Quote:
Originally Posted by Leo.G
Quick question, I recently saw a formula which throws the 500 rule out with the bath water and lists a 14mm maximum exposure time at 16.x seconds, would you happen to know this formula?
I lost the link I found it through.
Thanks!
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With any of these exposure "rules", more like guides, aside from focal length, FOV multiplier, it's all a question of a few extra things:
- the amount of star trailing you can tolerate
- the distance to the Celestial pole (centre of apparent star rotation)
- whether you like to zoom in for a tight view or happy to view image as a whole
I've used the 500 rule a few times, but personally much prefer something like 300 to 400 for untracked images on various wide and normal FF focal lengths that I've applied it to.
On your 14mm Samyang, depending on where you are pointing in the sky , you may prefer to use a lower numbered "rule". If you are shooting with your frame centre near the South Celestial Pole then your star trails will be shorter than if you are shooting with your frame centre well away from the SCP. This means significantly longer star trails for the same exposure duration in the latter case. It therefore might be better to go for something in the 200 to 300 range range, so your suggestion of 16 seconds (16 x 14 = 224), seems like a reasonable choice.
There is an exposure "rule" which also takes account of the distance (angle) to the SCP, involving the cosine of an angle, but your suggestion of 16 seconds will get you out of trouble, perhaps a little higher to 20 seconds if you need some more depth. If you are in a darker location, shoot to lower ISO as there's no need to increase the sensitivity too much, as all this will do is reduce dynamic range and if excessive, crush colours.
Best
JA