"Mick Maguire" <[EMAIL PROTECTED]> writes:
> Can somebody give me a "Push" 101?
I would really recommend Ansel Adams's excellent book series; "The
Camera", "The Negative" and "The Print". In the second of these, he
describes the techniques available in detail. However, a quick
summary of what's going on when you "push" (overdevelop) or "pull"
(underdevelop) B/W film:
You may have heard the old adage: "Expose for the shadows; develop for
the highlights". This holds the key to pushing and pulling. When you
develop an exposed negative film, the silver halide that was exposed
to light will be reduced to metallic silver, creating little clumps of
silver in the emulsion. What isn't reduced is washed out during the
fixing process. The remaining clumps of silver are what we see as
the "grain" that makes up the image.
Now, if you leave the film in the developer longer than you're
normally supposed to, the clumps of metallic silver will grow larger,
with the already large clumps growing, relatively speaking, more than
the smaller ones. The effect is a grainier, more contrasty and more
dense negative. The most transparent parts of the negative (the
deepest shadows in the image) will be left unchanged, while the
densest parts (the highlights) will change the most. If, instead, you
underdevelop, the highlights will once again be most strongly affected
(they will darken in the final image), while the shadow areas will
change little. Because of the nature of these changes, Ansel Adams
preferred to call the process "expansion" and "contraction" of the
contrast range, instead of using the words "pushing" and "pulling", as
we usually do today.
Adams worked with sheet film, and could choose to expose and develop
each negative optimally for the image he wanted to produce. Thus, he
explains how you should always give the film the proper amount of
exposure to achieve shadow detail, and then expand or contract the
contrast range during development, to place the highlights of the
image where you want them, resulting in an image of a full, pleasing
tonal range. For 35mm roll processing, you have to treat the whole
roll the same. Adams suggests, for situations where you have to
expose images of varying contrast ranges on the same roll, careful
exposure targeting of the shadows, combined with a slight
underdevelopment, thus using contraction to protect against "washing
out" of the highlights. This, he explains, will limit grain size and
make the more contrasty images manageable during printing. The softer
images can be corrected by using a harder grade of paper.
So -- it's not as easy as setting your ASA dial to 1600 instead of
400, leaving the film in the developer for about twice the normal
time, and expecting good images from low light conditions. If you do
this, you should realize that you'll get a grainy image, with more
contrast than usual, especially in the form of loss of shadow detail.
Still, it can be a very useful technique, of course.
As a rule of thumb, B/W negative film can be pushed or pulled up to
about two stops if you have to, but going beyond one stop in either
direction tends to reduce the quality of the negative more than you'd
normally appreciate. The amount of over- or underdevelopment needed
to produce expansion or contraction by one stop is dependent on both
film and developer, so you need to study the manufacturer's
recommendations.
Incidentally, while "normal" push and pull processing does not affect
the shadow areas, Adams does describe a method of increasing the
sensitivity of the film to achieve more shadow detail. This is a
technique that contracts the lower end of the contrast range upward,
gaining detail there, while leaving the upper end unchanged. The
trick is to pre-expose the film with _almost_ enough light to
register, thus lowering the threshold for the amount of light needed
during the actual image exposure in order to create shadow detail. It
can be done by photographing a smooth, uniform, evenly lighted
surface, under-exposing it by some six or seven stops (I'm guessing
now; I don't recall the actual level where modern film starts to
register an image), and then winding the shutter for the actual
exposure without winding on the film itself.
-tih
--
Puritanism -- the haunting fear that someone, somewhere, may be happy.
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