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Message: <blockquote data-quote="Moab Man" data-source="post: 660001" data-attributes="member: 11881">[MENTION=44173]Yan Lauzon[/MENTION]The fastest glass you can afford is a two fold benefit. First, You're trying to collect light that is ever so faint. Add to it, DX sensors have more ISO noise than full frame so the more light you can get is a shorter exposure and less noise. Second, exposure time. The longer you have to have the shutter open the more star movement (star trails) you have. It is a fine balancing act between long enough to collect enough light, but not too long to get noticeable movement, and not having to crank up the ISO to create a noisy image. It's like a waiter carrying a very full tray on one hand. When it's all balanced out it works well. Get it wrong and it all crashed down.</blockquote>