X-ray section topograph images of spherical strain centres in elastically isotropic crystals have been simulated using numerical solution of Takagi's equations. Variations in the image with the size of the deformation strain field, the defect position in the Borrmann fan and the crystal thickness have.. been investigated. Excellent agreement has been found between the simulated images and a wide variety of experimental images from hydrogen-induced strain centres in float-zone silicon.