Approximately, we assume their tracks are parallel lines in a synthetic aperturetime.
但在合成孔径时间内可以近似认为他们的轨道是直线且相互平行。
2
Usually one can first meter using a larger aperture (so that the metered exposure time is under 30 seconds), then stop down as necessary and multiply the exposure time accordingly.
Under those conditions photographers shoot with a fast exposure time and small aperture. That makes it impossible to capture faint objects in a dark background, such as stars.