500 Rule Calculator - Free Online Tool

Find the maximum shutter speed for sharp stars in your astrophotography

The 500 Rule is the most widely used shortcut in astrophotography for preventing star trails in night sky images. When you photograph stars, the Earth's rotation causes them to move across the sensor during a long exposure — producing streaks instead of pinpoints of light. The 500 Rule gives you a quick, reliable estimate of the maximum shutter speed you can use before this trailing becomes noticeable. Simply divide 500 by the effective focal length (focal length multiplied by the camera's crop factor) and you have your maximum exposure time in seconds. Whether you're capturing the Milky Way over a dark sky landscape, photographing star fields with a wide-angle lens, or shooting deep-sky objects with a tracking telescope, knowing your maximum untracked exposure time is essential. Exceeding it means blurry, streaked stars that ruin an otherwise beautiful shot. Our calculator goes beyond the basic 500 Rule by also computing the 300, 400, and 600 rule variants, the more accurate NPF Rule, and even a declination adjustment for your target's position in the sky. Different cameras benefit from different rules. Modern high-megapixel mirrorless cameras (42MP, 61MP, or more) resolve star trails at much shorter exposures than older lower-resolution sensors. That's why the 300 Rule — a more conservative variant — is often recommended for cameras above 24MP, while the 600 Rule was common in the film era and early digital days when sensors were far less resolving. Our calculator takes your megapixel count into account and highlights the most appropriate rule for your equipment. The NPF Rule is the most technically accurate formula for preventing visible star trails. Unlike the simple 500 Rule, the NPF formula accounts for your lens aperture, pixel pitch (derived from megapixels and sensor dimensions), and the declination of your target. Stars near the celestial equator appear to move fastest because they sweep across the sky in a full 360° circle over 24 hours. Stars near the celestial poles barely move. Dialing in the declination of your target (for example, 0° for equatorial Milky Way core shots, or 60°+ for circumpolar targets) gives you a longer maximum exposure when shooting toward the poles. The exposure triangle section of our calculator also computes the resulting Exposure Value (EV) for your chosen combination of shutter speed, aperture, and ISO. For Milky Way photography, most astrophotographers target approximately -7 to -8 EV. If your EV is significantly below -8, you risk high noise from underexposure; if it's above -5, you may be overexposing skyglow. Use the EV feedback to fine-tune your ISO and aperture settings alongside the shutter speed from the 500 Rule. The sensor reference table in the results panel lets you instantly see how the 500 Rule result changes across all common sensor formats at your chosen focal length — a handy comparison if you own multiple cameras or are deciding between sensor formats. From Medium Format with a crop factor of 0.79x, all the way to the compact 1-inch sensor at 2.7x crop, every popular camera type is covered with its real-world sensor dimensions for accurate pixel pitch calculations.

Understanding the 500 Rule

What Is the 500 Rule?

The 500 Rule is a simple astrophotography guideline that tells you the maximum shutter speed to use before star trailing becomes visible in your images. The formula is: 500 divided by the product of your focal length and your camera's crop factor. For example, on a full-frame camera with a 24mm lens, the 500 Rule gives 500 ÷ 24 = approximately 20.8 seconds. On an APS-C camera (crop factor 1.5x) with the same lens, the effective focal length is 36mm equivalent, so the result is 500 ÷ (24 × 1.5) = approximately 13.9 seconds. The 500 Rule is a well-established heuristic that has guided astrophotographers since the film era and remains widely used today despite the availability of more precise formulas like the NPF Rule.

How Is It Calculated?

The core 500 Rule formula is: Max Shutter Speed (seconds) = 500 ÷ (Crop Factor × Focal Length in mm). The crop factor accounts for the sensor size relative to a 35mm full-frame standard. Full-frame sensors have a crop factor of 1.0x. APS-C sensors for Nikon, Sony, and Fuji cameras have 1.5x; Canon APS-C is 1.6x; Micro Four Thirds is 2.0x; and 1-inch sensors are approximately 2.7x. Medium Format sensors are actually larger than 35mm film, giving a crop factor below 1.0x (0.79x for 44×33mm). Rule variants swap out 500 for 300 (conservative, for high-res cameras), 400 (moderate), or 600 (relaxed, for older lower-res cameras). The NPF Rule incorporates aperture and pixel pitch: NPF = (16.856 × aperture + 0.0997 × focal length + 13.713 × pixel pitch in µm) ÷ (focal length × cos(declination)).

