500 rule calculator
With a 24 mm lens on full frame, the 500 rule allows about 20.8 seconds before stars trail, and a 50 mm lens on a 1.5 crop body about 6.7: enter focal length and crop factor to get your limit. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Focal length
- 24
- Crop factor
- 1
- Max shutter seconds
- 20.8333333333
- Effective focal length
- 24
- Rule
- 500
- Precision
- 10
This 500 rule astrophotography calculator turns two simple camera numbers — lens focal length in millimetres and sensor crop factor relative to full-frame — into the classic maximum shutter time before stars begin to trail on a fixed tripod. The formula is fully deterministic: maximum shutter seconds equals five hundred divided by focal length times crop factor. A full-frame twenty-four millimetre wide lens yields about twenty point eight seconds; a fifty millimetre lens on a one point five crop APS-C body yields about six point seven seconds. The engine also reports the thirty-five millimetre equivalent effective focal length so you can compare lenses across formats.
What the 500 rule means and what you get
The 500 rule is a long-standing rule of thumb for still night-sky photography on a static tripod: it estimates the longest exposure you can use before the Earth's rotation turns point stars into short trails. In its common form the maximum shutter time in seconds is five hundred divided by the product of the lens focal length in millimetres and the sensor crop factor relative to a thirty-five millimetre full-frame frame.
Longer lenses and tighter crops both shrink the budget because the sky appears more magnified and star motion is easier to see across the frame. Non-positive crop factors are also rejected so the denominator stays meaningful. The 500 rule is intentionally simple and slightly conservative or aggressive depending on pixel density and how large you print; it is not a substitute for the NPF rule or for a star tracker.
Where the 500 rule fits in field work, teaching, and software
Night-sky photographers use the 500 rule when they want a quick upper bound on shutter time for Milky Way arches, constellation frames, and aurora stills without a tracking mount. Educators introduce the rule when teaching crop factor, equivalent focal length, and why a crop body forces shorter exposures for the same lens.
App and tool builders call the same contract from scripts to seed deterministic test vectors for exposure helpers, to normalise user-entered lens and sensor parameters before showing a recommended shutter, or to satisfy programming-challenge statements that demand the classic ceiling rather than a hand-waved guess. Workshop practice often needs a fast, offline source of second-scale limits so students can set bulb or timed exposures without trusting opaque phone apps that switch between four-hundred and six-hundred variants silently.
Product and education tools use the calculation when a learner types focal length and crop and the UI must show max shutter seconds plus the thirty-five millimetre equivalent focal length with a clear rule tag. Pair it with hyperfocal or thin-lens helpers when you are building a small lab of free, deterministic optics and photography calculators, and document that the 500 rule is a starting point: high-resolution sensors and large prints often need shorter times, while wider skies and lower magnification can sometimes go longer.
What you can do with it
Milky Way tripod planning
Compute the classic max shutter for a given lens and crop so stars stay points on a static tripod before you switch to stacking or tracking.
Teaching crop factor and night exposure
Show students how equivalent focal length shortens the 500-rule budget when moving from full-frame to APS-C or MFT.
FAQ
What does the 500 rule calculator cost?
It is free in your browser on this page.
What is the 500 rule formula?
Maximum shutter time in seconds equals 500 divided by (focal length in millimetres times sensor crop factor). Effective focal length is focal length times crop.
What happens if focal length is zero or negative?
Non-positive focal length is not a valid lens length in millimetres for this rule.
Is the 500 rule the same as the NPF rule?
No. This tool implements only the classic 500 rule. NPF uses pixel pitch, declination, and other factors for a tighter estimate on modern sensors.
What crop factor should I use?
Use 1.0 for full-frame, about 1.5 for many APS-C Nikon/Sony bodies, about 1.6 for common Canon APS-C, and about 2.0 for Micro Four Thirds. Check your camera manual if unsure.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/hobby/rule-500 \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"focal_length":24,"crop_factor":1}'const res = await fetch("https://api.kit.forhosting.com/hobby/rule-500", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"focal_length": 24,
"crop_factor": 1
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/hobby/rule-500",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"focal_length": 24,
"crop_factor": 1
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/hobby/rule-500", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"focal_length":24,"crop_factor":1}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"focal_length":24,"crop_factor":1}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/hobby/rule-500", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"focal_length": 24,
"crop_factor": 1
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "hobby.rule_500",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_abs_value | 1000000000000000 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
What the 500 rule means and what the response contains
This calculator returns the rounded inputs, the max_shutter_seconds value, the effective_focal_length equal to focal length times crop, a rule tag of five hundred, the method tag rule_500, the precision used, and a short formula reminder so API clients and worksheets can show the same identity. There is no approximate sampling path and no camera-body database: the engine either evaluates the closed form inside the supported magnitude range or rejects the input with invalid_input. Rejected cases include missing fields, non-finite values, magnitudes above the documented ceiling, and any non-positive focal length, which breaks the physical meaning of a lens focal length in millimetres.
How focal length and crop factor are read and stay reproducible
Optional leading plus signs and scientific notation on numeric strings are allowed when they parse as finite decimals; embedded spaces, hex prefixes, and non-numeric tokens are rejected with a clear message so a bad cell does not become a wrong night exposure. Crop factor is dimensionless: use one for full-frame, about one point five for many Nikon and Sony APS-C bodies, about one point six for common Canon APS-C, and about two for Micro Four Thirds. The same positivity rule applies to crop factor so the effective focal length stays defined. The preferred fields are focal_length for the lens focal length in millimetres and crop_factor for the sensor crop relative to full-frame, but aliases such as f, focal, and fl for focal length, and crop, cf, sensor_crop, and cropFactor for the crop factor are accepted so spreadsheets, query strings, and typed forms can reuse the same contract. Focal length must be strictly positive: zero or negative millimetre values are rejected with invalid_input because they are not valid lens focal lengths and would make the five-hundred-over-product formula undefined or nonsensical. An optional precision field, with aliases decimals and dp, chooses how many decimal places to expose for the numeric fields, defaulting to ten and capped at fifteen so JSON goldens stay stable. Pair the free browser run on this page with the priced API when you need the identical mathematical result inside an automated pipeline without re-implementing the astrophotography formula yourself.
Where the 500 rule fits in field work, teaching, and software
On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — matches the rest of the KIT tier-A catalog and lets you move from a classroom demo to an automated pipeline without changing the mathematical result.
Integration use cases
Deterministic test fixtures
Generate reproducible max_shutter_seconds and effective_focal_length values for unit tests, property checks, and CI assertions.