What is the NPF Rule in astrophotography?

The NPF Rule is a formula for calculating the maximum shutter speed before stars begin to trail in astrophotography. It's more accurate than the 500 Rule because it accounts for your camera's pixel pitch (sensor resolution) and lens aperture. The formula is: NPF = (35 × Aperture + 30 × Pixel Pitch) ÷ Focal Length.

How is NPF different from the 500 Rule?

The 500 Rule only considers focal length (500 ÷ focal length), while the NPF Rule also factors in your camera's sensor resolution and lens aperture. This makes NPF significantly more accurate, especially for modern high-megapixel cameras where the 500 Rule often results in star trails.

What is pixel pitch and why does it matter?

Pixel pitch is the distance between pixels on your camera sensor, measured in micrometers (μm). Cameras with smaller pixels (higher megapixels) have smaller pixel pitch and show star trailing more easily, requiring shorter shutter speeds. The NPF Rule accounts for this.

What shutter speed should I use for Milky Way photography?

Use the NPF Calculator to find your exact maximum shutter speed based on your camera and lens. Typically, for a 24mm lens at f/2.8 on a full-frame camera, you'll get around 10-15 seconds. Always use the 'Sharp' setting for critical work.

How do I calculate my camera's field of view?

Field of view is calculated using the formula: FOV = 2 × arctan(sensor dimension ÷ (2 × focal length)). For example, a full-frame camera (36mm wide sensor) with a 24mm lens has a horizontal FOV of about 73.7°. Use our calculator for instant results with any sensor and focal length combination.

What is crop factor and how does it affect field of view?

Crop factor is the ratio of a full-frame sensor (36mm) to your sensor size. APS-C has ~1.5x crop factor, Micro 4/3 has 2x. A 24mm lens on APS-C gives the same FOV as a 36mm lens on full frame (24 × 1.5 = 36mm equivalent). Smaller sensors have narrower FOV at the same focal length.

How big is the Orion Nebula compared to the Moon?

The Orion Nebula (M42) is about 1.5° wide, roughly 3 times the diameter of the full Moon (0.5°). The entire Orion constellation spans about 20° × 25°, or about 40-50 Moon diameters across. Use a 200-300mm lens to fill the frame with just the nebula, or 50-85mm to capture the whole sword region.

What focal length should I use for the Andromeda Galaxy?

The Andromeda Galaxy (M31) spans about 3.2° × 1° including its outer regions. For a frame-filling shot on full frame, use 200-300mm. At 85mm, Andromeda will be about 15% of frame width - good for context shots. At 400mm+, you'll capture detailed core structure but may crop the outer spiral arms.

How do I photograph constellations?

Most constellations are 15-40° across. Use 24-50mm on full frame to capture medium constellations like Orion (20°) or Scorpius (30°). For larger asterisms like the Big Dipper (26°), go wider at 20-24mm. The Southern Cross at 6° can be captured with 85-135mm for a tighter composition.

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Understanding Field of View

Field of View (FOV) is the angular extent of the scene that your camera can capture. It's determined by two factors: your sensor size and your focal length. Understanding FOV is crucial for planning astrophotography compositions.

The FOV Formula

Field of view is calculated using:

FOV = 2 × arctan(sensor size ÷ (2 × focal length))

For example, a full-frame sensor (36mm wide) with a 24mm lens: FOV = 2 × arctan(36 ÷ 48) ≈ 73.7° horizontal

Crop Factor Explained

Crop factor compares your sensor to a "full frame" 35mm sensor (36×24mm):

Full Frame: 1.0× (reference)
APS-C (Canon): 1.6× crop
APS-C (Sony/Nikon): 1.5× crop
Micro Four Thirds: 2.0× crop
Medium Format: 0.79× (larger than FF!)

A 24mm lens on APS-C gives the same FOV as a 36mm lens on full frame. Smaller sensors have narrower fields of view.

Celestial Object Sizes

Knowing how big objects appear in the sky helps you choose the right lens:

Object	Angular Size	Suggested FL (FF)
Moon	0.5°	400-800mm
Orion Nebula	1.5°	200-400mm
Andromeda Galaxy	3°	135-300mm
Orion Constellation	20°	35-85mm
Milky Way Core	30°	14-35mm

Planning Your Shot

Once you know your FOV, use the NPF Calculator to determine the maximum shutter speed for sharp stars, then the Exposure Calculator to balance your ISO and aperture settings.

Frequently Asked Questions

What focal length do I need for the Milky Way?

For the Milky Way core (about 30° wide), use 14-24mm on full frame or 10-16mm on APS-C. This captures the bright galactic center with room for landscape foreground. For a full horizon-to-horizon arc, you'll need 8-14mm or create a panorama from multiple shots.

Does crop factor affect image quality?

Crop factor itself doesn't affect quality—it's just geometry. However, smaller sensors typically have smaller pixels, which can show noise more readily at high ISO. Full-frame cameras generally perform better in low light due to larger pixels. The advantage of crop sensors is extra "reach" for planetary and lunar photography.

How do I photograph something larger than my FOV?

Create a panorama! Take multiple overlapping shots (30-50% overlap) and stitch them in software like PTGui, Microsoft ICE, or Photoshop. For the full Milky Way arc, you might need 8-12 vertical frames at 24mm. Many astrophotographers create stunning panoramas this way.

Why is my actual FOV different from calculated?

Several factors can cause differences: lens focal length varies with focus distance (especially at close range), some lenses have "focus breathing," and manufacturer specifications can be approximate. For critical work, test your specific lens at infinity focus on a known target.

What's the best focal length for astrophotography?

There's no single "best"—it depends on your target. 14-24mm is most versatile for nightscapes and Milky Way. 50-85mm is great for constellation portraits and large nebulae. 135-300mm excels at galaxies and medium nebulae. 400mm+ is ideal for lunar, planetary, and small deep-sky objects. Many astrophotographers own lenses across this range.

FOV Calculator

Field of View Results

What Fits in Your Frame

Size Reference

Focal Length Guide for Astrophotography

Understanding Field of View

The FOV Formula

Crop Factor Explained

Celestial Object Sizes

Planning Your Shot

Frequently Asked Questions

NPF Calculator

Exposure Calculator

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