Can anyone help me about the following?
Kodak produces a grey scale chart. It is a strip of semi rigid plastic
measuring approximately 20" long by 4" wide and is divided into 20 equal
vertical panels ranging from white in the left most panel to black in
the right most panel with progressively increasing gradations of grey
from white to black. The difference in grayness between each panel is
said to represent 1/2 of an f stop of exposure. Thus, the entire chart
represents 2 to the power 10 in terms of tonal differences. However,
photographic paper generally has a reproduction range of somewhere
between 1 to 40 or 1 to 50 [somewhere between 2 to the power 5 [ie 32]
and 2 to the power 6 [ie 64] so obviously the grey scale chart
represents a far greater reproduction range than can be accommodated on
photographic paper.
Middle grey is shown as the seventh of the 20 panels but nowhere on the
chart is there any data showing a density index. [I gather that Kodak
does produce a grey scale chart with density index data of sorts but a a
cost ten times that of the non-data chart].
In Ansel Adams' ten zone scale of grey, middle grey is shown as zone 5
[the sixth panel - see page 54 of "The Negative]. To create a density
index from zero to 1.0 based on Adams' ten zone scale, middle grey would
have a density index of 0.70 which is log 5, base 10, zone 2 would have
a density index of 0.30 which is log 2, base 10, zone 3 would have a
density index of 0.48 which is log 3, base 10 ..... and so on.
Looking at the opacity / density / relative log scale exposure graph on
page 86 of "The Negative" it is evident that Adams' zone 5 is shown as 5
exposure units with a relative log exposure of 0.7, a density index of
0.70 and an opacity of 5. However, the graph line is not linear and at
no point does it look to exceed a 40 degree tangential gradient.
This lack of linearity makes me wonder whether the Kodak grey scale
chart really does represent twenty equal gradations of grey the
difference between each one being equal to 1 / 2 of an f stop. After the
opacity index of 32 [2 to the power 5] and density index 1.50 [ this
being 5 f stops] on the page 86 exposure graph the curve flattens out
very quickly.
Examination of the graph on page 86 of "The Negative" leads me to wonder
whether the Kodak 20 panel grey scale is a particular accurate device at
anywhere other than in the middle third of the panels. The relative
flatness of the curve before 1.5 f stops and after 5 f stops makes me
think that Kodak's 20 panel grey scale is no particularly accurate.
Maybe this is why they do not publish any density index data on the
chart but instead only make it available on charts costing costing far
more than the widely available one.
Does anyone have any comments or information about the above?
To everyone who is bored stiff with technical questions I apologize.
John Hudson
Vancouver, BC
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