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.

What is a Sky Quality Meter (SQM) reading?

A Sky Quality Meter measures sky brightness in magnitudes per square arcsecond. Higher numbers mean darker skies. Bortle 1 reads 21.7-22.0, while inner-city Bortle 9 reads below 17.5. An SQM reading of 21.0+ is excellent for astrophotography.

Can I do astrophotography from a light-polluted city?

Yes, but with limitations. From cities (Bortle 7-9), focus on the Moon, planets, and bright stars. Use light pollution filters or narrowband filters for deep-sky objects. For Milky Way photography, you'll need to travel to Bortle 4 or darker locations.

Bortle Scale Guide - Is Your Sky Dark Enough? | Interactive Chart

Understanding the Bortle Scale

The Bortle Dark-Sky Scale was created by amateur astronomer John E. Bortle and published in Sky & Telescope magazine in February 2001. It provides a standardized way to compare the darkness of observing sites and communicate about sky quality.

Why Light Pollution Matters

Light pollution doesn't just wash out stars—it fundamentally changes what you can photograph:

Signal-to-noise ratio: Artificial light adds noise to every pixel, requiring shorter exposures
Dynamic range: Bright skies compress the range between sky and faint objects
Color accuracy: Sodium and LED lights create color casts that are hard to remove
Gradient handling: Light domes create uneven brightness across the frame

Measuring Sky Darkness

There are several ways to measure sky darkness:

Naked Eye Limiting Magnitude (NELM): The faintest star you can see. Requires star chart knowledge.
Sky Quality Meter (SQM): Electronic device measuring magnitudes per square arcsecond. Most accurate.
Light Pollution Maps: Satellite data showing artificial light at night. Good for planning.
Milky Way visibility: Quick visual assessment of sky quality.

Finding Dark Skies

To find Bortle 3 or darker skies, look for:

Dark Sky Parks: Designated areas with light pollution controls
Remote locations: 50+ miles from major cities
High altitude: Less atmosphere = less scattered light
Away from highways: Even rural highways create light corridors

🌐 Useful Resources

• Light Pollution Map - Interactive world map
• Dark Site Finder - Find dark sky locations
• International Dark-Sky Association - Dark Sky Parks list

Camera Settings by Bortle Class

As a general rule, darker skies allow higher ISO and longer exposures:

Bortle	ISO Range	Exposure Strategy
1-3	1600-3200	Use full NPF Rule time
4-5	3200-6400	NPF Rule, gradient removal needed
6-7	800-1600	Shorter exposures, heavy processing
8-9	100-400	Moon/planets only, or use filters

Frequently Asked Questions

What is the Bortle Scale?

The Bortle Dark-Sky Scale is a nine-level numeric scale that measures the night sky's brightness. Created by John E. Bortle in 2001, it ranges from Class 1 (excellent dark-sky site) to Class 9 (inner-city sky). It helps astronomers and astrophotographers plan observations and communicate about sky quality.

What Bortle class do I need to photograph the Milky Way?

For impressive Milky Way photography, aim for Bortle 4 or darker (lower number). You can capture the Milky Way at Bortle 5 when it's directly overhead, but it will be washed out with little structure. At Bortle 1-3, the Milky Way is stunning with visible dust lanes and can even cast shadows. Bortle 6+ makes Milky Way photography very difficult or impossible.

How do I find my local Bortle class?

Use online light pollution maps like lightpollutionmap.info or the Clear Outside app. You can also purchase a Sky Quality Meter (SQM) for precise measurements. As a rule of thumb: rural areas 30+ miles from cities are typically Bortle 3-4, suburbs are Bortle 5-6, and urban areas are Bortle 7-9.

Do light pollution filters really work?

Light pollution filters help in Bortle 4-7 locations by blocking common artificial light wavelengths (sodium and mercury vapor). They're most effective for emission nebulae. However, they can't replace a truly dark sky—traveling to a darker location will always produce better results than filtering heavy light pollution. Modern LED streetlights are also harder to filter.

Does the Moon affect Bortle class?

While the Bortle scale measures artificial light pollution, a full Moon can make even a Bortle 1 site perform like Bortle 4-5 for practical astrophotography. Always plan your Milky Way and deep-sky sessions around the new Moon. The Moon's effect varies by phase and altitude—a crescent Moon low on the horizon has minimal impact.

Bortle Scale Guide

Bortle Dark Sky Scale

Bortle 5: Suburban Sky

Milky Way Visibility

What You Can Shoot

Challenges

Example Locations

Light Pollution Tips

Check Light Pollution Maps

Moon Phase Matters

Shoot Away from Cities

Use Light Pollution Filters

Altitude Helps

Winter is Often Better

Equipment by Light Pollution Level

Bortle 1-3

Bortle 4-5

Bortle 6-7

Bortle 8-9