Astrophotography Exposure Calculator
How long can you expose the night sky before stars become trails? This calculator applies the classic 500 rule, the more precise NPF rule (aperture + pixel pitch), solves the Milky Way exposure triangle, and plans sub-exposure stacks — all computed instantly in your browser.
500 Rule Calculator
Max shutter ≈ 500 ÷ (focal length × crop factor). Simple, slightly optimistic — always round down.
NPF Rule Calculator
Max shutter ≈ (35 × aperture + 30 × pixel pitch) ÷ focal length. Accounts for your lens and sensor — stricter and more accurate than the 500 rule.
Declination 0° = celestial equator (worst case, stars move fastest). Pointing near the celestial pole (e.g. Polaris ≈ 89°) allows much longer exposures; the result is capped at 300 s.
Milky Way Exposure Triangle Solver
Pick any two settings and a sky target — the solver finds the third for a correct exposure.
Sky brightness values are practical starting points (the Milky Way preset is anchored to the classic f/2.8 · 25 s · ISO 6400 recipe). Check your histogram and adjust ±1 stop.
Sub-Exposure Stacking Planner
Stacking N identical sub-exposures improves signal-to-noise ratio by √N. Plan your total integration time here.
How the 500 Rule and NPF Rule Work
The Earth rotates 15° per hour, so stars drift across your frame during a long exposure. Two rules of thumb tell you how long you can expose before that drift becomes a visible trail:
- 500 rule:
max shutter (s) ≈ 500 ÷ (focal length × crop factor). A 24 mm lens on full frame gives 500 ÷ 24 ≈ 20.8 s, so use 20 s. It is simple but optimistic on modern high-resolution sensors — treat it as an upper bound. - NPF rule:
max shutter (s) ≈ (35 × aperture + 30 × pixel pitch) ÷ focal length. With a 24 mm f/2.8 lens on a 5.93 µm-pixel sensor: (35 × 2.8 + 30 × 5.93) ÷ 24 = 275.9 ÷ 24 ≈ 11.5 s. Because it accounts for aperture and pixel size, the NPF result is usually about half the 500-rule value — and noticeably sharper at 100% zoom. - Declination matters: both rules assume the worst case — stars on the celestial equator, which move fastest. Aim near the celestial pole (Polaris) and the same setup tolerates far longer exposures; the NPF tab above applies the
1 ÷ cos(δ)correction.
500 Rule Quick-Reference Table (max shutter, rounded down)
| Focal length | Full frame | APS-C 1.5× | MFT 2.0× |
|---|---|---|---|
| 14 mm | 35 s | 23 s | 17 s |
| 16 mm | 31 s | 20 s | 15 s |
| 20 mm | 25 s | 16 s | 12 s |
| 24 mm | 20 s | 13 s | 10 s |
| 28 mm | 17 s | 11 s | 8 s |
| 35 mm | 14 s | 9 s | 7 s |
| 50 mm | 10 s | 6 s | 5 s |
| 85 mm | 5 s | 3 s | 2 s |
Computed as 500 ÷ (focal × crop), rounded down. E.g. 14 mm APS-C: 500 ÷ 21 = 23.81 → 23 s; 85 mm MFT: 500 ÷ 170 = 2.94 → 2 s.
Worked Examples (Step by Step)
Example 1 — 24 mm on full frame (500 rule)
- Effective focal length: 24 mm × 1.0 = 24 mm.
- Apply the rule: 500 ÷ 24 = 20.83 s.
- Round down for safety: use 20 s.
- Pair with f/2.8 and ISO 6400 as a starting point (see the Milky Way solver).
Example 2 — NPF rule, 24 mm f/2.8, 5.93 µm pixels
- Numerator: 35 × 2.8 = 98; 30 × 5.93 = 177.9; sum = 275.9.
- Divide by focal length: 275.9 ÷ 24 = 11.50 s.
- Result: ≈ 11.5 s — roughly half the 500-rule value, visibly sharper when pixel-peeping.
Example 3 — 16 mm on APS-C 1.5× (500 rule)
- Effective focal length: 16 × 1.5 = 24 mm.
- 500 ÷ 24 = 20.83 → use 20 s.
Example 4 — Milky Way triangle: f/2.8, 20 s → which ISO?
- Milky Way core baseline ≈ EV₁₀₀ −7.67 (anchored to the f/2.8 · 25 s · ISO 6400 recipe).
- ISO = 100 × N² ÷ (t × 2^B) = 100 × 7.84 ÷ (20 × 2^−7.67).
- 2^−7.67 ≈ 0.004889; 20 × 0.004889 = 0.0978; 784 ÷ 0.0978 ≈ ISO 8000.
- Nearest standard ISO: 8000. Verify with your histogram — ETTR slightly, then pull down in post.
Example 5 — Stacking: 30 × 20 s subs
- Total integration: 30 × 20 = 600 s (10 min).
- SNR gain: √30 ≈ 5.48× versus a single frame.
- Equal total light with far less noise — the core technique of deep-sky imaging.
