A supplemental device for an injection device, the supplemental device for gathering dosage information concerning the use of the injection device, the supplemental device comprising: 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; 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 a coupling arrangement for coupling the supplemental device to the injection device such that the field of view of the first camera sensor portion and the field of view of the second camera sensor portion each comprise at least a part of a dosage window of the injection device, characterised in that the electronic supplemental device further comprises at least one processor configured: to use the first camera output to determine whether a rotational speed of a sleeve of the injection device visible through the dosage window in the field of view of the first camera sensor portion 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, to process the second camera output to identify a feature provided on the sleeve of the injection device visible through the dosage window 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, to refrain from attempting to identify the feature, wherein determining whether the rotational speed of the sleeve in the field of view exceeds the threshold speed is based on a measure of contrast of the first camera output. 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; 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 a coupling arrangement for coupling the supplemental device to the injection device such that the field of view of the first camera sensor portion and the field of view of the second camera sensor portion each comprise at least a part of a dosage window of the injection device, characterised in that the electronic supplemental device further comprises at least one processor configured: to use the first camera output to determine whether a rotational speed of a sleeve of the injection device visible through the dosage window in the field of view of the first camera sensor portion 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, to process the second camera output to identify a feature provided on the sleeve of the injection device visible through the dosage window 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, to refrain from attempting to identify the feature, wherein determining whether the rotational speed of the sleeve in the field of view exceeds the threshold speed is based on a measure of contrast of the first camera output. to use the first camera output to determine whether a rotational speed of a sleeve of the injection device visible through the dosage window in the field of view of the first camera sensor portion 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, to process the second camera output to identify a feature provided on the sleeve of the injection device visible through the dosage window 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, to refrain from attempting to identify the feature, wherein determining whether the rotational speed of the sleeve in the field of view exceeds the threshold speed is based on a measure of contrast of the first camera output.