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