Disabling iCloud photo library, effect on camera roll?

Hi, I am wondering how turning off iCloud photo library affects the camera roll, if at all, on an iDevice.

I recently turned it off on an iPhone (think that optimized storage was selected), got a message that all iCloud photos will be removed, but as far as I can tell the original camera roll is unaffected, all full res photos are there.

Is this always the case when iCloud photo library is disabled? Does the original camera roll (photos taken by the device) always remain intact afterwards, even if optimize was selected?