Example 6 — Pointing at declination 60°
- Base NPF result: 11.5 s (celestial equator).
- Correction: 11.5 ÷ cos(60°) = 11.5 ÷ 0.5 = 23.0 s.
Frequently Asked Questions
What is the 500 rule in astrophotography?
The 500 rule estimates the longest shutter speed you can use before stars show visible trails: divide 500 by your effective focal length (focal length times crop factor). For a 24 mm lens on full frame, 500 divided by 24 equals 20.83, so you shoot at 20 seconds. It is a film-era heuristic that is slightly optimistic on modern high-resolution sensors, so always round down.
What is the NPF rule and how is it different from the 500 rule?
The NPF rule is a more precise version of the 500 rule that accounts for your aperture and pixel pitch: shutter time equals (35 times aperture plus 30 times pixel pitch) divided by focal length. For a 24 mm f/2.8 lens on a sensor with 5.93 micron pixels, that gives about 11.5 seconds, roughly half the 500-rule value. Because it models pixel-level sharpness, the NPF rule produces noticeably sharper stars at 100 percent zoom.
Why does the NPF rule give a shorter exposure than the 500 rule?
The 500 rule was calibrated for film grain and prints, while the NPF rule targets pixel-level sharpness on digital sensors. On a 24 mm f/2.8 setup with 5.93 micron pixels, the NPF rule yields about 11.5 seconds versus 20.8 seconds for the 500 rule. The NPF value is the stricter, more honest limit if you inspect your images at full resolution.
How do I find my camera's pixel pitch?
Pixel pitch in microns equals 1000 times the sensor width in millimeters divided by the horizontal pixel count. A full-frame 24 MP sensor (36 mm wide, 6000 pixels across) has a pitch of about 6.0 microns. The NPF tab above includes approximate pitches for popular cameras such as the Sony A7 III at 5.93 microns and the Canon EOS R5 at 4.39 microns; for exact values check your manufacturer's sensor specifications.
What are the best camera settings for photographing the Milky Way?
A widely used starting point is a 20 to 25 second exposure at f/2.8 and ISO 6400 on a full-frame camera with a wide lens (14 to 24 mm). From there, use the 500 or NPF rule to set your maximum shutter for your specific lens, then solve the exposure triangle with this page's Milky Way solver: for example, f/2.8 at 20 seconds under dark skies needs roughly ISO 8000. Always confirm with your histogram and adjust by plus or minus one stop.
What ISO should I use for astrophotography?
Most modern cameras perform best between ISO 1600 and 6400 for night skies, where read noise is low and dynamic range is still reasonable. Higher ISOs like 8000 to 12800 are common for single Milky Way frames at f/2.8. Avoid your camera's maximum extended ISO, which adds noise without capturing more light; instead, stack more sub-exposures, since stacking improves signal-to-noise by the square root of the frame count.
Does the 500 rule work on crop-sensor cameras?
Yes, but you must multiply the focal length by the crop factor first. A 16 mm lens on a 1.5x APS-C body behaves like 24 mm, giving 500 divided by 24, or about 20.8 seconds, so use 20 seconds. Forgetting the crop factor is the most common mistake and produces visibly trailed stars.
How does target declination affect star trailing?
Stars near the celestial equator move fastest across the frame, while stars near the celestial pole barely move. Both rules assume the worst case of 0 degrees declination. Pointing at 60 degrees declination divides your trailing by the cosine of 60 degrees, effectively doubling the allowable exposure: an 11.5 second NPF limit becomes 23.0 seconds.
What is sub-exposure stacking and why do astrophotographers use it?
Stacking combines many short exposures into one image with software, improving the signal-to-noise ratio by the square root of the number of frames. Thirty 20-second exposures give 600 seconds of total integration and about 5.48 times better signal-to-noise than a single frame. Stacking also lets you reject airplane and satellite trails and keeps each sub short enough to avoid star trailing.
Can I just use one very long exposure instead of stacking?
No. A single very long exposure still trails stars beyond the 500 or NPF limit, saturates bright areas, and a single airplane crossing ruins the whole frame. Stacking many short, trail-free subs collects the same total light with far less noise and built-in outlier rejection, which is why it is the standard technique for both Milky Way panoramas and deep-sky imaging.
Should I use the 500 rule or buy a star tracker?
The 500 and NPF rules are for untracked, fixed-tripod shooting and cap you at roughly 10 to 30 seconds depending on focal length. A star tracker rotates your camera with the Earth, allowing minutes-long exposures at any focal length and dramatically cleaner images. If you shoot wide-field Milky Way panoramas a few times a year, the rules plus stacking are enough; if you are serious about deep-sky targets, a tracker is the single biggest upgrade you can buy.
Why are my stars still trailed even under the 500 rule limit?
The 500 rule is optimistic on high-resolution sensors, so switch to the stricter NPF rule, which typically halves the allowed time. Also check for tripod vibration, wind, mirror or shutter shock, and make sure you multiplied by the crop factor. If you are pixel-peeping at 100 percent, even NPF-level trailing can show; shorten the exposure further or stack more frames.