Should I use the NPF rule or the 500 rule?

For digital cameras over 20MP, the NPF rule is recommended. The 500 rule ignores pixel count, so for full frame 24MP at 14mm it gives about 36 seconds, but at that exposure stars trail about 6 pixels at 100% and look like lines. The NPF rule gives about 9 seconds (about 10 seconds toward the Milky Way), keeping stars almost as points even at 100%. For sizes you’d post on social media, the in-between “simplified NPF” (about 20 seconds) still looks like points.

What settings should I use for the Milky Way at 14mm f/2.8?

With full frame 24MP, the NPF rule gives about 10.7 seconds, so a shutter speed of 10 seconds, f/2.8 and around ISO 6400 is a good starting point (under a very dark sky). If you go for 20–25 seconds as the 500 rule suggests, ISO 3200 or so works, but stars trail slightly at 100%. Focus at the hyperfocal distance (about 2.35m) rather than infinity, and everything from about 1.2m out to the stars will be in focus.

How do exposure times change on APS-C or Micro Four Thirds?

At the same focal length the field of view is narrower and the pixels are finer, so exposure times get shorter. On APS-C (26MP) at 16mm f/1.4 toward the Milky Way, the NPF rule gives about 5.5 seconds, and the 500 rule gives about 23 seconds based on a 24.5mm equivalent. Just enter the focal length printed on the lens and the equivalent is calculated automatically.

Why do stars trail more easily on high-resolution cameras?

The smaller each pixel is, the more pixels the same star movement crosses. Comparing at 14mm f/2.8 toward the Milky Way, stars stay as points up to about 13.5 seconds at 12MP, about 10.7 seconds at 24MP and about 8.2 seconds at 61MP. If you don’t view at 100% and downsize the image instead, the effect of pixel count is smaller.

What is the hyperfocal distance, and how is it different from focusing at infinity for astrophotography?

The hyperfocal distance is the distance at which, if you focus there, everything from half that distance to infinity falls within the in-focus range. For full frame it is about 2.35m at 14mm f/2.8, about 4.8m at 20mm f/2.8 and about 6.9m at 24mm f/2.8. Infinity is fine if you only shoot stars, but for astrolandscapes that include foreground trees or rocks, focusing at the hyperfocal distance keeps things sharp closer to you.

My foreground is too close to get in focus. What should I do?

First stop down to shorten the hyperfocal distance. To include a foreground 1m away at 14mm on full frame, you need f/3.5 (hyperfocal distance about 1.9m). If that still isn’t enough, or stopping down makes the stars too dark, the reliable approach is “focus stacking”: take one frame focused on the foreground and one focused on the stars and blend them. Enter the distance to the foreground on the “Focus & depth of field” tab to see the required f-number.

How many frames and minutes do I need for star trails with lighten stacking?

Stars rotate about 15° per hour around the celestial pole, so you get an arc of about 15° in one hour and about 30° in two hours. With 30-second exposures and a 1-second gap, one hour is about 116 frames. A long gap breaks the trails up like dotted lines, so keep it around 1 second. On the “Trails & time-lapse” tab you can calculate the frame count and card space from the shooting time.

How many frames should I shoot for a starry-sky time-lapse?

You need video length in seconds × frame rate frames. At 30fps, a 10-second video needs 300 frames and a 20-second one needs 600. Shooting 300 frames with 15-second exposures and a 5-second gap (20-second interval) takes about 1 hour 40 minutes. With 24MP RAW files (about 26MB each), 300 frames use about 7.6GB.

Astrophotography Calculator | Shutter Speed Without Star Trails Using the NPF and 500 Rules

Just choose your camera’s sensor size and megapixels plus your lens’s focal length and f-number, and this tool calculates the shutter speed that keeps stars as sharp points, using the NPF rule (for high-resolution cameras) and the 500 rule. It shows how much longer you can expose depending on where you point (declination), such as the Milky Way core or the Big Dipper, how many pixels stars will trail at that exposure, and a suggested ISO for your sky darkness. You can also calculate the hyperfocal distance and f-number needed to get both the foreground and the stars sharp, plus the number of frames, shooting time, arc angle and card space for star trails (lighten stacking) and time-lapses.

