BSL Deep-Sky Imaging – The 500 Rule

The 500 Rule
The 500 Rule is a formula used in astrophotography and night photography
to calculate the maximum shutter speed (exposure time)
you can use before stars begin to trail or blur due to the Earth’s rotation.
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Maximum Shutter Speed (seconds) = 500 ÷ (Focal Length (mm) x Crop Factor)
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The Formula
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How the Components Work
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500 (Base Constant):
A standard baseline standardizing the Earth’s rotational speed relative to typical optical field sizes.
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Focal Length (mm):
The focal length of your camera lens. Wider lenses (smaller focal lengths, like 14mm or 24mm) cover a broader section of the sky, allowing for longer exposures before star movement becomes noticeable.
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Telephoto lenses (larger focal lengths, like 50mm or 85mm) magnify the sky, causing stars to cross pixels much faster.

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Crop Factor (Sensor Multiplier):
Adjusts for your camera’s sensor size relative to a standard 35mm full-frame sensor:

Camera Sensor Type	Lens Focal Length	Calculation	The 500 Rule Max Shutter Speed	My recommend (For better results)
Full-Frame (1.0x)	14mm	500 ÷ (14 x 1.0)	35 seconds	27 seconds
Full Frame (1.0x)	24mm	500 ÷ (24 x 1.0)	20 seconds	15 seconds
Full-Frame (1.0x)	50mm	500 ÷ (50 x 1.0)	10 seconds	8 seconds
APS-C (1.6x) (Canon)	16mm	500 ÷ (16 x 1.5)	20 seconds	17 seconds
APS-C (1.5x) (Sony /Nikon / Fuji)	16mm	500 ÷ (16 x 1.6)	20 seconds	17 seconds
Micro 4/3 (2.0x)	12mm	500 ÷ (12 x 2.0)	20 seconds	17 seconds
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Please note – ISO does not affect the 500 Rule calculation.

How ISO Fits In
While ISO doesn’t affect exposure time or star trailing, it plays a separate role in night and deep-sky photography:
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Shutter Speed (via 500 Rule):
Dictates how long your shutter can stay open without causing blurry star trails.

Aperture (f-stop):
Set as wide as possible (e.g., f/1.4, f/2.8) to let in maximum light.

ISO:
Set after your shutter speed and aperture are determined to properly brighten the image (typically between ISO 1600 and 6400, depending on your camera’s noise performance).

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