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.

Do I need a light pollution filter for astrophotography?

It depends on your sky darkness and target. In Bortle 1-3 skies, filters often reduce signal without benefit. In Bortle 5+ for emission nebulae, filters significantly help. For galaxies and star clusters, even in light pollution, broadband filters or more integration time may be better than narrowband.

What is the difference between L-eXtreme and L-eNhance filters?

L-eXtreme has narrower 7nm bandpasses for Hα and OIII, blocking more light pollution but requiring more integration time. L-eNhance has wider ~24nm bandpasses, allowing more signal through but less aggressive LP rejection. L-eXtreme is better for Bortle 7-9, L-eNhance for Bortle 5-7.

Can I use narrowband filters for galaxies?

Generally no. Galaxies emit broadband light (starlight), not emission lines. Narrowband filters will block most of the galaxy signal, resulting in very dim images requiring massive integration times. Use broadband filters like L-Pro or no filter with more exposure time.

What filter should I use for the Orion Nebula?

The Orion Nebula (M42) is bright and Hα-dominant. In dark skies (Bortle 1-4), no filter often gives best results. In moderate LP (Bortle 5-6), L-eNhance or L-Pro works well. In heavy LP (Bortle 7-9), L-eXtreme or similar duoband filter is recommended.

Why do some filters produce false color images?

Duoband and narrowband filters only pass specific wavelengths (like Hα red and OIII teal). Your camera's color filter array tries to interpret these as full-spectrum color, creating odd magenta/green tints. You need to process these images differently, often using HOO or SHO palettes.

NPF CalculatorAstrophotography Tools

Filter Advisor

Interactive spectrum analyzer with honest, personalized filter recommendations.

See exactly how light pollution affects your imaging and which filters actually help. Sometimes the best advice is: you don't need a filter.

Light Pollution Spectrum Analyzer

Show Emission Lines

Light Pollution

The red area shows light pollution intensity at Bortle 6. Vertical lines show emission wavelengths from nebulae. Select a filter to see what it blocks.

Select a filter below to see how it affects the spectrum

Tell Us About Your Setup

Your Sky Darkness (Bortle Class)

Bortle 6Moderate LP

Bright Suburban - Milky Way only visible at zenith

What Do You Want to Photograph?

Examples: Orion Nebula (M42), Lagoon Nebula (M8), Rosette Nebula

Your Camera

IR filter removed for Ha sensitivity

Budget Range

Prioritize natural star colors over LP rejection

💡 Before You Buy a Filter...

🚗

Drive to darker skies

A 30-60 minute drive to Bortle 4-5 skies will beat any filter. Use lightpollutionmap.info to find spots.

Filter Recommendations

BEST MATCHbroadbandbroadband

CLS (City Light Suppression)

Entry-level filter that blocks sodium and mercury lines while preserving natural star colors.

✓Good balance for moderate light pollution

✓Passes key emission lines for Hα-Dominant Emission Nebulae

Match95%

LP Rejection: 40%

Integration: 1x

$ Budget

broadbandbroadband

Optolong L-Pro

Popular broadband filter maintaining natural colors while blocking common light pollution wavelengths.

✓Good balance for moderate light pollution

✓Passes key emission lines for Hα-Dominant Emission Nebulae

Match95%

LP Rejection: 55%

Integration: 1x

$$ Mid-range

triband24nm/12nm

Optolong L-eNhance

Middle-ground filter passing Hβ, OIII, and Hα. Good balance of light pollution rejection and color.

✓Good balance for moderate light pollution

✓Passes key emission lines for Hα-Dominant Emission Nebulae

Match95%

LP Rejection: 80%

Integration: 1x

$$ Mid-range

duoband7nm

Optolong L-eXtreme

Aggressive dual-narrowband filter for emission nebulae in heavily light-polluted areas. Produces false color.

✓Passes key emission lines for Hα-Dominant Emission Nebulae

Match75%

LP Rejection: 95%

Integration: 1.5x

$$$ Premium

duoband12nm

IDAS NBZ

High-quality Japanese dual-narrowband with excellent coatings and minimal halos.

✓Passes key emission lines for Hα-Dominant Emission Nebulae

Match75%

LP Rejection: 90%

Integration: 1.5x

$$$ Premium

uvirbroadband

UV/IR Cut

Basic filter to restore color balance on modified cameras. Not for light pollution.

Match50%

LP Rejection: 5%

Integration: 1x

$ Budget

Understanding the Science

Why Filters Work

Light pollution is concentrated at specific wavelengths (LED peaks, sodium lines). Astronomical objects emit at different wavelengths (Hα, OIII). Filters block LP while passing the signal you want.

Broadband vs Narrowband

Broadband filters pass a wide range of colors - good for galaxies and natural color. Narrowband filters only pass specific emission lines - better LP rejection but false color output.

The Trade-Off

More aggressive filters block more LP but also block signal. A 7nm filter might need 2-3x more exposure than broadband. Sometimes more integration beats expensive filters.

Key Emission Wavelengths

Hα (656nm)

OIII (501nm)

Hβ (486nm)

SII (672nm)

Frequently Asked Questions

Do I actually need a light pollution filter?

Honestly? Maybe not. It depends on three factors:

Your sky: Bortle 1-3 = usually no filter needed
Your target: Emission nebulae benefit most; galaxies often don't
Your alternatives: Driving to darker skies or more integration time might be better

Use the advisor above to get specific guidance for your situation.

Why do my narrowband images look magenta/green?

This is normal! Duoband filters (L-eXtreme, L-eNhance) only pass Ha (red) and OIII (teal) wavelengths. Your camera's Bayer filter tries to interpret this limited spectrum as full color, creating false colors.

Solutions: Process using HOO palette (Ha→Red, OIII→Green+Blue) or use specialized tools like PixInsight's NBRGBCombination or Photoshop channel manipulation.

What's the difference between 7nm and 12nm bandwidth?

The bandwidth is how wide the "window" is for letting light through:

7nm: Narrower window = blocks more LP but also blocks some signal. Best for severe LP (Bortle 7-9). Needs more integration time.
12nm: Wider window = more signal through but less LP rejection. Better for moderate LP (Bortle 5-7).

Think of it as a tradeoff: aggressive filtering vs faster image acquisition.

Can I use the same filter for galaxies and nebulae?

Not ideally. Galaxies are broadband targets (starlight), while emission nebulae glow at specific wavelengths (Ha, OIII).

For galaxies: L-Pro or similar broadband LP filter, or no filter
For emission nebulae: L-eNhance, L-eXtreme, or narrowband
Compromise: L-eNhance works "okay" for both but isn't optimal for either

If you only buy one filter, L-Pro is the safest choice for variety.

I have a stock (unmodified) DSLR - which filter should I get?

Stock DSLRs have an IR-cut filter that blocks about 75% of Ha light. This limits your options:

Best choice: L-Pro or CLS (broadband) - maintains what sensitivity you have
Okay: L-eNhance - wider Ha band helps partially compensate
Not recommended: L-eXtreme or 7nm filters - too little Ha gets through

Consider getting your camera modified for serious nebula work - it's usually under $300 and makes a huge difference.