The 500 Rule is a simple mathematical formula that guides us in choosing the right exposure times, thus avoiding the unwanted star trails effect.

The 500 Rule states that you should divide the number 500 by the focal length in use. The result will be the maximum number of seconds of exposure before the stars start appearing as lines instead of dots.

Example:

500 : 18mm = 27s

This means that if you're taking the photo using a focal length of 18mm, you'll have to set an exposure time no longer than 27 seconds.

Remember to also take into account the Crop Factor. What is that?

Well, if you don't own a camera equipped with a Full Frame sensor, the formula will have to change slightly.

If you have a camera with an APS-C sensor (like the majority of digital cameras), you'll need to multiply the focal length in use by 1,6 (if the camera is a Canon) or by 1,5 (for all the other brands).

Therefore, the formula becomes:

500 : (18mm x 1,6) = 17s

But what if your camera doesn't have a Full Frame sensor or an APS-C? What if it has a Micro 4/3 sensor (found in many Panasonic cameras)?

In that case, you will need to multiply the focal length in use by 2.

The formula then becomes:

500 : (18mm x 2) = 14s

In other words, the longer the focal length, the less exposure time you'll be able to use before the stars start appearing as lines (due to Erath's rotation).

In those cases you will need to open the aperture to the maximum and raise the ISO value, while being careful not to introduce too much digital noise.

OR, you can rely on technology. Let's see what kind of technology:

A loss of 10 seconds (as happens in the case of the Crop Factor with APS-C sensors) is HUGE when it comes to nighttime photography.

Apart from opening the aperture and increasing the ISO, there isn't much else you can do. UNLESS, you buy yourself a photography tracker.

A photography tracker is a smart piece of equipment that mounts between the tripod and your camera, and is used to compensate for Earth's rotation.

By doing so, the camera will remain fixed on the portion of the sky you want it to be looking at. This means virtually unlimited exposure times and NASA-like photos.

Like these ones:

Of course, the tracker alone won't work miracles. You'll still need a camera capable of handling relatively high ISO values without producing excessive digital noise, and above all, a fair amount of post-processing.

Keep in mind that photos like the ones above never come out of the camera looking like that. They need a lot of post-production work to achive such results.

1. What is the 500 Rule?

The 500 Rule states that you should divide the number 500 by the focal length in use. The result will be the maximum number of seconds of exposure before the stars start appearing as lines instead of dots.

Full Frame sensor:

500 : focal length = Max. exposure time

APS-C sensor:

500 : (focal length x 1,6) = Max. exposure time (Canon)

500 : (focal length x 1,5) = Max. exposure time (other brands)

Micro 4/3 sensor:

500 : (focal length x 2) = Max. exposure time

2. What is the Crop Factor?

The Crop Factor indicates how much a non-Full Frame sensor resizes the image compared to the Full Frame format. The Crop Factor influences the field of view and creates a zoom-like effect on the photos because it's like taking them with a longer focal length.

3. What is the tracker?

A tracker is a smart piece of equipment that mounts between the tripod and your camera, and is used to compensate for Earth's rotation, allowing you to use virtually infinite exposure times.