Astrophotography
Photography of the night sky, from wide-field starscapes to magnified images of deep-sky objects.
Formula / rule
Rule of 500: t ≈ 500 / focal length
Measured in
seconds
In depth
Everything follows from extreme light scarcity: fast lenses, high ISO, long exposures and stacking are all attempts to gather enough signal. Earth's rotation sets a hard limit on single-exposure length before stars trail, which is why serious deep-sky work uses a tracking mount and combines dozens of frames with calibration frames.
Examples in practice
Twenty stacked 20-second frames at f/1.8 and ISO 3200 reveal Milky Way structure invisible in any single exposure.