Formula & Calculator
Astrophotography NPF Rule (Simplified)
More precise star-trail exposure limit than the 500 rule, accounting for aperture and sensor pixel size.
Interpretation
t_max = (35 × f_number + 30 × pixel_pitch) / focal_length. More accurate exposure time to avoid trailing, accounting for sensor pixel size and aperture.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| t_max | Maximum exposure time | s |
| f_number | Lens aperture (f-number) | |
| pixel_pitch | Camera sensor pixel pitch | µm |
| focal_length | Lens focal length | mm |
What it means
The NPF (or 500‑rule alternative) is a more precise formula for maximum exposure time in untracked astrophotography. It considers the aperture (f‑number) and pixel size (in µm), giving a shorter, more realistic time than the 500 rule, especially for modern high‑resolution sensors. It reduces trailing at the pixel level. It is used by advanced astrophotographers to capture sharp images with minimal trailing. Understanding this helps in achieving the best possible image quality in wide‑field astrophotography.
Worked example
NPF Rule (Simplified) – Two Detailed Examples
Real‑World| Parameter | Value |
|---|---|
| f_number | 2.8 |
| pixel_pitch (µm) | 4.3 |
| focal_length (mm) | 24 |
| Parameter | Value |
|---|---|
| f_number | 1.8 |
| pixel_pitch | 5.9 |
| focal_length | 35 |
Common mistakes
- NPF rule: t_max = (35 × f_number + 30 × pixel_pitch) / focal_length.
- f_number: The aperture f‑ratio – dimensionless.
- Pixel pitch: The size of one pixel in microns (µm).
- Focal length: In mm.
- Result: Maximum exposure time in seconds – more accurate than 500 rule.
Applications
The NPF rule (simplified) provides a more accurate formula for maximum exposure time before star trailing, considering pixel size and aperture. t_max = (35 * f_number + 30 * pixel_pitch) / focal_length. This accounts for the actual resolution of the camera sensor. Advanced astrophotographers use it for precise exposure planning to avoid trailing when using high‑resolution sensors and long focal lengths. By applying the NPF rule, photographers can maximise exposure time without star trails, essential for capturing faint objects in tracked or untracked setups. This improves image quality and reduces the need for stacking many short exposures.
- Precise exposure time calculation for high‑resolution astrophotography
- Optimisation of settings for tracked and untracked imaging
- Selection of appropriate lenses and camera settings
- Reduction of star trailing in deep‑sky images
- Education on modern astrophotography techniques
Frequently Asked Questions
t_max = (35 × f_number + 30 × pixel_pitch) / focal_length. This gives a maximum exposure time (in seconds) that accounts for both the aperture (f‑number) and sensor pixel size, providing a more accurate estimate than the 500 rule.
Pixel pitch is the distance between the centres of adjacent pixels on a sensor, typically in micrometres (µm). It determines the sensor's resolution and how much sky is covered by one pixel.
The NPF rule is more rigorous: it considers the sensor's pixel pitch and the lens aperture. It gives shorter exposure times for high‑resolution sensors (small pixel pitch) and faster lenses (lower f‑number) allow longer exposures.
Full‑frame cameras: 4‑6 µm. APS‑C: 3‑5 µm. High‑resolution cameras (e.g., 24 MP full‑frame) have ~5.9 µm; 50 MP has ~4.1 µm. Micro Four Thirds: ~3.7 µm.
Pixel pitch ≈ 5.9 µm. t_max = (35×1.8 + 30×5.9) / 50 = (63 + 177) / 50 = 240 / 50 = 4.8 seconds. So you should not exceed about 5 seconds to avoid trails.
The NPF rule includes a term (35 × f_number) in the numerator; a smaller f‑number gives a smaller contribution, thus a longer t_max. This reflects that larger aperture (lower f/#) collects more light, but the trailing effect is primarily due to focal length and pixel pitch.
It is still an approximation; it assumes a certain acceptable trailing (usually about 1‑2 pixels). The actual acceptable trailing depends on the final image size and viewing distance.
500 rule: t_max = 500/20 = 25 s. NPF (assume pixel pitch 5.9 µm): t_max = (35×2.8 + 30×5.9)/20 = (98 + 177)/20 = 275/20 = 13.75 s. The NPF rule is much stricter.
Yes, as long as you know the pixel pitch and focal length. It works for any sensor size, though the constants may be adjusted for very small pixels or large apertures.
Use the NPF rule to get a starting point, then test and adjust. You can also use a star tracker to allow much longer exposures. Alternatively, you can take multiple shorter exposures and stack them to improve signal‑to‑noise.