Why Does It Matter?

Star trailing is one of the most common and frustrating problems in astrophotography. The Earth rotates at approximately 0.00417807 degrees per second — a sidereal rotation rate that completes 360° every 23 hours and 56 minutes. During a long exposure, stars appear to arc across the sensor due to this rotation. At short focal lengths and with wide-angle lenses, this motion is slow enough to be invisible at reasonable ISO levels. But as focal length increases, even a few seconds of trailing becomes clearly visible in the final image. Understanding your maximum safe shutter speed before shooting allows you to choose the right ISO and aperture to get a properly exposed Milky Way shot without blurry stars ruining the foreground or background star field.

Limitations and Caveats

The 500 Rule is a simplification — it does not account for lens aperture, sensor resolution, or the sky position of your target. High-megapixel cameras (above 24MP) will reveal star trailing at significantly shorter exposures than the 500 Rule predicts, making the 300 Rule or NPF Rule more appropriate. The NPF Rule is more accurate but still assumes a stationary camera without equatorial tracking. For truly pinpoint stars across an entire wide-angle frame, a star tracker or equatorial mount is the only reliable solution, especially at focal lengths beyond 35mm. Additionally, the 500 Rule was originally developed for 35mm film cameras with much lower resolving power than modern digital sensors, so treating it as a guaranteed guide rather than a conservative starting point is recommended for modern mirrorless and DSLR cameras.

Key Astrophotography Exposure Formulas

500 Rule (Standard)

Max Exposure (s) = 500 / (Focal Length × Crop Factor)

The classic astrophotography rule for maximum shutter speed before star trails become visible. Divide 500 by the effective focal length (actual focal length multiplied by the sensor's crop factor).

NPF Rule (Precision)

Max Exposure (s) = (16.856×N + 0.0997×f + 13.713×p) / (f × cos(δ))

The more accurate formula accounting for aperture (N), focal length (f in mm), pixel pitch (p in µm), and target declination (δ). Gives shorter, safer results than the 500 Rule for high-resolution sensors.

Pixel Pitch from Megapixels

Pixel Pitch (µm) = Sensor Width (mm) × 1000 / √(MP × Aspect Ratio)

Derives the physical size of a single pixel from the sensor dimensions and megapixel count. Smaller pixel pitch means the sensor resolves finer detail and star trails become visible sooner.

Star Trail Threshold

Trail Length (px) = (Exposure × 15 × cos(δ) × 3600) / (Plate Scale × 3600)

Calculates the length of a star trail in pixels for a given exposure time. Earth rotates at 15 arcseconds per second; dividing by plate scale converts angular motion to pixel displacement.

Astrophotography Reference Data

Crop Factors by Camera Sensor Type

Sensor dimensions and crop factors for common camera formats, from medium format to compact 1-inch sensors.

Sensor Format	Dimensions (mm)	Crop Factor	Typical Pixel Pitch (µm)
Medium Format (44×33)	43.8 × 32.9	0.79×	5.3 (51 MP)
Full Frame (36×24)	36.0 × 24.0	1.0×	4.4 (45 MP), 5.9 (24 MP)
APS-C Nikon/Sony/Fuji	23.5 × 15.6	1.5×	3.9 (26 MP), 4.8 (20 MP)
APS-C Canon	22.3 × 14.9	1.6×	3.7 (32 MP), 4.3 (24 MP)
Micro Four Thirds	17.3 × 13.0	2.0×	3.3 (25 MP), 3.8 (20 MP)
1-inch Sensor	13.2 × 8.8	2.7×	2.4 (20 MP)

Max Exposure by Focal Length (500 Rule, Full Frame)

Quick-reference table showing maximum shutter speeds for common focal lengths on a full-frame (1.0× crop) camera using the 500 Rule.

