Manually manage previews
You can delete previews or update previews to regenerate them when you want.
Delete all the JPEG previews for a project
In the Library inspector, select the project whose JPEG previews you want to delete, then choose Photos > Delete Previews for Project.
Update all the JPEG previews in a project
In the Library inspector, select the project whose JPEG previews you want to update, then choose Photos > Update Previews for Project.
If previews are already up to date and you choose Update Previews, Aperture alerts you that no work is required. However, you can force the rebuilding of previews (see the following steps).
Force the rebuilding of JPEG previews in a project
In the Library inspector, select the project whose JPEG previews you want to force Aperture to update, hold down the Option key, then choose Photos > Generate Previews for Project.
Delete JPEG previews for a photo selection
In the Browser, you can select one or more photos and either delete or update the JPEG previews.
In the Browser, select a photo or group of photos whose JPEG previews you want to delete.
Choose Photos > Delete Previews.
Update JPEG previews for a photo selection
In the Browser, select a photo or group of photos whose JPEG previews you want to update.
Choose Photos > Update Previews.
Force the rebuilding of JPEG previews for a photo selection
In the Browser, select a photo or group of photos whose JPEG previews you want to force Aperture to update.
Hold down the Option key, then choose Photos > Generate Previews.