Astrophotography Exposure Calculator
Astrophotography Exposure Calculator
The focal length of your lens or telescope in millimeters
Your lens f-number (required for NPF Rule)
Used to auto-derive pixel pitch if not entered directly
Pixel size in micrometers — auto-calculated from MP + sensor if left blank
Declination of your target (0° = celestial equator, 90° = Polaris)
Common Target Declinations
Orion Nebula (M42): -5°
Andromeda Galaxy (M31): +41°
Milky Way Core: -29°
Pleiades (M45): +24°
Polaris: +89°
Enter Your Camera Settings
Fill in your focal length, sensor size, and aperture to get the maximum safe exposure time and compare all calculation rules side by side.
Find the perfect shutter speed to capture sharp, trail-free stars
Astrophotography is a patience-testing craft that rewards those who master the relationship between time, light, and the relentless rotation of the Earth. At the heart of every successful night-sky image lies one critical question: how long can you expose before stars turn from pinpoints into streaks? The answer depends on your focal length, sensor size, aperture, pixel density, and even the direction of the sky you are pointing toward. This free astrophotography exposure calculator solves that question instantly, whether you are shooting the Milky Way on a tripod or planning a deep-sky session with a tracked equatorial mount. For photographers using a fixed tripod, the goal is to maximize the single-frame exposure so that enough light is collected without visible star trails. The traditional 500 Rule divides 500 by the effective focal length (focal length × crop factor) and gives a rough-and-ready answer in seconds. It was designed when 10–12 megapixel cameras were common; on a modern 45-megapixel full-frame body, the 500 Rule often allows too much trailing. That is where the NPF Rule comes in. Developed by astrophotographer Frédéric Michaud, the NPF Rule incorporates lens aperture and pixel pitch to produce results that are two to four times more conservative and far more accurate on dense sensors. This calculator computes all four standard rules — 500, 400, 300, and NPF — simultaneously, so you can compare them side by side and choose the one that fits your priorities. For deep-sky imagers using an equatorial mount, the challenge is entirely different. Your mount tracks the sky, cancelling Earth's rotation, so you can theoretically expose for minutes or hours. However, light pollution from the sky background introduces noise into each sub-frame. The question becomes: how long should each individual sub-exposure be before sky glow dominates the signal and stacking gives diminishing returns? This tool's Sub-Exposure Planner answers that question using the Robin Glover (SharpCap) method, which takes the camera's read noise and the Bortle-scale light pollution at your site and computes the optimal sub-exposure in seconds. It also applies correct multipliers for monochrome cameras, DSLR/OSC colour sensors, and narrowband filters. The calculator supports declination correction for all rule-based results. Stars near the celestial equator (declination 0°) move fastest across the sensor and produce the longest trails; stars near Polaris (declination +89°) barely drift at all. By entering the declination of your target — Orion is about -5°, Andromeda is +41°, the Milky Way core is around -29° — you can reclaim several extra seconds of allowable exposure at objects higher in the sky. Pixel pitch is the key variable the 500 Rule ignores. If you do not know your camera's pixel pitch in micrometers, the calculator can derive it automatically from your megapixel count and sensor format. Simply select your sensor size (Full Frame, APS-C, Micro 4/3, etc.) and enter your megapixel count. The tool calculates sensor dimensions, divides by the number of pixels, and extracts the per-pixel size. This derived value is then fed directly into the NPF formula and the Plate Scale method. The results section shows you the maximum exposure from each rule as a horizontal bar chart, making it instantly clear which formula is most conservative. You also get the effective focal length, pixel scale in arcseconds per pixel (a key metric for understanding image resolution), field of view in degrees, and — for the Sub-Exposure Planner — the recommended frame count and total integration time to reach a useful signal-to-noise ratio. Whether you are a beginner trying the Milky Way for the first time with a kit lens or an experienced imager planning a narrowband session on the Horsehead Nebula, this calculator gives you all the numbers you need to make confident exposure decisions before you ever leave the house.
Understanding Astrophotography Exposure
What Is Star Trailing?
