A sphere is placed in front of a convex lens of focal length f. The radius of the sphere is much smaller compared to f. The image of the sphere would look spherical if the object distance is

If the object is placed at a distance twice of focal length of convex lens, then magnification is one. Image size is same as object size. Hence if the centre of sphere is at a distance twice of focal length from convex lens, there will not be any distortion due to uneven magnification and the image of sphere is also spherical .