Focal Length (mm)	500 Rule (s)	300 Rule (s)	600 Rule (s)
14	35.7	21.4	42.9
20	25.0	15.0	30.0
24	20.8	12.5	25.0
35	14.3	8.6	17.1
50	10.0	6.0	12.0
85	5.9	3.5	7.1
135	3.7	2.2	4.4
200	2.5	1.5	3.0

Worked Examples

24mm Lens on Full Frame Camera

A photographer shoots the Milky Way with a 24mm f/1.4 lens on a 24MP full-frame camera (crop factor 1.0×, sensor 36×24mm).

500 Rule: Max exposure = 500 / (24 × 1.0) = 20.8 seconds

300 Rule (conservative): 300 / (24 × 1.0) = 12.5 seconds

Pixel pitch: 36mm × 1000 / √(24M × 1.5) = 5.97 µm

NPF Rule: (16.856 × 1.4 + 0.0997 × 24 + 13.713 × 5.97) / (24 × cos(0°)) = (23.6 + 2.4 + 81.9) / 24 = 4.5 seconds

The 500 Rule suggests 20.8 seconds, but the NPF Rule recommends only 4.5 seconds for truly pinpoint stars. A practical starting point is 12–15 seconds at ISO 3200 for this setup.

50mm Lens on APS-C Crop Sensor

An astrophotographer uses a 50mm f/1.8 lens on a 26MP APS-C camera (Nikon, crop factor 1.5×, sensor 23.5×15.6mm).

Effective focal length: 50 × 1.5 = 75mm equivalent

500 Rule: Max exposure = 500 / (50 × 1.5) = 6.7 seconds

300 Rule: 300 / (50 × 1.5) = 4.0 seconds

Pixel pitch: 23.5mm × 1000 / √(26M × 1.5) = 3.76 µm

NPF Rule: (16.856 × 1.8 + 0.0997 × 50 + 13.713 × 3.76) / (50 × cos(0°)) = (30.3 + 5.0 + 51.6) / 50 = 1.7 seconds

At 50mm on APS-C, even the 500 Rule allows only 6.7 seconds. The NPF Rule limits you to 1.7 seconds — a star tracker is highly recommended for focal lengths above 35mm on crop sensors.

Targeting Circumpolar Stars at High Declination

Shooting at 24mm f/2.8 on full frame (24MP), targeting stars near Polaris at declination +70°.

500 Rule (does not account for declination): 500 / 24 = 20.8 s

cos(70°) = 0.342

NPF Rule with declination: (16.856 × 2.8 + 0.0997 × 24 + 13.713 × 5.97) / (24 × 0.342)

= (47.2 + 2.4 + 81.9) / 8.2 = 16.0 seconds

Targeting stars at +70° declination allows 16.0 seconds with the NPF Rule — over 3× longer than the 4.5 seconds allowed for equatorial targets. Stars near the poles barely move, giving you significantly more exposure time.

How to Use the 500 Rule Calculator

Enter Your Focal Length

Type your lens's actual focal length in millimeters — for example, 24mm, 35mm, or 50mm. Use the prime or zoom focal length you plan to shoot at, not the 35mm equivalent. The calculator multiplies by your crop factor automatically.

Select Your Sensor Size

Choose your camera's sensor format from the dropdown. This sets the crop factor automatically: 1.0x for Full Frame, 1.5x for APS-C (Nikon/Sony/Fuji), 1.6x for APS-C (Canon), 2.0x for Micro Four Thirds, or 2.7x for 1-inch sensors. Medium Format cameras with a 0.79x crop factor are also supported.

Add Megapixels and Aperture for NPF Results

Enter your camera's megapixel count and your shooting aperture (f-stop) to unlock the more accurate NPF Rule result. The calculator automatically derives your sensor's pixel pitch from the megapixels and sensor dimensions, eliminating the need for manual lookup.

