ISO 517:2008涉及影响图像中心照度的摄影镜头的光圈和相关特性。 ISO 517:2008规定了静止相机中使用的所有类型镜头的光圈标记，并给出了光圈编号的公差。它还定义了光圈光圈、入射光瞳、焦距、相对光圈和光圈数，并给出了它们的测量或确定方法。 ISO 517:2008仅适用于聚焦在无限远处物体上的透镜；也就是说，至少是镜头焦距的50倍。

ISO 517:2008 pertains to apertures and related properties of photographic lenses affecting the illuminance at the centre of the image. ISO 517:2008 specifies aperture markings for all types of lenses used in still cameras, and gives tolerances for the stop numbers. It also defines aperture stop, entrance pupil, focal length, relative aperture and stop numbers, and gives methods for their measurement or determination. ISO 517:2008 applies only to lenses focused on objects at infinity; that is, at least 50 times the focal length of the lens.