The editing tools in Photos are surprisingly sophisticated and easy to intuit, even if you have little experience editing photos. As with iPhoto, adjustments are applied to the entire image, not selective areas, as is possible with Aperture's Quick Brushes.
At the surface level, you can drag sliders for Light, Color, and Black & White and let Photos do the computation to improve the image. It doesn't simply turn up the exposure for Light, for example.
In fact, clicking the expansion button that appears at the right edge of the control reveals separate sliders for Exposure, Highlights, and so on. As you drag the master Light slider, for example, the other controls adapt to balance the image. For photos with decent exposure, I wasn't usually able to blow out the highlights by just maxing out the Light slider; the software kept the levels in check.
And if those feel limiting, several other controls are tucked in the Add menu, including White Balance, Vignette, Sharpen, and Levels.
When working with Raw+JPEG pairs (where the camera captures both a raw image and a full-size JPEG image, and presents them as one photo), Photos gives you the option of choosing which version to use as the original.
I was also impressed with the Retouch tool, primarily because of its design: it smartly combines two tools, an automatic retoucher and a clone tool. Click once on a spot to attempt an auto fix. Or, to sample from a nearby area, Option-click there first before clicking the spot to be repaired.
iCloud Photo Library
Building a new flagship photo application on the Mac is a tall order, and yet Apple is being more ambitious by incorporating iCloud Photo Library. The feature is not turned on by default — there's no requirement that your photos be stored on Apple's iCloud servers.
If you do enable it, Photos uploads copies of your original images to iCloud, which are then shared with any device on which you've enabled the feature. In this way, your entire photo library can be accessible on your iPhone or iPad.
There are, of course, some caveats. Most people's photo libraries are larger than even the highest-capacity device (currently 128GB for iPhone and iPad, 512GB for a maxed-out MacBook Air). In that case, you have the option of optimizing the photo storage, which stores only low-resolution versions on the device for when you're browsing the library. When you tap or click a photo thumbnail to view it, the high-resolution version is downloaded as needed.
This implementation is both clever and frustrating. Opening an optimized photo makes a small status wheel appear, which immediately fills to 25 percent and then, in my experience, often idles for a bit while waiting for the data to arrive — in some cases longer than a minute. I experienced this on a variety of Wi-Fi and cellular networks. However, I'll also note that performance seemed to improve as the release date drew closer, so I can't rule out that the lag I witnessed was due to testing or optimization before Photos was available. How the service performs under load when potentially hundreds of thousands of people are accessing it is an open question.
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