ABOUT

About Astrophotography Calculator | Shutter Speed Without Star Trails Using the NPF and 500 Rules

The Astrophotography Calculator works out the longest shutter speed that keeps stars as points without trailing, using the NPF rule and the 500 rule. It derives the pixel pitch from your sensor size (full frame, APS-C, Micro Four Thirds, 1-inch, medium format) and megapixels, and once you enter the focal length, f-number and shooting direction (declination), it lists the NPF rule, simplified NPF and 500 rule exposure times, the value to set on your camera, and how many pixels stars will trail at that exposure. It can also give a suggested ISO for your sky darkness, the hyperfocal distance and required f-number to get both the foreground and the stars sharp, and the number of frames, shooting time, arc angle and card space for star trails (lighten stacking) and time-lapses.

How to use Astrophotography Calculator | Shutter Speed Without Star Trails Using the NPF and 500 Rules

1On the “Max exposure without trails” tab, choose your sensor size and effective megapixels (enter 24 for 24MP)
2Enter the lens focal length (the actual number printed on the lens) and f-number
3Choose the direction you want to shoot (Milky Way core, Orion, the Big Dipper, etc.). If unsure, set declination to 0°
4Check the NPF rule exposure and the “Camera setting”, then choose your sky darkness to see the suggested ISO
5On the “Focus & depth of field” tab, enter the distance to the nearest foreground object and check the required f-number and focus distance
6For star trails or time-lapses, enter the exposure time and number of frames (or shooting time) on the “Trails & time-lapse” tab and check the interval setting and card space

Great for

Find out how many seconds you can expose before stars trail when shooting the Milky Way with a 14mm f/2.8 lens
See why the 500 rule causes star trails on 45MP and 61MP high-resolution cameras
Get exposure times that account for the equivalent focal length on APS-C and Micro Four Thirds cameras
Check how much longer you can expose when pointing toward the Big Dipper or Polaris
Get a suggested ISO that matches your chosen shutter speed and f-number
Decide the aperture and focus distance to get both foreground trees or rocks and the starry sky in focus
Plan the number of frames, shooting time and arc angle for star trails with lighten stacking
Find out how many frames you need for a 10- or 20-second starry-sky time-lapse and how many GB of card space it takes

Good to know

The NPF rule is a formula devised by Frédéric Michaud of the Société Astronomique de France, calculated here as t = (16.856 × f-number + 0.0997 × focal length + 13.713 × pixel pitch µm) ÷ (focal length × cos declination). It is a strict guide under which stars stay almost as points (1–2 pixels of trailing) even at 100% view.
The simplified NPF uses (35 × f-number + 30 × pixel pitch) ÷ focal length with a declination correction (÷ cos declination). It is a guide for stars that look like points at social-media or phone screen sizes.
The 500 rule is 500 ÷ (35mm-equivalent focal length), a guideline from the days of film and low-resolution digital cameras. Because it ignores pixel count, stars stretch by about 5–8 pixels and look like lines at 100% on cameras of 24MP or more.
Pixel pitch is calculated from the sensor width and pixel count (with the aspect ratio taken from the sensor dimensions). Full frame 24MP gives 6.0µm, 45MP gives 4.4µm, and APS-C 26MP gives 3.75µm.
The speed at which stars move is proportional to cos(declination): fastest at the celestial equator (0°) and slower closer to the celestial poles. Toward the Milky Way core (Sagittarius, declination −29°) you can expose about 1.14× as long as at the equator, and toward the Big Dipper (+55°) about 1.7× as long.
The ISO guide is based on “f/2.8, 20 s, ISO 3200” for a very dark sky, “ISO 1600” for suburbs and “ISO 800” under moonlight, converting differences in f-number and exposure into brightness and rounding to 1/3-stop values. In practice, take a test shot and adjust using the histogram.
Depth of field and hyperfocal distance are calculated with a circle of confusion of 0.03mm for full frame, 0.02mm for APS-C and 0.015mm for Micro Four Thirds. If you check at 100% on a high-resolution camera, choose “Strict” (half the value).
Star-trail angles are calculated from the fact that stars rotate around the celestial pole by about 15.04° per hour (one full turn in 23 h 56 min). With long-exposure noise reduction on, each frame is followed by a wait as long as the exposure, so turn it off for lighten stacking and time-lapses.
All calculations run in your browser and nothing you enter is sent to a server. Only your camera and lens settings are saved in this browser.

