A supplemental device for gathering dosage information concerning the use of an injection device, the supplemental device comprising: a coupling arrangement for coupling the supplemental device to an injection device; a first camera sensor portion for generating a first camera output indicative of a scene in the field of view of the first camera sensor portion; and a second camera sensor portion for generating a second camera output indicative of a scene in the field of view of the second camera sensor portion; and characterised in that the supplemental device further comprises at least one processor configured: to use the first camera output to determine whether a rotational speed of a sleeve forming part of the injection device and in the field of view exceeds a threshold speed during dose dialling or dose delivery; if it is determined the rotational speed of the sleeve does not exceed the threshold speed during dose dialling or dose delivery, to process the second camera output to identify a feature provided on the or another sleeve forming part of the injection device and determine an amount of dose dialled in or an amount of dose delivered based on the identified feature; and if it is determined the rotational speed of the sleeve does exceed the threshold speed during dose dialling or dose delivery, to refrain from attempting to identify the feature. a coupling arrangement for coupling the supplemental device to an injection device; a first camera sensor portion for generating a first camera output indicative of a scene in the field of view of the first camera sensor portion; and a second camera sensor portion for generating a second camera output indicative of a scene in the field of view of the second camera sensor portion; and characterised in that the supplemental device further comprises at least one processor configured: to use the first camera output to determine whether a rotational speed of a sleeve forming part of the injection device and in the field of view exceeds a threshold speed during dose dialling or dose delivery; if it is determined the rotational speed of the sleeve does not exceed the threshold speed during dose dialling or dose delivery, to process the second camera output to identify a feature provided on the or another sleeve forming part of the injection device and determine an amount of dose dialled in or an amount of dose delivered based on the identified feature; and if it is determined the rotational speed of the sleeve does exceed the threshold speed during dose dialling or dose delivery, to refrain from attempting to identify the feature. to use the first camera output to determine whether a rotational speed of a sleeve forming part of the injection device and in the field of view exceeds a threshold speed during dose dialling or dose delivery; if it is determined the rotational speed of the sleeve does not exceed the threshold speed during dose dialling or dose delivery, to process the second camera output to identify a feature provided on the or another sleeve forming part of the injection device and determine an amount of dose dialled in or an amount of dose delivered based on the identified feature; and if it is determined the rotational speed of the sleeve does exceed the threshold speed during dose dialling or dose delivery, to refrain from attempting to identify the feature.