Astrophotography Exposure Time Calculator

What this astrophotography exposure calculator does

This calculator helps you estimate the maximum exposure time you can use for night sky and Milky Way photos before stars start to show noticeable trails. It is based on the popular 500 Rule, which relates exposure time to your lens focal length and your camera sensor’s crop factor.

By entering just two values – focal length in millimetres and sensor crop factor – you get a suggested maximum shutter speed (in seconds). The calculation is done entirely in your browser; nothing is uploaded or stored.

How the 500 Rule formula works

The 500 Rule is an empirical guideline, not a strict physical law. It assumes that a small amount of star movement on the sensor is acceptable at typical viewing sizes and resolutions. The central idea is that the longer the focal length and the smaller the sensor, the shorter your exposure must be to keep stars looking like points instead of streaks.

The plain-text formula used by this tool is:

T (seconds) = 500 ÷ (focal length × crop factor)

where:

• T is the maximum recommended exposure time in seconds.
• focal length is your lens focal length in millimetres (mm).
• crop factor is the ratio of your sensor size to a 35 mm full-frame sensor.

In MathML form, the same relationship can be written as:

Here f is the focal length in millimetres and C is the sensor crop factor. If you use a constant other than 500 (for example, 400 or 300 for more demanding image quality), just replace 500 with that constant in the formula.

Understanding sensor crop factor

Different camera systems use different sensor sizes. The crop factor compares the diagonal of your sensor to the diagonal of a 35 mm full-frame sensor. A crop factor larger than 1 means a smaller sensor with a narrower field of view for the same focal length.

Common crop factors include:

A higher crop factor effectively magnifies star movement in your image. That is why the same 24 mm lens allows a longer exposure on full-frame than on an APS-C or Micro Four Thirds body.

How to use this exposure time calculator

1. Find your focal length. Use the actual focal length printed on the lens (e.g., 14 mm, 24 mm, 35 mm). For zooms, use the value you have selected.
2. Enter the crop factor. Use 1.0 for full-frame, or the value appropriate for your camera model (e.g., 1.5, 1.6, 2.0).
3. Click Calculate. The tool will output the maximum exposure time in seconds according to the 500 Rule.
4. Round down slightly. In practice, choose a shutter speed equal to or slightly shorter than the result (for example, use 13 s instead of 13.9 s) for cleaner stars.

For privacy, all calculations run locally in your browser; no data is sent to any server.

Worked example: 24 mm lens on APS-C

Suppose you are photographing the Milky Way with a 24 mm lens on an APS-C camera with a crop factor of 1.5. You want to know the longest shutter speed you can use before star trailing becomes obvious.

Step 1: compute the product of focal length and crop factor:

24 mm × 1.5 = 36

Step 2: divide 500 by that product:

500 ÷ 36 ≈ 13.9 seconds

According to the 500 Rule, your maximum exposure is around 14 s. To be safe, you might choose a shutter speed of 13 s. In the calculator, you would enter 24 for focal length, 1.5 for crop factor, and then use the result it returns in seconds.

For comparison, here is how the recommended exposure changes for a few common setups, all using the 500 constant:

Interpreting the results

The number returned by the calculator is a guideline, not a rigid limit. Use it as a starting point, then refine your settings in the field.

• If your stars already look slightly elongated at the suggested exposure, try reducing the shutter speed by 20–30 percent.
• If you are mainly sharing small web images or social media posts, you may tolerate a bit more trailing than someone printing large wall art. In that case, you might occasionally push slightly beyond the calculated value.
• For very high-resolution sensors (e.g., 40+ megapixels), it is common to replace the 500 constant with 400 or 300 to reduce trailing.

Remember that exposure time is only one piece of the exposure triangle. Once you know your maximum shutter speed, you still need to set aperture and ISO to achieve a bright enough image.

500 Rule vs. more conservative constants (400, 300, 200)

Many modern guides suggest lowering the 500 constant for sharper, trail-free stars, especially on dense sensors and for large prints. The basic structure of the rule stays the same; only the constant changes.

