As you work with Aperture and Photoshop, it is useful to understand how changes made in each application to an image affect what you see in the other. Here's a basic example workflow:
Below is an example image that you might plan to edit with both Aperture and Photoshop:
Start by making a few adjustments in Aperture including straightening the image, adjusting the blue color of the sky, and adding some vibrancy:
Before sending the image to be edited in Photoshop, make sure that the export settings are correct in Aperture > Settings > Export. Select Photoshop as the External Photo Editor. In this example the External Editor File Format is also set to "PSD (8-bit)":
Send the Aperture version to Photoshop for editing by choosing Photos > Edit with Adobe Photoshop...
Before sending the image to Photoshop, Aperture creates a new master. This master renders into the image all Aperture adjustments you've made so far. The new master will be either a PSD or TIFF, depending on your preference settings in Aperture. When the image opens in Photoshop, all Aperture adjustments will be visible because they have been rendered into the image.
Edit the image in Photoshop. This may involve pixel edits to the image, addition of new layers, and so on. When you save the image, changes are written directly to the master. Be sure to enable the Maximize Compatibility option when saving a PSD in Photoshop.
When you return to Aperture, it detects that the master has been updated, and creates a new thumbnail and preview to reflect the changes. In the stack depicted below, you can see from left to right: the Aperture-adjusted image, and the updated master sent back from Photoshop.
You can now proceed to add adjustments to the image in Aperture. In this example a vignette has been added:
You can now send the image to Photoshop for further editing. When the image opens again in Photoshop, the Aperture adjustments added after the initial round trip are not visible in Photoshop. This is because Aperture does not flatten or render the image again after the initial creation of the external master. This preserves Photoshop layers, allowing you to come back and work with them as needed at any point in your workflow. In the image below, you have changed the font of the text in Photoshop.
When you ave the image again in Photoshop and return to Aperture, the thumbnail and preview are updated to reflect the current state of the master, and all Aperture adjustments are visible.
You may find it helpful to apply certain adjustments in Aperture before sending the image to Photoshop. For example, if you plan to add text to the image in Photoshop, be sure to apply the straighten adjustment in Aperture prior to initially sending it. Otherwise the text will be affected by the straighten adjustment. Basically, any Aperture adjustment that you need to see to perform a desired edit in Photoshop should be applied before the initial round trip.
If necessary, you can create a new rendered master with all edits rendered by holding the Option key as you choose Photos > Edit a Copy with Adobe Photoshop... Remember that if you do so, Aperture will flatten the image, and you will no longer have access to separate layers in Photoshop.