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SO I need :
- Chromatic abberation adjustments in Aperture,
- Eyedropper tool (to check the RGB values)
- CMYK and 300 dpi output.
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Of these the only one that's partly possible for now is 300 DPI output - you can define a percentage of output under custom export options, so you could define a percentage that would give you a 300 DPI image. The EXIF will not be tagged as 300 DPI which is why I say it's a partial solution. Also of course if your images have all been cropped differently this is not a practical solution, and since you need to perform all three tasks it's probably not a practical solution for you as you'll still need Photoshop.
Aperture does not let you select CMYK for an output profile (only RGB profiles) though interestingly you can use CMYK for onscreen proofing.
The only option for an RGB eyedropper is the built in "Digital Color Meter" (under Applications/utilities). Not sure how well that will work though as was pointed out it measures just the color being sent to the screen.
It seems like it would be easy enough to export a set of images and do a photoshop batch alteration of all files to be 300DPI CMYK images.