How to combine or merge several images at different focal planes into a single image from a microscope or other image series so that everything appears in focus. This would be for brightfield images (dark features, bright background).
How to do Extended Focus in GIMP
Extended focus in GIMP requires a great deal of manual intervention. Use ImageJ instead.
How to accomplish Extended Depth of Field (Extended Focus) in ImageJ
Download Extended Depth of Field plugin and install*
Open all images at incremental focus planes as stack: File > Import > Image Sequence
Plugins > Extended Depth of Field > Easy Method
Choose settings iteratively based on results
*To install plugins find plugin directory in ImageJ and place plugin here. Exit Image J (if it’s open) and open it again.