You might use these controls this way: A mistake the unwary often make is shooting a picture of their nearest and dearest against a bright background — an unclouded mid-day sky is a common problem. Invariably, Rosco is dark and the background far brighter. If you did nothing but crank up the exposure you'd better expose your boyfriend, but the background would also brighten to the point where it entirely overwhelms the image. But try editing it with the Light control. Drag its smart slider to the right and Rosco becomes brighter but the background stays as it is or darkens, thus leaving you with the kind of image you could share with your mother versus one that shows him in a poor light. You can do this same kind of thing with the Color and Black & White controls.
But this can be just a starting point (or you can ignore it altogether). Click on the downward pointing triangle next to each control and you'll see a series of other controls — the Exposure, Highlights, Shadows, Brightness, Contrast, and Black Point controls I mentioned earlier. You can adjust each of these individually using their sliders or by clicking in the appropriate field and entering numeric values. So, if the global Light control gets you close, but not quite there, you can dive down a level and tweak as necessary.
And if that's not enough control for you, click the Add button in the edit area and you find such additional options as Histogram, Sharpen, Definition, Noise Reduction, Vignette, White Balance, and Levels. Adobe Photoshop (or even Lightroom) it may not be, but if you find iPhoto's editing controls a little underwhelming, you should be far happier with what Photos offers.
About the cloud
When Photos was first announced a number of people fell into a panic. "Apple can't force me to keep my images in the cloud!" they cried. And they're correct. Apple can't force you to keep your images in the cloud, nor will it attempt to. Using iCloud storage is entirely optional. If you want to only store your images locally and not have them beamed into the cloud, you can. All you need do is ensure that the iCloud Photo Library option is unchecked in the iCloud preference within Photos. Or, if you want your images both in the cloud and on your Mac, enable the Download Original To This Mac option, also within Photos' iCloud preference.
This latter option has advantages. When you edit an image or movie within Photos for OS X or in the updated version of Photos that will ship with iOS 8.2, those edits are automatically synced with all your devices and your iCloud.com account. For instance, if I convert one of my beach landscape images to black and white for a more dramatic effect, that version will appear on all the devices linked with my iCloud Photos Library. (And if I revert an image, that change will also be populated to my other devices.)
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