Hello andyroo,
The expected workflow should contain these steps:
- enable Keep Key Points option in the Advanced Preferences tab,
- add images and run Align Photos command (which contains of key point detection, image matching and camera alignment steps),
- add new subset of images,
- run Align Photos dialog with "reset current alignment" option disabled;
you will see that the feature points are being detected for the newly added images only.
If you remove key points from the Workspace pane (the option is available in the Tie point cloud context menu), there will be no way to use "incremental alignment approach".
In case you are following this procedure, but the result is considerably worse than the results of complete re-alignment for the joint set of images, please share the example sub-sets to support@agisoft.com, so that we could re-produce the problem.