Crop, Then Auto-Enhance in Aperture
When I'm working through a batch of "casual" photos, I use the Auto White Balance and Auto-Enhance (magic wand) tools in Aperture 3.3 quite a bit. In many cases, it gives me the basics of what I want, and is a great time saver. I'm also a reasonably heavy Crop user. And, as the order of the Adjustment layout in Aperture suggests, it's best to crop first, then apply the automated adjustments.
The automated adjustments honor the cropping of your photo, and examine only the segment of the photo visible after the crop. Makes sense, really. By considering only the information post-crop, the resulting adjustment should yield a better result. Why bother considering information that's effectively thrown away when cropped out?
Here's an example of differences in the resulting Curves adjustment from an Auto-Enhance pre- and post-crop, using an image from Half Moon Bay I talked about last week. Let's look at pre- and post-cropped versions of this image, each with the Auto-Enhance "magic wand" applied:
Now, compare the resulting Curves adjustment for each image:
You can see the difference in the curve, both the curve's contour and in/out values. Again, makes sense - check out the overall histogram of the the images as well. Quite different pre- and post-crop.
I suspect the same types of differences can be seen in the other adjustments included in Auto-Enhance (Enhance, Highlights & Shadows, etc.) as well as with the Auto White Balance.
If you use the automatic adjustments, crop your image first and get the best adjustment possible.