Re: [APML] Exposure Question

[Roland Roberts <roland@astrofoto.org>]

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>>>>> "Brian" == Brian Larmay <astrobri@ameritech.net> writes:

    Brian> Hi all,
    Brian> Ive been slipping on everything concerning astrophoto since spring and will
    Brian> finally be getting out tonight for some long overdue practice.
    Brian> My question is concerning exposure time...I always stop my lens to f/4 with
    Brian> exposure times of 7 mins, but this time I want to stop down to f/5.6 ...
    Brian> what would the correct exposure time be then?

    Brian> Whats the formula?

Each f-stop will approximately double your exposure time.  Reciprocity
failure on E200 is pretty low, so to a single f-stop, doubling is a
pretty good estimate.  Remember that anything less than a factor of 2
generally doesn't matter much, so if you want to double and then
"round" to 15 minutes, it isn't going to show much of a difference
from must doubling to 14 minutes.

    Brian> Will this sharpen my images noticably vs @ f/4?

Yes, it should, assuming both are focussed as best you can get.

    Brian> Will this reduce Vignette and help me to create mosaics
    Brian> alot easier?

Yes, vignetting should be reduced.

    Brian> All exposures are made using Kodak E200 tonight.

I'm quite suprised you get away with only 7 minutes at f/4.  My first
shot of Cygnus was pretty mediocre with 4 minutes at f/4, but I was
clueless about what to try.  From my dark sky site (which is not
exactly pristine since we are talking about the northeast US), I can
go 20--60 minutes at f/4 depending on the altitude of what I'm
shooting.  For stuff near the zenith, 60 minutes is about it.  For
stuff down around 30-degrees up, 20 minutes is pushing it.  This is
with normal development.  Also, I don't have a "real" film scanner, I
have one of those flatbeds that doubles as a slide scanner (the Epson
2450).  It needs a well exposed negative to give good results.

Do you push you film?  What do you use to scan it?  I'm just having
trouble figuring out how 7 minutes works for you.

regards,

roland
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		       PGP Key ID: 66 BC 3B CD
Roland B. Roberts, PhD                             RL Enterprises
roland@rlenter.com                            6818 Madeline Court
roland@astrofoto.org                           Brooklyn, NY 11220

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