[davidmanze]

You should be able to do exposures up to around 30 seconds before star trailing becomes apparent when using a ultra wide lens.

With the Samyang 14mm star trails are just visible at 15 seconds on the D750, 20 secs clearly so. I limit it to 15 seconds!

 

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 "But I guess, the "wow" milky way shots are done by exposure stacking and at shorter times."

 

Personally I can't be bothered with shot stacking, I take a single shot and boost areas with the adjustment brush and full on "clarity", if you keep on adding with the adjustment brush after about 3-4 passes the milky ways stands out nicely!