prints, including the standard which will be
neutral, and the others showing three degrees of colour casts of blue,
green, cyan, yellow, magenta and red respectively.
It
is possible (but unlikely under normal circumstances) that you would not be
able to remove the full 30
value e.g. If your filter pack
for the best print was 10Y+20M. Then to make the change of reducing 30Y+30M
would require a filter pack of
10M+20C
i.e.
Y M C
Filters for best print
10 20
0
Change to make
-30 -30
0
-------------
Required filtration
-20 -10
0
Add neutral content
20 20
20
--------------
Final filter pack
0 10
20 (10M+20C)
Do
not forget, of course, to adjust the exposure of each print to take educated
guess account of the filter pack density changes. You should now have a set
of prints with various colour casts and you can see how the various colour
casts affect the colours and grey scale of the test card.
USING
THE SET OF TEST CARDS
When
you come to make your first print from a new film etc. make an educated
guess at the filtration required and make a trial print.
Unless you are very lucky the print will have some colour bias. If
this is very strong you will have to make another guess and try again. Now
by looking at the print and comparing it with your set of test cards you
should be able to see which one corresponds to the colour cast in your
print, paying particular notice of the colour content of your picture and
comparing it with the similar colours on your test cards. For example if it
is a portrait, see how the pink and brown colours are affected. If the
picture is of a landscape then look carefully at the blue and green areas.
Greys are not always present in the average photo-graph. Now once you have
decided which of the test card prints matches, as closely as possible, your
trial print, all you have to do is make the change to your filter pack
corresponding to the change indicated on the test card print. If you find
the colour cast lies between two test prints, then a simple matter of inter-polation
will produce the correct filter change.
The
more prints you make the easier it ill become to identify the corrections
needed to make good prints and eventually you will be able to dispense with
these aids. However, one advantage with their use is that they can be used
in artificial light. Try them for yourself and let me know how you get on.