star field shots- question on exposure times

[DJDD]

hi all,

my wife is in Nepal (lucky!) and wants to take some star field and star trails shots. what exposure, ISO and focal lengths would be best, do you think?

her equipment =:
- Nikon D80
- 12-24 mm f/4 lens
- 18-200 f/3.5-5.6 lens
- inca tripod (light and sort of stable)

teh things I am unsure of are:
ISO settings.
- 800 or 400?

exposure times.
- for star fields, is it reasonable to work with the formula 700/fl = max exposure before trailing?
- for star fields, will she see colour in the stars at the exposure times suggested from the first question?
- we can stack later if needs be

- for star trails, what would be the minimum exposure time to get some decent star trails? It is probably about minus 10 degrees Celcius or less where she is! brrr

- what focal length is best for star trails, given that she probably cannot leave the camera unattended and it will be perhaps too cold to stay outside for long periods?

any other advice is welcome.

thanks, in advance, for any assistance.

DJDD

[rogerg (Roger)]

Being fixed tripod you'd be wanting to do mostly very wide (maximum open) stuff, but concentrating on nice framing of forground objects.

For startrails the ISO isn't important, I'd go for about ISO 200, and exposure times of about 30 minutes to give a nice length trail. You don't want too much ISO or it'll just all be noise, and you don't need high ISO because you're after the trail effect that'll show no problem, not faint nebulas.

For non-trailing shots, I'd go for 800 ISO and maximum of about 30 seconds. I find 30 seconds at 17mm gives minimal trailing while longer it starts to get anoying. Similarly zooming in more pronounces the trails more, so I'd keep to the 25mm or wider kind of range.

I'd set the f-stop to either full open or just a stop or 2 down from full open to get a bit better quality shot (but less light so there's a trade-off).

I'm not good with formula's so can't help with anything specific, but that's from my own practical experience.

Roger.

[DJDD]

thanks, Roger, for the advice.
good tip on the ISO for star trails.

i am not sure she will want to get out for a 30 minute exposure but star fields are always nice.



cheers,
DJDD