[APML] To Push or Not To Push

["Thomas W. Earle" <twade@bmi.net>]

To all,

I've been concerned lately that pushing film may be a bad thing,
especially since our main goal is to capture faint nebula.  First,
no matter what you do to the film during development while it is
exposing it will ALWAYS "record" at its rated speed.  For example,
one of our favorite slide films is Kodak E200.  While exposing this
film to a faint celestial object such as M8, it will generally
capture photons at an ISO rating of 200.  If I expose the film to a
background sky of 21.0 magnitude per square arc-second, it will take
approximately 130 minutes to do so.  This exposure will likely just
about all the faint "stuff" that's possible with film; however,
under extremely dark skies, you might can get away with 160 minutes.
The film would be given normal development.  Now let's take the
example of pushing the film 
1-stop during development.  One of the key problems with pushing
film is the loss of shadow detail.  Something we definitely want to
avoid since this is our primary goal.  Why does film that is pushed
lose shadow detail?  The simple answer is you don't expose the film
long enough when pushing film.  As noted above, an ISO 200 film is
ALWAYS an ISO 200 film during its photon capturing phase.  Since
pushing 1-stop raises the background density by 1-stop (i.e.
lightens the overall image), you are forced to expose your image
nearly 1/2 as long (i.e. E200 pushed 1-stop has an effective ISO of
320, not 400); otherwise, you would have an overexposed image.  What
this means is your exposure becomes approximately 75 minutes.  This
results in a magnitude per square arc-second of only 20.4.  As a
result, you have lost a good portion of your "shadow" recording
power; hence, you lose the fainter stuff.  Pushing 2-stops is really
bad at capturing the fainter nebula since you expose the film for
only 40 minutes which results in a sky background of 19.8 magnitudes
per square arc-second.  This is a significant drop in recording
power.  In summary, if you really have to push, DO NOT push more
than 1-stop.  Keep in mind, the loss is more pronounced the darker
the sky you have access too.  If you are limited to a 19.8 magnitude
per square arc-second sky, you lose nothing by pushing 2-stops;
however, a 19.8 magnitude per square arc-second sky is quite light
(i.e. a Bortle scale of around 4.8-5.0).

Clear as mud right?

Wade


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