An adapter that can connect to an objective has a receiving area for receiving the objective. The adapter has holding structures which extend essentially in a plane directed transversely to the objective axis of the objective and which are designed to engage corresponding structural elements of an adapter receptacle while the adapter is being inserted into the adapter receptacle, thereby preventing the adapter from already moving in the direction of the objective axis while the adapter is being inserted into the adapter receptacle. A device for holding an adapter, can include an adapter receptacle for receiving the adapter and an adapter and also for moving an optical element into the objective axis. An adjusting device can include the adapter and the adapter receptacle.