The filters introduced in iOS 7 are here to stay, and though they gain no new counterparts, iOS 8 does introduce a new adjustments control that lets you alter a photo's light, color, and (if you choose to go the greyscale route) its black and white intensity.
Tapping on one of those three options will bring up a thumbnail slider to increase or decrease the option as a whole; however, if you tap the list icon above the thumbnail slider, you can see the individual values being adjusted (for instance, the Color slider controls Saturation, Contrast, and Cast). Tap one of those, and you'll get a detail slider to adjust those specific values alone.
All these edits sync across your devices, so that the fixed image appears in your library immediately. They'll also be non-destructive: if you decide you preferred your image unfiltered, you'll be able to revert it.
On top of that, iOS's new Actions options for developers means that your favorite third-party apps can provide filters and adjustments that you can use from within the Photos app just by tapping the Edit button, then tapping the circle icon with ellipses in it to access the app.
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