Questions this tool answers

Use Astrophotography Calculator | Shutter Speed Without Star Trails Using the NPF and 500 Rules for searches like these:

Calculate the shutter speed that keeps stars from trailing
Get the exposure time with the NPF rule
Understand the difference between the 500 rule and the NPF rule
Find the shutter speed, aperture and ISO for shooting the Milky Way
Find astrophotography exposure times for APS-C and Micro Four Thirds
Understand why stars trail on high-resolution cameras
Get both the foreground and the stars in focus in astrolandscape photos
Calculate the hyperfocal distance
Calculate the frame count and shooting time for lighten stacking
Find the number of frames and card space needed for a time-lapse

FAQ

Should I use the NPF rule or the 500 rule?: For digital cameras over 20MP, the NPF rule is recommended. The 500 rule ignores pixel count, so for full frame 24MP at 14mm it gives about 36 seconds, but at that exposure stars trail about 6 pixels at 100% and look like lines. The NPF rule gives about 9 seconds (about 10 seconds toward the Milky Way), keeping stars almost as points even at 100%. For sizes you’d post on social media, the in-between “simplified NPF” (about 20 seconds) still looks like points.
What settings should I use for the Milky Way at 14mm f/2.8?: With full frame 24MP, the NPF rule gives about 10.7 seconds, so a shutter speed of 10 seconds, f/2.8 and around ISO 6400 is a good starting point (under a very dark sky). If you go for 20–25 seconds as the 500 rule suggests, ISO 3200 or so works, but stars trail slightly at 100%. Focus at the hyperfocal distance (about 2.35m) rather than infinity, and everything from about 1.2m out to the stars will be in focus.
How do exposure times change on APS-C or Micro Four Thirds?: At the same focal length the field of view is narrower and the pixels are finer, so exposure times get shorter. On APS-C (26MP) at 16mm f/1.4 toward the Milky Way, the NPF rule gives about 5.5 seconds, and the 500 rule gives about 23 seconds based on a 24.5mm equivalent. Just enter the focal length printed on the lens and the equivalent is calculated automatically.
Why do stars trail more easily on high-resolution cameras?: The smaller each pixel is, the more pixels the same star movement crosses. Comparing at 14mm f/2.8 toward the Milky Way, stars stay as points up to about 13.5 seconds at 12MP, about 10.7 seconds at 24MP and about 8.2 seconds at 61MP. If you don’t view at 100% and downsize the image instead, the effect of pixel count is smaller.
What is the hyperfocal distance, and how is it different from focusing at infinity for astrophotography?: The hyperfocal distance is the distance at which, if you focus there, everything from half that distance to infinity falls within the in-focus range. For full frame it is about 2.35m at 14mm f/2.8, about 4.8m at 20mm f/2.8 and about 6.9m at 24mm f/2.8. Infinity is fine if you only shoot stars, but for astrolandscapes that include foreground trees or rocks, focusing at the hyperfocal distance keeps things sharp closer to you.
My foreground is too close to get in focus. What should I do?: First stop down to shorten the hyperfocal distance. To include a foreground 1m away at 14mm on full frame, you need f/3.5 (hyperfocal distance about 1.9m). If that still isn’t enough, or stopping down makes the stars too dark, the reliable approach is “focus stacking”: take one frame focused on the foreground and one focused on the stars and blend them. Enter the distance to the foreground on the “Focus & depth of field” tab to see the required f-number.
How many frames and minutes do I need for star trails with lighten stacking?: Stars rotate about 15° per hour around the celestial pole, so you get an arc of about 15° in one hour and about 30° in two hours. With 30-second exposures and a 1-second gap, one hour is about 116 frames. A long gap breaks the trails up like dotted lines, so keep it around 1 second. On the “Trails & time-lapse” tab you can calculate the frame count and card space from the shooting time.
How many frames should I shoot for a starry-sky time-lapse?: You need video length in seconds × frame rate frames. At 30fps, a 10-second video needs 300 frames and a 20-second one needs 600. Shooting 300 frames with 15-second exposures and a 5-second gap (20-second interval) takes about 1 hour 40 minutes. With 24MP RAW files (about 26MB each), 300 frames use about 7.6GB.

Special Thanks

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