An optical device (301, 401, 701) for modifying light distribution, the optical device comprising: - a first optical element (102, 202, 302, 402, 502, 602, 702) being a first piece of transparent material and comprising a first surface (104, 204, 304, 404) for modifying a distribution of light exiting the first optical element through the first surface, and - a second optical element (103, 203, 303, 403, 503, 603, 703) being a second piece of transparent material and comprising a second surface (105, 205, 305, 405) facing towards the first surface and for further modifying the distribution of the light entering the second optical element through the second surface, wherein the second optical element is rotatable with respect to the first optical element around a geometric optical axis of the optical device, and one of the first and second surfaces comprises convex areas (106, 206) and another one of the first and second surfaces comprises concave areas (107, 207) for at least partly compensating for an optical effect of the convex areas when the second optical element is in a first rotational position with respect to the first optical element so that the convex areas and the concave areas are aligned with respect to each other, and wherein a combined optical effect of the first and second surfaces is changeable by rotating the second optical element from the first rotational position towards a second rotational position in which the concave areas and the convex areas are non-aligned with respect to each other, wherein: - the first and second optical elements comprise sliding surfaces (309, 310, 409, 410) for sliding with respect to each other and for mechanically supporting the first and second optical elements with respect to each other in radial directions perpendicular to the geometric optical axis, - the first optical element comprises a cavity concentric with the geometric optical axis and the second optical element comprises a projection concentric with the geometric optical axis and being in the cavity of the first optical element, walls of the cavity and the projection constituting the sliding surfaces (309, 310) for supporting the first and second optical elements with respect to each other in the radial directions, and characterized in that a bottom of the cavity of the first optical element constitutes a part of the first surface of the first optical element and an end-surface of the projection of the second optical element facing towards the bottom of the cavity constitutes a part of the second surface of the second optical element, and in that the projection of the second optical element is hollow. - a first optical element (102, 202, 302, 402, 502, 602, 702) being a first piece of transparent material and comprising a first surface (104, 204, 304, 404) for modifying a distribution of light exiting the first optical element through the first surface, and - a second optical element (103, 203, 303, 403, 503, 603, 703) being a second piece of transparent material and comprising a second surface (105, 205, 305, 405) facing towards the first surface and for further modifying the distribution of the light entering the second optical element through the second surface, - the first and second optical elements comprise sliding surfaces (309, 310, 409, 410) for sliding with respect to each other and for mechanically supporting the first and second optical elements with respect to each other in radial directions perpendicular to the geometric optical axis, - the first optical element comprises a cavity concentric with the geometric optical axis and the second optical element comprises a projection concentric with the geometric optical axis and being in the cavity of the first optical element, walls of the cavity and the projection constituting the sliding surfaces (309, 310) for supporting the first and second optical elements with respect to each other in the radial directions, and characterized in that a bottom of the cavity of the first optical element constitutes a part of the first surface of the first optical element and an end-surface of the projection of the second optical element facing towards the bottom of the cavity constitutes a part of the second surface of the second optical element, and in that the projection of the second optical element is hollow. a bottom of the cavity of the first optical element constitutes a part of the first surface of the first optical element and an end-surface of the projection of the second optical element facing towards the bottom of the cavity constitutes a part of the second surface of the second optical element, and in that the projection of the second optical element is hollow.