Adjust Declination and Review the Exposure Triangle

Drag the declination slider to match your target's position in the sky — 0° for equatorial targets (Milky Way core), or higher values for circumpolar targets. Add your ISO to see the resulting Exposure Value and check whether your full exposure triangle is within the optimal range for Milky Way photography.

Frequently Asked Questions

What is the 500 Rule in astrophotography?

The 500 Rule is a simple formula for calculating the maximum shutter speed you can use before star trails become visible in a night sky photograph. The formula is: 500 divided by the effective focal length (focal length × crop factor). For example, with a 24mm lens on a full-frame camera, the 500 Rule gives approximately 20.8 seconds. Exceeding this time means the Earth's rotation will cause stars to streak across the sensor, appearing as short arcs rather than pinpoints of light. The rule was originally derived from observations with 35mm film cameras and remains a quick and widely-used starting point for astrophotography beginners and experienced photographers alike.

What is the difference between the 300, 400, 500, and 600 rules?

All four variants use the same formula structure — divide the constant by the effective focal length — but differ in how conservative the result is. The 300 Rule gives the shortest (most conservative) safe exposure, recommended for modern high-resolution cameras with 24MP or more. The 400 Rule is a moderate compromise. The 500 Rule is the traditional standard that most astrophotographers learn first. The 600 Rule allows the longest exposures and was more appropriate for older lower-resolution film and early digital cameras that could not resolve the fine star trailing that modern sensors capture. For cameras with 40MP or more, the 300 Rule is strongly recommended.

What is the NPF Rule and is it more accurate than the 500 Rule?

The NPF Rule is a more mathematically rigorous formula for calculating maximum star-trail-free exposure. Unlike the 500 Rule, the NPF formula incorporates your lens aperture (N), the sensor's pixel pitch in micrometers (P), and the declination of your celestial target (F for declination adjustment). The full formula is: (16.856 × aperture + 0.0997 × focal length + 13.713 × pixel pitch) ÷ (focal length × cos(declination)). The NPF Rule is consistently more accurate for high-resolution cameras and telephoto lenses, giving shorter and safer results than the 500 Rule predicts. Our calculator auto-derives pixel pitch from your megapixels and sensor dimensions, so you don't need to look it up manually.

How does declination affect the maximum shutter speed?

Stars at the celestial equator (declination 0°) sweep across the sky at the maximum angular speed because they travel the full circumference of the celestial sphere in one sidereal day. Stars near the celestial poles travel in much smaller circles and appear to barely move. The declination correction divides the base NPF result by cos(declination), so a target at 60° declination allows twice as long an exposure as the same target at 0°. For Milky Way core photography, your target is near declination −30° to −30°, so the correction is modest. For circumpolar targets like star-trail photography around Polaris at 89°N declination, you can expose for many minutes without noticeable trailing.

What Exposure Value (EV) should I target for Milky Way photography?

Experienced Milky Way photographers typically target an Exposure Value of approximately −7 to −8 EV for optimal results. This range captures enough light from the faint diffuse glow of the Milky Way and individual stars without overexposing the brighter parts of the sky or introducing excessive light pollution. An EV below −8 often indicates underexposure — you may need to raise ISO or widen your aperture. An EV above −5 suggests possible overexposure or that skyglow is brightening the frame. The standard EV formula is: EV = log₂(aperture² ÷ (shutter speed × ISO ÷ 100)). Our calculator computes this automatically from your input values.

Why do high-megapixel cameras require shorter exposures than the 500 Rule suggests?

High-megapixel cameras have smaller individual pixels packed more densely across the sensor. This smaller pixel pitch means each pixel captures light from a narrower angle of sky, making it more sensitive to the angular movement of stars during an exposure. A 61MP Sony camera has a pixel pitch of roughly 3.76µm, while a 12MP camera of the same sensor size has a pitch of about 8µm — more than double. Even the same tiny angular movement of a star translates into a proportionally larger shift across more pixels, making trailing visible sooner. The NPF Rule accounts for this by incorporating pixel pitch directly into the formula, and the 300 Rule was developed as a simpler heuristic to compensate for the limitations of modern high-resolution sensors.