The present disclosure relates to medicament injection devices. An injection device includes: a movable dosage programming component comprising a rotary encoder system having a predefined angular periodicity, a sensor arrangement including a first optical sensor configured to detect movement of the movable dosage programming component relative to the sensor arrangement during dosing of a medicament, wherein the first optical sensor is configured to operate in a strobe-sampling mode at a first frequency, a second optical sensor configured to detect movement of the rotary encoder system relative to the second optical sensor wherein the second optical sensor is configured to operate in a strobe-sampling mode at a second frequency lower than the first frequency, and a processor arrangement configured to, based on the detected movement, determine a medicament dosage administered by the injection device.