Aperture has a Loupe tool (also called the Centered Loupe) that allows you to check the focus and details of image thumbnails in the Browser without having to open the photos in the Viewer, zoom in to check details, and then switch back to scanning through thumbnails in the Browser. (For information about using the Viewer to zoom in to images, see View photos at full resolution.)
You can position the Loupe over a portion of the photo to see a magnified view of that area. In addition to checking a photo’s focus, the Loupe is useful for making quick checks for sensor dust, flaws, fingerprints on film scans, and other issues. You can use the Loupe on any photo or thumbnail in the Aperture library.
Aperture also provides a second version of the Loupe for your convenience. For more information, see Use the alternate Loupe.