Update on April 8, 2014: OS X 10.10.3 is now available, bringing with it Photos app to the general public as a free download. You can download the update via the Mac App Store under the Updates tab. For more information on getting started with Photos an importing your iPhoto Library to the new app, check out Apple's getting started guide for Photos.
Last month, Apple raised the curtain on its new Photos app for OS X. Our Jason Parker took it for a spin and found it to bring a level of simplicity more commonly found on iOS to Apple's desktop platform OS X. Since that time, Photos has been in a somewhat private beta wherein only registered developers had access to OS X Yosemite 10.10.3, which includes the new applications.
Apple had promised a public beta for everyday users to test, experiment and report any bugs found in the app back to the Cupertino-based company. And today, Apple made good on that promise by releasing a public beta for OS X 10.10.3.
If you had signed up to test OS X Yosemite last summer, you should still be enrolled in the program. If so, an update will be available for you in the Mac App Store under the Updates tab.
For those who never registered for the original Yosemite beta, you can still sign up. You'll need to visit Apple's OS X Beta Program site and sign up using your Apple ID.
During the signup process, you're reminded to create a backup of your Mac using Time Machine (or the like) and are walked through potential pitfalls of using beta software.
Once you agree to the terms of the beta, you need to download and install a small program that registers your Mac with the beta program. From there, all future updates are done through the Mac App Store.
The entire process from signing up until you have 10.10.3 installed will vary depending on your connection speed and Mac, but I'd suggest setting aside an hour for the initial installation of the beta. You'll need even more time if you've yet to back up your Mac.
