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Re: [APML] To Push or Not To Push
Hey all,
I've always been confused about pushing, what it gains and what it
loses for you. It appears from the conversation below that it is NOT a
good idea to shorten exposures when you are planning to push. If I
normally expose for 150 minutes, then I should expose for that length,
right?
If this is the case, I'm willing to give it a try and see if there's
anything to gain. I've been looking around for some Velvia 100F, but
haven't come across any yet. Is this one of those hard to find films?
Of course all this is academic if there's no one in the valley who will
do a push. There's only one place that is willing to mount negatives as
slides. My usual place sends slide film to Eugene for developing and
mounting, and I'm not sure if they will even push film.
Alan
--- Jerry Lodriguss <lists4@astropix.com> wrote:
>
> >I've been concerned lately that pushing film may be a bad thing,
> >especially since our main goal is to capture faint nebula.
>
>
> Hi Wade,
>
> It's only a bad thing if you think a 75 minute exposure is going to
> record
> the same amount of faint stuff as a 150 minute exposure, regardless
> of the
> development!
>
>
>
> > First,
> >no matter what you do to the film during development while it is
> >exposing it will ALWAYS "record" at its rated speed.
>
> Except for those films that experience reciprocity failure...
>
>
> 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.
>
>
> Then don't underexpose it!
>
>
>
> >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.
>
> It would not necessarily be overexposed if you kept the same exposure
> and
> push processed it. This would mainly apply to daytime images, and
> only
> those images that spanned the entire tonal range, not necessarily
> astronomical images.
>
> For astronomical images, most of the faint detail is down in the
> shadow end
> of the tonal curve, so exposing the same, and pushing helps for two
> reasons: 1) it puts this shadow detail up higher onto the more linear
>
> portion of the tonal curve where it has more contrast with the sky
> background than instead of where it would be if you didn't push, down
> in
> the flat-shaped toe of the curve, and 2) it makes it easier to scan
> because
> the sky fog background is lighter.
>
> For astronomical images that contain subjects with large brightness
> ranges,
> such as M42, M31, some of the highlight detail may become clipped or
> blown
> out by the pushing process, but this is a special case because there
> are
> not that many objects like this, and in this case you would
> consciously be
> deciding to sacrifice the highlight detail to better record the
> shadow
> detail, possibly planning on making a masked composite of two
> different
> exposure lengths.
>
> It is not necessarily true that pushing raises the brightness of the
> sky in
> terms of actual "detail" because part of this brightness increase is
> from
> chemical fogging.
>
>
>
> > 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.
>
>
>
> No, if you push, do not shorten your exposure!
>
>
>
>
> >Clear as mud right?
>
>
> No, what you say is true, IF you shorten the exposure thinking you
> can get
> something for nothing!
>
> Jerry
>
>
>
>
>
>
>
>
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