[alt-photo] Re: Your Approach to Making Negs for Platinum Printing? Ideal Negative Contrast and Dmax?
photographeur at nerdshack.com
Sun Oct 9 22:12:59 GMT 2011
One more comment on practical sensitometry/densitometry:
Let's assume one has fully characterized one's print process and
found its ES. Now, one desires to make in-camera negatives with a DR
equal to the print process's ES (or, some other DR if one does not
want a full-scale print).
Many people contact print a step wedge onto a piece of their camera
film, and adjust their exposure and processing to yield a negative
with the target DR. This can be a useful approximation, but for
several reasons it will usually be in error by as much as 0.6 density
when you actually make in-camera negatives using the process
constants you determined:
(1) Lens flare. Real camera lenses have flare, which means that
some of the light that was intended to end up on areas of the film
representing highlights actually ends up on areas of the film
representing shadows. This means that the highlight areas are
exposed a bit less, and the shadow areas a bit more, than you expect
from metering the scene. In extreme cases (uncoated, soft-focus
lenses) this can reduce your expected negative DR by over 0.6
density. Even with modern, multi-coated optics the reduction can be
on the order of 0.3 density. This is by far the largest factor if
you are using panchromatic film, and results in negatives with a DR
that is lower than expected -- usually by 0.2 to 0.6 density.
(2) Spectral composition of exposing light. If you shoot mostly
outdoors in daylight (5000 K + color temperature), and contact print
the step wedge under an enlarger (generally 2700-3000 K), the film's
response will be different. This is most noticeable with
orthochromatic (green/blue sensitive) or non-color-sensitized (blue
only) emulsions, but affects panchromatic film (to a lesser degree), as well.
(3) Exposure time. If you normally shoot using electronic flash and
make the contact exposure using, say a 1/2 second exposure (or
vice-versa), the reciprocity factor of the film will produce an error.
So, how can one test one's film in a real-world manner? Here is one way:
I constructed a test target that is a step wedge with 15 steps x 0.15
(total DR 2.25) (this equals or exceeds the ES of almost all
available camera films). Each square is 6" by 6". However, the
wedge range is not from 0.0 to 2.25 -- rather, it is from 1.0 to
3.25. The squares are arranged on a piece of opal diffusing
plexiglass in a 4 x 4 array, total size 2' x 2', surrounded by a
black mask that is 4' x 6'. The 16th square (one of the center
squares) is clear. So, the wedge has 15 steps spaced at 0.15, plus a
16th square spaced 1.0 above the second-lightest square. The
plexiglass is illuminated from behind (being careful that the
illumination is very even -- better than 0.05, or 1/6 stop, across
its surface). I generally use the sun. Aim the camera at the
illuminated target, making sure that nothing outside of the 4' x 6'
mask is on film, and expose your test sheets.
You will probably want to do this for each lens you use, in case the
flare is significantly different.
After extensive use, this method has worked well -- but I do not
think it creates as much flare as most outdoor scenes (or night
scenes containing lights). Therefore, I intend to make another one
with the squares cut down by 1/2", leaving them on the same centers
so that there is an additional latticework of clear between the squares.
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