what is the best brand & iso for shooting star trails?
at which direction do I direct the camera?
what exposure times provide the best results ie for 180deg and 360deg star trails?
with thanks neil
If this isn't a false question here goes.
The earth rotates around its north south axis (appox) once every 24 hours (approx), so to get a 180deg star trail you would need a 12 hour exposure, not really very practical in most places. To achieve your 360 star trail, I believe is even more difficult, as you will need an exposure of 24 hours, this could only be achieved at certain times of the year in the arctic or antarctic regions. And with special equipment, both photographic and positioning.
On a more positive note regular trails are very easy to capture if you are in an area not blighted by light pollution. If you want the trails to seem to spin around one spot within the frame, you need to point your securely mounted camera at the pole star. Buy my reasoning a 90 deg trail would take six hours. You will need a clear night and be well away from towns and cities, even then don't shoot in there direction. I live 30 miles from an island whos halo effect can be seen clearly most nights. If you want the stars to streak through your picture, position the pole star outside the frame, the longer your lens and the further away from north you point the straighter the trails will appear.
On the question of brands and speeds, the answer is as always, it depends on what you want the finished result to look like. The more sensative films (400-1600) will show more stars, the slower films will enlarge better with less grain, but won't show as many stars.
The last big factor is the moon, for good results there must be no moon in the sky, not only the photo, the moon will appear elongated after a relatively short exposure time (1/4-2 sec) depending on lens length etc. It also reflects a lot of light from the sun, this will have the same polluting effect as the towns light.
Hope this is all of help, the important thing is to just go out and try.
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