Star trailing occurs because the Earth rotates at approximately 15 arcseconds per second (one full revolution in 23 hours, 56 minutes, 4 seconds — the sidereal day). When you photograph the night sky from a fixed tripod, every star traces a short arc across your sensor during the exposure. If the arc is shorter than one pixel, it is invisible and the star looks like a perfect point. If the arc exceeds one or two pixels, you will see an elongated streak instead of a pinpoint, especially noticeable in high-resolution crops. The maximum exposure time before trailing becomes visible is what every exposure rule attempts to calculate. On a 24-megapixel full-frame camera at 24mm f/1.8, that limit is roughly 12–14 seconds using the NPF Rule — far shorter than the 500 Rule suggests.
How Are Exposure Times Calculated?
The classic 500 Rule computes: Exposure = 500 ÷ (Focal Length × Crop Factor). The NPF Rule is more precise: Exposure = (35 × Aperture + 30 × Pixel Pitch µm) ÷ Focal Length. With declination correction: divide the result by cos(declination). The Plate Scale method converts one pixel's angular size (Pixel Scale = 206.265 × Pixel Pitch µm ÷ Focal Length) into the time a star takes to cross one pixel at the sidereal rate of 15 arcsec/s. For SNR-based sub-exposure planning, the Robin Glover formula is: Sub-exposure = (Constant × Read Noise²) ÷ Light Pollution, where the constant is 25 for 2% noise tolerance, 10 for 5%, and 5 for 10%. Color cameras use a 3× multiplier; narrowband filters use a 25× multiplier.
Why Does the Right Exposure Matter?
Using too short an exposure on a tripod wastes light and forces you to capture more frames, increasing thermal noise and battery drain. Using too long an exposure produces star trails that cannot be corrected in post-processing. For sub-exposure planning on a tracked mount, the optimal frame length is critical for signal-to-noise ratio: too short and read noise (which is fixed per frame) dominates; too long and sky background noise saturates the sub, wasting data. Getting the exposure right per the sky conditions and camera means every minute of your session is scientifically efficient — maximising the signal you collect on your target while minimizing the noise floor of each individual frame.
Limitations and Caveats
All rule-of-thumb methods (500, 400, 300) were originally calibrated for low-resolution cameras and film. They are useful starting points but may overestimate safe exposure times on modern 24–61 megapixel bodies. The NPF Rule is more accurate but still uses approximate coefficients; actual trailing tolerance is subjective and depends on your output size and viewing distance. The SNR-based sub-exposure method assumes a stable sky background; passing clouds, dew, or atmospheric turbulence can invalidate the Bortle-derived light pollution estimate. Mount periodic error, atmospheric dispersion, and wind vibration — none of which are modelled here — can cause additional trailing even within the calculated limits. Always bracket exposures around the calculator's recommendation on your first visit to a new site.
Formulas
Classic rule of thumb for maximum untracked shutter speed. Divides 500 by the effective focal length. Calibrated for older, lower-resolution cameras (10-12 MP); tends to overestimate safe exposure on modern high-resolution sensors.
Developed by Frédéric Michaud, this formula incorporates lens aperture and pixel pitch (sensor photosite size in micrometers) to produce more accurate results on modern high-resolution cameras. Typically 30-60% more conservative than the 500 Rule.
Calculates the angular size of one pixel in arcseconds. One pixel's worth of star drift at the sidereal rate of 15 arcsec/s gives the maximum exposure: Exposure = Pixel Scale / 15.
Stars near the celestial poles move slower across the sensor than stars at the equator. Dividing by cos(declination) extends the allowable exposure for targets away from declination 0°. At +60° declination, you gain roughly 2× more exposure time.
