Skip to main content
Calcimator

Astrophotography Exposure

Calculate the maximum untracked exposure time to avoid star trails using the NPF rule, 500 rule, and declination correction.

Inputs

mm
μm
°

Results

NPF Rule Exposure

7.7 sec

Declination-Adjusted

7.7 sec

500 Rule Exposure2.5 sec
300 Rule Exposure1.5 sec
Image Scale4.95 arcsec/px
Trail Length at NPF Time23.39 pixels
Sky Rotation0.0322°
Assumed Aperture (f/5)40 mm
How to Use This Calculator
  1. Enter your lens or telescope Focal Length (mm) — wider focal lengths allow longer untracked exposures.
  2. Look up your camera's Pixel Size (µm) in its specifications; typical DSLRs range from 3.7 to 6.5 µm.
  3. Enter the Target Declination (°) of your subject — stars near the celestial pole trail less, so longer exposures are possible.
  4. Use the NPF Rule Exposure (sec) as your maximum untracked shutter time to avoid visible star trails.
  5. Compare the 500 Rule and 300 Rule exposures — the NPF rule is more accurate for modern high-resolution sensors.

How the result changes with Pixel Size

Pixel SizeNPF Rule ExposureDeclination-Adjusted
2.457.4 sec7.4 sec
7.328.1 sec8.1 sec
139 sec9 sec
189.7 sec9.7 sec

What each input means

Focal Length
Lens or telescope focal length in millimeters (wider = longer exposures allowed)
Pixel Size
Camera sensor pixel pitch in micrometers (check camera specs; typical DSLR: 3.7-6.5 μm)
Target Declination
Declination of target in degrees (0 = celestial equator, +90 = north pole; higher = longer exposures)

How this is calculated

Formula

t = (35×aperture + 30×pixel_size) / focal_length (NPF Rule)

Worked example, using the default values

  1. Identify Input Parameters
    Focal Length = 200, Pixel Size = 4.8, Target Declination = 0 = 3 input(s) provided
  2. Calculate NPF Rule Exposure
    NPF Rule Exposure
    7.7 = 7.7
  3. Calculate Declination-Adjusted
    Declination-Adjusted
    7.7 = 7.7
  4. Calculate 500 Rule Exposure
    500 Rule Exposure
    2.5 = 2.5
  5. Calculate 300 Rule Exposure
    300 Rule Exposure
    1.5 = 1.5

Related Calculators

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Astronomy

Telescope Limiting Magnitude

Calculate the faintest star visible through a telescope based on aperture and sky brightness conditions.

Astronomy

Telescope Magnification

Calculate telescope magnification, true field of view, exit pupil, and resolution from focal length, eyepiece, and apparent field of view.

Specialized Photography

Astrophotography Calculator

Star trail vs point: max exposure from focal length and declination.

Astronomy

Asteroid Impact Energy

Calculate the kinetic energy and destructive potential of an asteroid impact from diameter, velocity, and density. Estimates crater size and earthquake equivalent.

More in Science & Physics.