An optical apparatus as a first optical apparatus used in a camera system including the first optical apparatus and a second optical apparatus detachably and communicably attachable to the first optical apparatus, one optical apparatus of the first and second optical apparatuses including an optical element movable for an image blur correction, and another optical apparatus of the first and second optical apparatuses including an image sensor movable for the image blur correction, the first optical apparatus comprising: a setting means configured to set in each of four different blur directions (a) a first ratio of an image blur correctable amount provided by movement of the image sensor to a total image blur correctable amount in the camera system and (b) a second ratio of an image blur correctable amount provided by movement of the optical element to the total image blur correctable amount using information on a shift between a center position of an image circle formed by the one optical apparatus and a center position of a mechanical movable range of the image sensor , information on a mechanical movable amount of the image sensor and information on a size of a signal readout area of the image sensor , wherein the four different blur directions comprise two first directions and two second directions, the two first directions are distinguished with respect to a sign of the respective first direction as a negative first direction and a positive first direction, the two second directions are distinguished with respect to the sign of the respective second direction as a negative second direction and a positive second direction; the two first directions and the two second directions are orthogonal to an optical axis of the one optical apparatus; wherein for each of the four different blur directions, the total image blur correctable amount is a sum of the image blur correctable amount provided by the movement of the image sensor and the image blur correctable amount provided by the movement of the optical element . a setting means configured to set in each of four different blur directions (a) a first ratio of an image blur correctable amount provided by movement of the image sensor to a total image blur correctable amount in the camera system and (b) a second ratio of an image blur correctable amount provided by movement of the optical element to the total image blur correctable amount using information on a shift between a center position of an image circle formed by the one optical apparatus and a center position of a mechanical movable range of the image sensor , information on a mechanical movable amount of the image sensor and information on a size of a signal readout area of the image sensor , wherein the four different blur directions comprise two first directions and two second directions, the two first directions are distinguished with respect to a sign of the respective first direction as a negative first direction and a positive first direction, the two second directions are distinguished with respect to the sign of the respective second direction as a negative second direction and a positive second direction; the two first directions and the two second directions are orthogonal to an optical axis of the one optical apparatus; wherein for each of the four different blur directions, the total image blur correctable amount is a sum of the image blur correctable amount provided by the movement of the image sensor and the image blur correctable amount provided by the movement of the optical element . <EOS>