You can think of it this way:

• 500 Rule – classic guideline; usually acceptable for web and small prints, wide-angle lenses, and typical viewing distances.
• 400 Rule – slightly stricter; reduces trailing for newer high-resolution cameras while keeping reasonable exposure times.
• 300 or 200 Rule – very conservative; good when you plan to crop heavily or make large, detailed prints, at the cost of higher ISO or wider apertures.

Numerically, they all follow the same pattern:

T = constant ÷ (focal length × crop factor)

where the constant is 500, 400, 300, 200, or your preferred value.

500 Rule vs NPF Rule

For more advanced planning, some astrophotographers use the NPF Rule. Unlike the simple 500 Rule, the NPF Rule factors in:

• lens focal length,
• aperture (f-number),
• pixel pitch or sensor resolution, and
• the declination of the stars you are imaging.

The NPF Rule typically yields shorter exposure times than the 500 Rule, especially on high-resolution cameras. It aims to limit not just visibly obvious trails, but also very small star shape distortions noticeable when zoomed in.

However, the NPF Rule is also more complex to compute and requires details (like pixel size) that many photographers do not remember in the field. That is why the 500 Rule remains popular as a quick, practical reference when setting up shots under the night sky.

Connecting exposure time with ISO and aperture

Once you know your maximum exposure time from the 500 Rule, you still need to balance ISO and aperture to get a usable image. A typical workflow might look like this:

1. Use the calculator to find your maximum shutter speed.
2. Set your lens to its widest aperture (e.g., f/1.4, f/1.8, or f/2.8) to gather as much light as possible.
3. Raise ISO until the sky and Milky Way look bright enough in your test shot (commonly ISO 1600–6400 for modern cameras in dark locations).

In light-polluted areas, you may need to lower your ISO or shorten your exposure further to avoid washing out the sky. In very dark locations, you can often use the full exposure time from the 500 Rule and a high ISO to record faint structures in the Milky Way.

Limitations and assumptions of the 500 Rule

The 500 Rule is intentionally simple, but that simplicity comes with important limitations and assumptions you should keep in mind.

• Wide-angle assumption: The rule works best for wide-angle lenses (typically 14–35 mm on full-frame). At longer focal lengths, even small amounts of trailing become more obvious, and the rule may overestimate acceptable exposure time.
• Typical viewing sizes: The 500 constant assumes images will be viewed at common sizes and distances. If you plan to make very large prints or allow close inspection on high-resolution screens, you may see trails sooner than the rule predicts.
• Sensor resolution: The rule predates today’s high-megapixel cameras. On sensors with very small pixels, star elongation is more visible, which is why many photographers now prefer 400, 300, or even shorter constants.
• Star position in the sky: The Earth’s rotation causes stars near the celestial equator to move faster across your frame than stars near the celestial poles. The 500 Rule does not explicitly account for this, so results are an average compromise.
• Compositional choices: If your composition places bright stars or constellations close to the edges of the frame, apparent motion can be amplified by projection effects. Trailing may appear sooner in those areas than in the centre.
• Tracking mounts and star trackers: The calculator assumes no tracking. If you use a star tracker, you can typically expose for much longer than the 500 Rule suggests, but foreground elements will then blur instead.
• Atmospheric and optical effects: Seeing conditions, lens aberrations, and focus accuracy also affect star sharpness. The 500 Rule does not compensate for these factors.

Because of these limitations, treat the 500 Rule result as a starting estimate. Review your shots at high magnification in the field and adjust your exposure time, ISO, or focal length based on what you observe.

Frequently asked questions

Does the 500 Rule work with crop-sensor cameras?

Yes. You simply need to include the crop factor in the formula. That is exactly what this calculator does for you. Enter your focal length and crop factor, and it returns an exposure time that already accounts for the smaller sensor.

Is the 500 Rule accurate for modern high-resolution cameras?

It is accurate enough as a field guideline, but it is often optimistic for 40+ megapixel sensors. If you find visible trailing at the default result, try recalculating with a smaller constant such as 400 or 300 (by mentally scaling the result down), or simply reduce the suggested exposure by 20–40 percent.

What constant should I use instead of 500?

There is no single correct answer. For most modern cameras and web-sized output, 500 or 400 works reasonably well. For critical work, big prints, or heavy cropping, many astrophotographers move to 300 or below. The key is to test a few values with your specific camera and viewing needs, then adopt a constant that consistently gives you results you like.