Astrophotography Calculator

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

Focal length (mm)

Crop factor

Aperture (f/)

ISO

Sensor width (mm)

Megapixels

Number of sub-exposures

Tab to move between fields

Recommended exposure (sec)

10.1

500 Rule max (sec)13.9

NPF Rule max (sec)10.1

Total integration (min)5

Scene EV (ISO 100)4.64

Pixel pitch (µm)4.8

Approx. limiting magnitude9.7

SNR improvement (×)5.48

How to Use This Calculator

Enter your focal length in mm and camera sensor pixel pitch in micrometers.
Set the target object (Milky Way, nebula, star trails) to load recommended exposure parameters.
Input your sky darkness rating on the Bortle scale from 1 to 9.
Review the 500 Rule Max Exposure, Recommended ISO, and Tracking Required flag.
Use the Image Scale in arcsec per pixel to match your setup to the desired level of detail.

How the result changes with Focal length (mm)

Focal length (mm): Lens focal length in millimeters.
Crop factor: Sensor crop factor (1.0 for full-frame, 1.5 for APS-C, 2.0 for MFT).
Aperture (f/): Lens aperture f-number (e.g. 2.8).
ISO: Camera ISO sensitivity setting.
Sensor width (mm): Physical sensor width in mm (23.5 for APS-C, 36 for FF).
Megapixels: Sensor resolution in megapixels.
Number of sub-exposures: Number of frames to stack for noise reduction.

500 Rule max (sec): Maximum exposure time per the classic 500 Rule.
NPF Rule max (sec): Maximum exposure per the NPF Rule (better for high-res sensors).
Recommended exposure (sec): Conservative recommended single exposure time.
Total integration (min): Total stacked exposure time in minutes.
Scene EV (ISO 100): Equivalent exposure value at ISO 100.
Pixel pitch (µm): Calculated pixel pitch of your sensor.
Approx. limiting magnitude: Estimated faintest star magnitude captured per frame.
SNR improvement (×): Signal-to-noise ratio improvement from stacking (√N).

Identify Input Parameters
4 parameters
Focal length (mm) = 24, Crop factor = 1.5, Aperture (f/) = 2.8, ISO = 3200 = 7 input(s) provided
Calculate Recommended exposure
Recommended exposure = min(maxExposure500, maxExposureNpf)
10.1 = 10.1
Calculate 500 Rule max
500 Rule max = 500 / (focalLength * cropFactor)
13.9 = 13.9
Calculate NPF Rule max
NPF Rule max = (35 * aperture + 30 * pixelPitch) / focalLength
10.1 = 10.1