Apparatus to acquire an image of roots of a plant located in a target rhizotron, said apparatus comprising: a turntable (12) having a rotation axis (X) and intended to support said target rhizotron； a lightening device (14)； an first camera (16) having an optical axis (O) substantially oriented toward said rotation axis (X)； a focus sensor (18) arranged so as to provide a measure (M) of at least one parameter of the first camera； a control module (20), configured to receive said measure (M), compare said measure (M) to comparison data (C), and inform an operator accordingly, the comparison data comprising data related to reference rhizotrons different than said target rhizotron, and/or data related to one or several previously acquired images of the target rhizotron.
