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<blockquote data-quote="PaulPosition" data-source="post: 315435" data-attributes="member: 25700">It all depends what you want to achieve. You want somewhat fast shutter for fixing the stars sharply. You want slow or very slow shutter (minutes) for start trails and for exposing the foreground if it's interesting enough.When you go very slow, however, like a night shoot with low ISO and a 10 stops ND filter, you risk not even capturing the low light emitting stars AND you get another kind of noise dubbed 'hot pixel' which you need to remove. That can be done in camera but it means for every exposure you do the camera does another one that's exactly as long (see long exposure noise reduction) which means your star trails become dotted lines. It can also be done in post process, you'll need a 'dark frame' which you'll get by shooting for the same shutter speed with the lens cap ON.BTW, placing a filter on the lens will not &quot;decrease ISO&quot; a anymore than boarding a plane will make you fly ; for practical purposes it might accomplish the same but there will be a day where &quot;I can fly&quot; will only cause confusion.&nbsp; -&nbsp; ISO sets the sensors sensitivity. ND filter reduces the light that gets in, but so does faster shutter speed or smaller aperture.</blockquote>

[QUOTE="PaulPosition, post: 315435, member: 25700"] It all depends what you want to achieve. You want somewhat fast shutter for fixing the stars sharply. You want slow or very slow shutter (minutes) for start trails and for exposing the foreground if it's interesting enough. When you go very slow, however, like a night shoot with low ISO and a 10 stops ND filter, you risk not even capturing the low light emitting stars AND you get another kind of noise dubbed 'hot pixel' which you need to remove. That can be done in camera but it means for every exposure you do the camera does another one that's exactly as long (see long exposure noise reduction) which means your star trails become dotted lines. It can also be done in post process, you'll need a 'dark frame' which you'll get by shooting for the same shutter speed with the lens cap ON. BTW, placing a filter on the lens will not "decrease ISO" a anymore than boarding a plane will make you fly ; for practical purposes it might accomplish the same but there will be a day where "I can fly" will only cause confusion. - ISO sets the sensors sensitivity. ND filter reduces the light that gets in, but so does faster shutter speed or smaller aperture. [/QUOTE]

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