Aperture: Master image may appear sharper than a version
When you toggle between a master and a version, the master may look sharper, even after the version has been sharpened. The master isn't really sharper; this is an illusion caused by image scaling.
When viewed at 100 percent, the sharpness of a master and unsharpened version will be the same. Aperture scales masters to fit your screen using an algorithm that's different from the one used to scale versions, hence the difference in appearance. Because it's a scaling issue, the degree of difference can vary with the exact size of the Viewer or, in the case of full-screen mode, the size of your display.
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Tips for avoiding this issue:
Use the loupe when comparing master and version.
Toggle the image to 100 percent (Z key) to check actual sharpness.
Resize the Viewer until you find a size at which the difference between master and version is minimized.
Try full-screen mode. Depending on the size of your screen, the difference may not even be noticeable.