Diaphragm (optics) curved blades improve the roundness of the iris opening. In a photograph, the number of blades that the iris diaphragm has can be guessed by counting the number of diffraction spikes converging from a light source or bright reflection. For an odd number of blades, there are twice as many spikes as there are blades. In case of an even number of blades, the two spikes per blade will overlap each other, so the number of spikes visible will be the number of blades in the diaphragm used. This is most apparent in pictures taken in the dark with small
Diaphragm (optics) by using an automatic ND filter. Unlike a real diaphragm, this has no effect on depth of field. A real diaphragm when more-closed will cause the depth of field to "increase" (i.e., cause the background and the subject to both appear more in-focus at the same time) and if the diaphragm is opened up again the depth of field will "decrease" (i.e., the background and foreground will share less and less of the same focal plane). In his 1567 work "La Pratica della Perspettiva" Venetian nobleman Daniello Barbaro (1513-1570) described using a camera obscura with a biconvex lens as a