[Leica] How to read film data sheets

[Erwin Puts <imxputs@knoware.nl>]

A comment has been made that Velvia is better than K'chrome because the
resolution figure of V is higher than K (±160 versus ± 125). No doubt that
the cited figure is put in the data sheets. Has it any relevance?
No. I will as usual give a solid explanation why not.

Film manufacturers produce data sheets with info about resolution. They
give resolution figures for low (1:80) and high (1:1000) contrast targets.
And they give an MTF graph. For optical analysis the resolution data are
completely obsolete and so they should also be buried for film emulsions.
Because of the same reasons. I do not have to recall these, as they are
amply documented.
Now look at the high contrast figure. What does it mean. The test pattern
is the well known and much abused barline target: black and white lines of
diminishing width per mm produce a test pattern of ever increasing spatial
frequency. (more lines per mm). This target is illuminated in such a way
that the luminance dofference between adjacnet black and white lines is
1:1000 or 10 stops contrast difference. This type of contrast you might
encounter when taking a silhouet aganst blue sky. But than we have a low
resolution target (only the silhouet line has the contrast figure). It is
nay impossible to find in high res targets. Look at any picture you took
the last decade and see if you can find a detail with very fine structures
in it and ask yourself: do I see two adjacent very small object details
that differ in luminance by more than 10 stops? You will not find any
detail! So the high contrast figure is meaningless. If you are in need of a
figure go for the low contrast value and now we see that V and K are
identical. NO advantage for either one.
Take a look at the MTF graph and now you see a big difference, The K graph
tells you that from 1 to 20 lp.mm the MTF value is far above 100%! The same
values for V are lower. So in the critical areas where  sharpness  is all
there is the K wins. Why the threshold of 20 lp/mm. That is exact the value
Leica lenses are calibrated for!!
Why then is K for many purposes the better film: it is grain based where
the V is dye cloud based. Recall that a dye cloud image is being generated
by arificially restraining the growth of clumps of grain and replacing them
by dye clouds of about the same dimension at about the same location. Note
the vagueness here? A grain image is an exact replica of the optical image
falling onto the emulsion. The dye cloud image is a chemical interpretation
of this image.
The capture of fine detail is better preserved with the grain based image
and its 'hard' edges against the finer (smaller) dye cloud based image with
the soft edges.

This is same the reason why fine grain developers in fact kill fine detail
and acutance developers enhance fine detail up to the limit of grain noise.
Recall the Rodinal discussion?

As in optical evaluation we must become accustomed to the fact that
resolution figures are out and MTF graphs are in. Thats reality.


Erwin