Every time you connect a camera to your Mac, or insert a memory card, the new Photos app opens automatically.
That’s great if you use the new Photos app. But what if you still prefer iPhoto, Aperture, or Lightroom because they have more features? In that case, Photos popping open all the time is just an annoyance.
The solution: Once Photos opens, look for the “Open Photos for this device” checkbox in upper left. Turn it off. You have to do that for every memory card and every camera, but only once apiece. After that, Photos will keep its peace.