# Calculating the focal length

> Source: https://topodrone.com/knowledge/topodrone-post-processing/calculating-the-focal-length/
> Section: TOPODRONE Post Processing
> Updated: 2025-11-26

This tool is not directly related to GNSS data processing, but it helps to calculate the focal length of the camera, which is necessary for data processing in photogrammetric programs.  
Instructions for calculating the focal length:

1. Click the Tools tab and select Focal Length Calculation.

2. In the opened window specify the following parameters: flight altitude of your aircraft, previously obtained focal distance in millimeters (from the result of aerotriangulation in photogrammetric software) and error values by height of your control points.

![Calculating the focal length](https://topodrone.com/knowledge/i/4ac056115d02eeb6.jpg)

3. Click the "Calculate" button. The program will calculate average height error and focal length, which can be used in photogrammetric programs.

![Calculating the focal length](https://topodrone.com/knowledge/i/28da74145434b50a.jpg)

4. If necessary, the new focal length can be written to the EXIF tags of your photos by clicking the "Write to Tags" button. This will help in further photogrammetric data processing.

5. Specify the folder with the photos.

![Calculating the focal length](https://topodrone.com/knowledge/i/e599c357c540910e.jpg)

6. The program will write the new focal length to all photos in the specified folder.

> **Warning:** The program replaces the focal length in the original files without creating copies of the photos.

![Calculating the focal length](https://topodrone.com/knowledge/i/01884a1df04ac9ac.jpg)