In this article the workflow for exporting undistorted images from Metashape is described. Since Metashape 1.8.1 the parameters have been changed when exporting images without distortion using Convert images feature. The described workflow is for versions 1.8.1 and later.

This article closely considers the process of images export by using the parameter Transform to initial calibration. 
The information about other parameters in the Convert Images dialog window can be found in the User manual available for download here:

Metashape supports Convert Images command, that is available from Export submenu of the File menu (File > Export > Convert Images) and allows to export free of lens distortions and geometrically corrected images. To export images without distortion enable Transform to initial calibration option on the Convert Images dialog window: 

Transformation to initial calibration includes the following steps: remove distortion, center principal point and square pixels. When the option Transform to initial calibration is enabled the calibration is recalculated, and the resulting values are reflected in the Initial tab of the Camera calibration dialog window, as an example below.

In Metashape versions before 1.8.1 the focal length value from the Adjusted tab was used.

Therefore it is important to specify Pixel size and Focal length for the camera in the initial tab. The values are taken from EXIF meta data, but if the values are not specified due to the lack of EXIF data, then unspecified focus value is taken into account and may lead to an error.

What to do if meta data is missing?

If calibration parameters are missing, as shown on the image below, it is recommended to either use focal length from the Adjusted tab or to specify the camera calibration data manually.

To use focal length from the Adjusted tab open Camera calibration dialog window (select Tools > Camera Calibration...), navigate to Adjusted tab and copy the focal length value:

Then open Initial tab and change the Type of calibration from Auto to Precalibrated, paste the value for the focal length to the corresponding field:

Click OK button and navigate to the Convert Images... command available from menu File > Export (File > Export > Convert Images). In the Convert Images dialog window enable Transform to initial calibration option and apply the command. On the images below the same section of the building is presented. The image on the left represents the case when the focal length value (on the Initial tab) was not calculated correctly due to the lack of calibration parameters. While the image on the right is exported using the focal length value from the Adjusted tab. The values differ in 3 times approximately.

If you know the correct value for Focal length in mm and sensor Pixel size in mm, then it is best to specify these values in the Camera Calibration dialog window (use the Type of calibration - Auto): 

Then the f value in the Initial tab will be calculated correctly and the results of export images with Adjusted focal length value will be as shown below: