One of the most important variables in long exposure night photography is exposure time! Why? Exposure time defines the overall amount of light that will reach the sensor of your camera, making the starry summer sky visible in your photos. In principle, the longer you expose your photo, the higher the level of detail of the stars and the landscape features.
However, too long exposure with respect to the focal length of your lens (one of your lens features given in mm, e.g. Nikkor 24-70mm f2.8), can ultimately contribute to a lack of detail and definition of the starry sky in your photograph.
Earth, just like any other celestial body, is a subject to constant motion with respect to other celestial bodies. If you expose your photograph long enough you shall start observing the aforementioned motion in a form of star-trail effect.
An obvious question arises, how long can one expose a photograph in order to acquire maximum amount of light, yet with no visible star-trail effect?
The answer to the above question depends heavily on the choice of the focal length on your lens and your camera body!
Note: I have intentionally omitted the effect of your geographical localization which also influences the “apparent” motion of stars in a long exposure photograph. I intend to address this issue in an iOS app that will use your actual GPS localization to provide the most accurate result!.
In order to make our lives as easy as possible, I have compiled a little tool below that given your camera model and a creative choice of you focal length should yield the maximal exposure time, that should still allow you to take a photograph without any visible trailing effect!
Please remember you can use the tool in both ways, e.g. very quickly calculate the expected minimal time that should yield nice star trail for a range of tele-photo lenses!
If you have any extra questions, or want me to add extra cameras to the tool, please leave a comment below!