Core the apples and slice crosswise 1/16 to 1/8 inch thick. Keep every batch at one thickness.
Pat away surface juice. Toss with cinnamon and optional lemon or maple sugar.
Preheat the air fryer to 300°F for 3 minutes.
Place slices in a mostly single layer. Slight edge overlap is acceptable, but do not form a stack.
Air fry 10 minutes, turn and separate the slices, then cook 6 to 10 minutes more. Remove thin slices first.
Transfer to a rack in one layer for 10 minutes. Judge crispness only after cooling.