I love looking at the night sky. It's breathtaking, both visually and scientifically. I also love thinking about taking pictures of the night sky (haven't done it quite yet). You see, something interesting sometimes happens when you try to snap a picture of the night sky. The stars appear blurry. But how can that be? All the camera's settings seem correct: aperture, ISO and shutter speed. So how can the stars be blurry? In this case, the error isn't in the focus setting, it's in the shutter speed. This is an error of time.
The Earth is round and it rotates. That is not a question, it's a fact. A fact that can actually be shown using a camera. The Earth completes one full rotation every 23 hours and 56 minutes. So as seen from Earth, all stars rotate one full circle and return to the same place in the sky after this period of time (at least for now, who knows what'll happen in the future). This revolution, with respect to the stars, is what astronomers call a sidereal day. It is also the reason that if the camera's shutter speed is too slow, the stars will have moved during the frame and appear blurry. But it's so dark at night, the only way to let more light into the lens is by lengthening the shutter speed. Is there any way to confidently calculate the correct shutter speed as to keep the stars in focus? Yes there are, a couple actually. I've included two of the more popular ones below.
The 500 Rule is a simple, classic formula: divide 500 by your effective focal length (focal length factoring in for crop factor). The result is the maximum exposure time in seconds before stars begin to blur and turn into trails. It's quick and works reasonably well for wide-angle lenses, but it doesn't account for a camera's sensor resolution or aperture.
The NPF Rule is a more advanced calculation that considers your camera's megapixels, aperture, and sensor size for a more precise result. By accounting for pixel pitch (how densely packed pixels are on your sensor), it provides better recommendations for higher-resolution cameras. NPF typically gives shorter exposure times than the 500 Rule for modern high-res sensors.
Disclaimer: This calculator uses a simplified version of the NPF rule for educational purposes.
Actual optimal exposure times may vary depending on your camera's exact sensor dimensions, pixel pitch,
atmospheric conditions, and post-processing tolerance for minor star trailing.
For precise astrophotography planning, consult dedicated NPF calculators that account for your camera model's
sensor data.
Have a good one!
-Michael