Re: [APML] To Push or Not To Push

[Jerry Lodriguss <lists4@astropix.com>]

>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








_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo