The “rule of 500” refers to two completely different formulas depending on the field. In astrophotography, it’s a quick way to calculate the longest shutter speed you can use before stars blur into trails. In diabetes management, it’s a formula for estimating how many grams of carbohydrate one unit of insulin will cover. Both share the same name because both involve dividing 500 by a key number. Here’s how each one works.
The Rule of 500 in Astrophotography
When you photograph the night sky with a long exposure, Earth’s rotation causes stars to shift position during the shot. Leave the shutter open too long and stars stretch into short lines instead of appearing as sharp points. The rule of 500 gives you a ceiling for your exposure time:
500 ÷ focal length of your lens = maximum shutter speed in seconds.
With a 16mm lens on a full-frame camera, that’s 500 ÷ 16 = 31.25 seconds, which you’d round down to 30. With a 24mm lens, it’s about 20 seconds. A 50mm lens drops you to just 10 seconds. Always round down to the nearest whole second, not up, because even a small overshoot can produce visible trailing.
Adjusting for Crop-Sensor Cameras
The basic formula assumes a full-frame sensor. If you’re shooting with an APS-C or Micro Four Thirds camera, you need to account for the crop factor first. Multiply your lens’s focal length by the crop factor, then divide 500 by that result.
- Canon APS-C (1.6x crop): A 24mm lens becomes 24 × 1.6 = 38.4. Then 500 ÷ 38.4 = about 13 seconds.
- Nikon APS-C (1.5x crop): A 24mm lens becomes 24 × 1.5 = 36. Then 500 ÷ 36 = about 14 seconds.
- Micro Four Thirds (2x crop): A 24mm lens becomes 24 × 2 = 48. Then 500 ÷ 48 = about 10 seconds.
The smaller your sensor, the shorter your maximum exposure. This is why astrophotographers working with crop-sensor bodies often choose the widest lens they can afford.
When the Rule of 500 Falls Short
The rule of 500 originated in the film era and works as a rough guide, but modern high-resolution sensors can expose its limitations. A 60-megapixel camera captures far more detail than a 12-megapixel one, which means even tiny amounts of star movement become visible when you zoom in. On cameras like the Sony a7CR, trails that the rule says shouldn’t exist will show up at 100% crop.
The formula also ignores the direction your camera is pointing. Stars near the celestial equator move faster across the frame than stars near the poles, so the same shutter speed that works aimed at Polaris may produce trails aimed at Orion. It doesn’t factor in aperture-related diffraction either.
A more precise alternative called the NPF rule accounts for your sensor’s pixel pitch, your aperture, and the direction you’re facing. Many astrophotographers now treat the rule of 500 as a starting estimate and then test a shot at full zoom on their camera’s screen before committing to a final exposure time. Some photographers use 400 or even 300 in place of 500 to get sharper stars on high-resolution sensors.
The Rule of 500 in Diabetes Management
For people managing diabetes with insulin, the rule of 500 estimates how many grams of carbohydrate one unit of rapid-acting insulin will cover. The formula is:
500 ÷ Total Daily Dose of insulin = grams of carbohydrate covered by 1 unit.
If your total daily insulin dose (basal plus bolus combined) is 50 units, the formula gives 500 ÷ 50 = 10. That means 1 unit of insulin would cover roughly 10 grams of carbohydrate. Knowing this ratio lets you calculate how much insulin to take before a meal based on what you’re eating.
How Total Daily Dose Is Estimated
The formula depends on knowing your Total Daily Dose (TDD), which is the sum of all insulin you take in a day. For someone just starting insulin, clinicians often estimate this from body weight: your weight in pounds divided by 4, or your weight in kilograms multiplied by 0.55. A 180-pound person would have an estimated TDD of about 45 units. Roughly 40 to 50 percent of that goes toward background (basal) insulin, and the rest covers meals.
Why the Number 500 Isn’t Always Right
The rule of 500 is a starting point, not a fixed prescription. Research published in the Journal of Diabetes Investigation found that the constant in this formula can range anywhere from 300 to 500 depending on the individual and even the time of day. One study of people with type 1 diabetes on insulin pumps found that 300 was more accurate at breakfast, 500 at lunch, and 400 at dinner. This variation reflects real shifts in how sensitive your body is to insulin throughout the day, with morning insulin resistance being higher for many people.
Some clinicians use 450 instead of 500 as a default, and more recent proposals have suggested that 300 may be more appropriate in certain populations. Age, physical activity, body composition, and individual insulin sensitivity all affect the true ratio. The calculated number is always meant to be refined over time based on how your blood sugar actually responds after meals. If your post-meal glucose consistently runs high with a ratio derived from the rule of 500, the constant likely needs to come down.
Two Formulas, One Principle
Despite coming from completely different worlds, both versions of the rule of 500 work the same way: divide 500 by a personal variable to get a practical limit. In photography, the variable is focal length, and the result is seconds. In diabetes care, the variable is total daily insulin, and the result is grams of carbohydrate per unit. Both are designed to give you a usable estimate in the field, with the understanding that real-world fine-tuning will follow.