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Old 21-01-2013, 05:07 PM
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OzEclipse (Joe Cali)
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Join Date: Jun 2012
Location: Young Hilltops LGA, Australia
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Quote:
Originally Posted by AstroJunk View Post
I'm still pretty new at this having similarly just bought a 650d, but I have already come up with my own simplified equation for exposure without star trails. Its easier to use that the others and much along with Gregs comments. Its:

30 seconds or less / conditions on the night.

The 600 rule gives 8mm @ f4.5 = 16 seconds which is better but will require higher ISO and corresponding noise issues.

I haven't done Joes' maths, but of course the key factors are fl, pixel size (chip size is sort of irrelevant) and rate of movement of the field. I'll stick to my equation for the timebeing
Hi Jonathan,

In which universe does 600/8 = 16 ? Is this some sort of corollary to Douglas Adams 6x9=42?

None of these things are perfect, they are just a guide. My formula takes no account of the light spread of brighter stars which makes a trail longer but also wider nor does it give you trail free images. Of course if you zoom right in then N, the number of pixels drift has to be made smaller and your time becomes much shorter with commensurate increasing noise problems.

Tripod photography is always best expressed using a small print or small screen image to minimize visibility of trailing.

All these rules fall over when you go to these ultra wide and fisheye lenses. The focal lengths stated are obviously not the real focal lengths. Your lens is trailing more like a 12mm focal length lens than an 8mm.

Joe
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