Introduction to starry sky and milky way photography: 500 rule/NPF rule, focus on infinity and star field noise reduction

1. Calculation of Earth’s rotation and star trails (Star Trails)

The Earth spins 360 degrees every 24 hours, meaning stars in the sky move relative to the line of sight from the ground at a rate of 15 degrees per hour (15 arc minutes per minute). In a long exposure, if the shutter time is slightly longer, the originally clear and bright light spot will be stretched into an ugly short white dotted stick on the pixel array.

1. Classic 500 Rule:

• Formula: Maximum shutter time without tailing (seconds) = 500 / lens equivalent focal length (mm)
• Case: Using a full-frame 14mm ultra-wide-angle lens, the maximum shutter time is 500 / 14 ≈ 35 seconds; if using a 24mm lens, the maximum shutter time is 500 / 24 ≈ 20 seconds.

2. Precise NPF rules for the modern high-pixel era:

• The 500 rule is a product of the low resolution of the film era. On a 40-60 million pixel mirrorless camera, the pixel density is extremely high. When shooting according to the 500 rule, it will inevitably smear when zoomed in!
• Modern industry standards use stricter NPF rules (combined aperture value N, pixel pitch P and focal length F). The empirical correction is: Directly use the "300 rule" or the "250 rule" (for example, the 14mm lens safety shutter is recommended to converge to 15 - 20 seconds).

2. 100% absolutely accurate focusing technique in the dark night

In the wilderness where you can't see your fingers, the autofocus is completely paralyzed. It is also a big mistake to directly twist the focus ring to the position marked with infinity (∞) (modern lenses reserve space for thermal expansion and contraction, and usually go "beyond infinity" causing the entire film to be out of focus).

1. Switch the lens to MF (manual focus);
2. Open the aperture to the maximum (such as f/1.8 or f/2.8) and temporarily increase the ISO to 12800 or even 25600 (only to brighten the viewfinder);
3. Find the brightest star in the night sky (such as Vega, Jupiter or Venus) and turn on the maximum digital magnification focus assist (10x or 15x) of the camera screen;
4. Turn the focus ring very slowly and focus on the bright star on the screen until the light spot shrinks from a big blurry circle to the smallest and sharpest needle-level bright spot**! After focusing is completed, tear off a piece of masking tape to stick the lens focus ring tightly to prevent accidental touching, and adjust the ISO back to 3200 or 6400 to start shooting.

3. The ultimate image quality upgrade for Hoshino Photography: Equatorial Mount vs. Landscape Stacker (Starry Landscape Stacker)

• Limitations of high-sensitivity single shots: Single ISO 6400 exposure for 20 seconds, the picture is extremely noisy;
• Multiple Alignment and Stacking: Continuously shoot 10 to 16 starry skies with the same parameters and import them into professional software (Starry Landscape Stacker for Mac, Sequator for Windows). The software will intelligently track the star displacements and automatically align them, and use the median algorithm to offset and smooth out random high-sensitivity noise, instantly obtaining magazine-level views as pure and deep as ISO 200, with the Milky Way's level of detail gushing out.

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