Reference Tables
Sensor Crop Factors and Pixel Pitch
| Sensor Format | Crop Factor | Typical Pixel Pitch (µm) | Example Camera |
|---|---|---|---|
| Medium Format | 0.64× | 5.3 | Fujifilm GFX 100 |
| Full Frame (35mm) | 1.0× | 4.3–5.9 | Sony A7 III (5.93), Nikon Z6 (5.94) |
| APS-C (Nikon/Sony) | 1.5× | 3.9–4.2 | Nikon Z50 (4.22), Sony A6400 (3.92) |
| APS-C (Canon) | 1.6× | 3.7–4.3 | Canon R7 (3.76), Canon 90D (3.20) |
| Micro Four Thirds | 2.0× | 3.3–3.7 | OM-1 (3.34), GH6 (3.52) |
| 1-inch | 2.7× | 2.4–2.6 | Sony RX100 VII (2.41) |
Bortle Scale Light Pollution Reference
| Bortle Class | Sky Description | Naked-Eye Limiting Mag | Sky Brightness (mag/arcsec²) |
|---|---|---|---|
| 1 | Excellent dark site | 7.6–8.0 | 21.99–22.0 |
| 2 | Truly dark site | 7.1–7.5 | 21.89–21.99 |
| 3 | Rural sky | 6.6–7.0 | 21.69–21.89 |
| 4 | Rural/suburban transition | 6.1–6.5 | 20.49–21.69 |
| 5 | Suburban sky | 5.6–6.0 | 19.50–20.49 |
| 6 | Bright suburban | 5.1–5.5 | 18.94–19.50 |
| 7 | Suburban/urban transition | 4.6–5.0 | 18.38–18.94 |
| 8 | City sky | 4.1–4.5 | 18.00–18.38 |
| 9 | Inner city sky | <4.0 | <18.00 |
Worked Examples
Milky Way with a 24mm Lens on Full Frame
500 Rule: 500 / (24 × 1.0) = 20.8 seconds
NPF Rule: (35 × 1.4 + 30 × 5.93) / 24 = (49 + 177.9) / 24 = 9.5 seconds
Declination correction: 9.5 / cos(-29°) = 9.5 / 0.8746 = 10.9 seconds
Pixel Scale: 206.265 × 5.93 / 24 = 50.95 arcsec/px → 50.95 / 15 = 3.4 sec per pixel of drift
Deep-Sky Sub-Exposure Planning (Tracked Mount)
Robin Glover formula: Sub = (Constant × ReadNoise²) / LightPollution
At Bortle 5, estimated sky background flux ≈ 0.27 e⁻/s/pixel (typical for mono CCD at f/5)
Constant for 5% noise tolerance = 10
Sub = (10 × 2.5²) / 0.27 = (10 × 6.25) / 0.27 = 62.5 / 0.27 ≈ 231 seconds
Mono camera multiplier = 1× (no color correction needed)
APS-C Camera with Telephoto for Andromeda
Effective focal length: 200 × 1.5 = 300mm
NPF Rule: (35 × 2.8 + 30 × 4.22) / 200 = (98 + 126.6) / 200 = 1.12 seconds
Declination correction: 1.12 / cos(41°) = 1.12 / 0.7547 = 1.48 seconds
How to Use This Calculator
Choose Your Mode
Select 'Star Trailing' if you are shooting handheld or on a fixed tripod and need the maximum shutter speed before stars trail. Select 'Sub-Exposure Planner' if you have a tracking equatorial mount and want to find the ideal per-frame exposure length for stacking deep-sky images.
Enter Camera and Lens Settings
Input your focal length in millimeters, choose your sensor size from the dropdown (this sets the crop factor automatically), and enter your aperture f-number. If you know your camera's pixel pitch in µm, enter it directly. Otherwise enter your megapixel count and the calculator will derive it automatically from your sensor dimensions.
Add Declination and Review the Chart
For the most accurate result, enter the declination of your target object in degrees (Orion ≈ -5°, Andromeda ≈ +41°, Milky Way core ≈ -29°). The comparison bar chart instantly shows all four rule results side by side — pick the most conservative value (NPF or Plate Scale) for the sharpest stars on a modern sensor.
Export and Plan Your Session
Click 'Export CSV' to save all inputs and results as a spreadsheet you can take to the field. For sub-exposure planning, set the Bortle scale to match your site and choose your camera type (Color, Mono, or Narrowband). The planner returns the recommended seconds per frame, helping you decide how many subs to collect for a useful integration time.
