Hi Oshyan,
This problem can simply be reproduced with the default scene that load when opening Terragen, for example :
- Set the camera altitude to 100
- set the sun elevation at 10
- Set the base colors all white
- For the depth of field, set the focus distance at 500 with an aperture of 10
You should see the 2 dark lines. If you change the focus distance, these lines will move accordingly farther or closer: