Apple Aperture 2 - Manual (Page 350)

350 Part III Performing Image Adjustments
Using an External Editor
If you need to perform more advanced image operations, such as compositing, you can
set an external editor for use within Aperture. First you have to specify which application
Aperture should use and the 16-bit file format (TIFF or PSD). You only have to do this
once. Then you select an image and choose a menu command to have the image open
in the application designated as the external editor.
To set an external editor in Aperture:
1 Choose Aperture > Preferences, or press Command-Comma (,), then click Export.
2 Click the Choose button below the External Image Editor field, navigate to an
application in the Select Application dialog, then click Select.
The application’s name appears in the External Image Editor field.
3 Choose the appropriate 16-bit file format (TIFF or PSD) from the External Editor File
Format pop-up menu.
This is the file format Aperture uses when opening files in external editors.
To use an external editor in Aperture:
1 Select an image in the Browser.
2 Choose Images > Edit With > [Application Name] (or press Command-Shift-O).
Aperture creates a new master (leaving the original unchanged), converts it to the
chosen file format, and then opens the image file in the selected external editor. The
image file is tracked as a new master stacked with the original master.
3 When you’ve finished modifying the image in the external application, save the
image file.
The saved image file is automatically updated in Aperture.