Frequently Asked Questions
What is the difference between the 500 Rule and the NPF Rule?
The 500 Rule is a quick rule of thumb: divide 500 by your effective focal length and you get a rough maximum exposure in seconds. It was calibrated for low-resolution film and early digital cameras (10–12 MP). The NPF Rule, developed by astrophotographer Frédéric Michaud, adds your lens aperture and pixel pitch to the formula, producing a result that is typically 30–60% more conservative on modern high-resolution cameras. For a Sony A7R IV (61 MP) at 24mm f/1.4, the 500 Rule gives about 14 seconds while the NPF Rule gives around 5–6 seconds — a dramatic difference. For best results on sensors above 20 MP, always prefer the NPF Rule over the classic 500 Rule.
How does declination affect the maximum exposure time?
Stars near the celestial equator (declination 0°) move at the full sidereal rate of 15 arcseconds per second relative to a fixed sensor. Stars near the celestial poles move much slower because they trace tighter circles. The correction factor is cos(declination): at 60° declination the apparent motion is only half as fast, doubling your allowable exposure. At Polaris (+89°) the correction factor is virtually zero, allowing very long exposures. For Orion (-5°) the correction is negligible. For Andromeda (+41°) you gain about 25% more exposure time. Entering your target declination into this calculator automatically applies this adjustment to the NPF and Plate Scale results.
What is pixel pitch and how do I find mine?
Pixel pitch is the physical size of each individual photosite on your camera's sensor, measured in micrometers (µm). It is the most important variable the 500 Rule ignores. A Sony A7 III has 5.93 µm pixels; a Sony A7R IV has only 3.76 µm pixels — meaning the A7R IV will show star trailing nearly 60% sooner at the same focal length. You can find your camera's pixel pitch on DxOMark, DigicamDB, or the manufacturer's spec sheet. Alternatively, enter your megapixel count and sensor format into this calculator and it will derive the pixel pitch automatically using the known sensor dimensions for each format.
What is the Bortle scale and why does it matter for sub-exposures?
The Bortle scale rates night sky darkness from 1 (pristine dark sky, no artificial light pollution) to 9 (inner city sky where only the brightest stars are visible). For tracked deep-sky imaging, the sky background is the main noise source competing with your target signal. In darker skies (Bortle 1–3), the sky is very faint, so you need longer sub-exposures to ensure sky noise exceeds read noise per frame. In bright suburban or city skies (Bortle 6–9), even short exposures are dominated by sky glow. The Robin Glover sub-exposure formula uses the Bortle-mapped light pollution value and your camera's read noise to compute the scientifically optimal sub-frame length, minimising the number of frames needed for a given final image quality.
When should I use a narrowband filter and how does it change my sub-exposure?
Narrowband filters (Ha, OIII, SII) transmit only a very narrow slice of light (3–10 nm bandwidth), blocking most sky glow from artificial light sources. This dramatically improves contrast on emission nebulae from light-polluted sites. However, because the filter blocks so much light, your sensor needs a much longer exposure to accumulate sufficient sky background photons for the Robin Glover threshold to be met. The narrowband multiplier in this calculator is 25× compared to monochrome. In a Bortle 5 suburban sky with 3 e⁻ read noise, you might need only 120 seconds per sub with a colour camera but 3,000 seconds per sub with a narrowband filter — essentially meaning you should use very long exposures (30–60 min subs) when narrowband imaging.
Does tracking completely eliminate star trailing?
A well-polar-aligned equatorial mount cancels the bulk of Earth's rotation, allowing exposures of minutes to hours without star trails from the sidereal rate. However, residual periodic error in the mount's worm gear, atmospheric refraction near the horizon, autoguider corrections, flexure in the optical train, and wind vibration can all cause minor trailing even with tracking engaged. For this reason, most deep-sky imagers still keep individual sub-exposures under 5–20 minutes and stack many frames instead of taking one very long exposure. The Sub-Exposure Planner in this calculator gives you the scientifically optimal frame length based on noise theory, not trailing concerns — combine both tabs to fully plan your session.