<P> 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 bright spots, for example night cityscapes . Some cameras, such as the Olympus XA or lenses such as the MC Zenitar - ME1, however, use a two - bladed diaphragm with right - angle blades creating a square aperture . </P> <P> Similarly, out - of - focus points of light (circles of confusion) appear as polygons with the same number of sides as the aperture has blades . If the blurred light is circular, then it can be inferred that the aperture is either round or the image was shot "wide - open" (with the blades recessed into the sides of the lens, allowing the interior edge of the lens barrel to effectively become the iris). </P> <P> The shape of the iris opening has a direct relation with the appearance of the blurred out - of - focus areas in an image called bokeh . A rounder opening produces softer and more natural out - of - focus areas . </P> <P> Some lenses utilize specially shaped diaphragms in order to create certain effects . This includes the diffusion discs or sieve aperture of the Rodenstock Tiefenbildner - Imagon, Fuji and Sima soft focus lenses, the sector aperture of Seibold's Dreamagon, or the circular apodization filter in the Minolta / Sony Smooth Trans Focus or Fujifilm APD lenses . </P>

Where is the field diaphragm located on the light microscope