Building an Astrophotography Exposure Comparison Tool

By Charlie Groves

I’m a keen hobbyist astrophotographer, and I was always surprised that no tools existed to compare relative brightness of different exposure settings. For example, how much brighter is ISO 800/10s/f2.8 compared to ISO 400/20s/f4? This is impossible to do in your head in the field, and I wanted to build a tool for myself.

This is why I built astrocompare.com, a simple tool to compare the brightness of different exposure settings.

Logic Behind the Tool

Comparing ISO and shutter speed is easy, as they are linear. However, f-stop is a bit more complex because it is logarithmic. To handle this, I created a relative aperture brightness difference dictionary that maps f-stop values to brightness differences.

const relativeApertureDict: Record<string, number> = {
  "f1.4": 1,
  "f2": 1.41,
  "f2.8": 2,
  "f4": 2.83,
  "f5.6": 4,
  ...
};

It’s important to note that when ISO and shutter speed increase, brightness also increases. But when f-stop increases, brightness decreases. To get the relative brightness difference, you calculate:

shutterSpeed * iso / relativeApertureDict[fStop];

To go one step further, I added a "number of exposures" field. In astrophotography, you often take multiple exposures and stack them to reduce noise. This feature helps account for that in the calculation.

I hope you find this tool useful, and feel free to reach out if you have any questions or feedback.