The aperture is the opening in the lens through which light gets to the sensor (or film). Adjusting its size—"stopping down" or "opening up"—controls the amount of light that reaches the sensor or film. The aperture is described by an ƒ-number, which is the ratio between the diameter of the aperture opening and the focal length of the lens; ƒ/4 means the diameter of the opening is one-quarter the focal length—25mm on a 100mm lens. From a practical standpoint, the effective aperture is the ratio between the size of the entrance pupil (the aperture as viewed through the front of the lens) and the focal length. Movie lenses are often calibrated in T (true) stops, which are based on the measurement of light actually transmitted by that individual lens rather than calculated.
Obviously, the larger the opening, the more light transmitted. So fast lenses—those with large maximum apertures—are desirable when shooting in dim light: You can shoot at a faster shutter speed or use a lower ISO setting in a given light level. Wider apertures also produce shallower depth of field, handy when you want to concentrate the viewer’s attention on a specific subject (or portion of the subject). On the downside, fast lenses are bulkier and more expensive than slower ones.
Number Of Aperture Blades
Lenses with a greater number of aperture blades have "rounder" apertures, even when stopped down, which produces a more pleasant blur effect in backgrounds ("bokeh") and foregrounds than the polygonal-shaped apertures produced by fewer blades. Rounder apertures also produce rounder, more pleasing specular highlights.
Simple lens elements focus different wavelengths at different distances behind the lens (longitudinal chromatic aberration) and light rays traveling at an angle through the lens at different positions, depending on their wavelengths (lateral chromatic aberration). These aberrations appear as colored artifacts in color images, but also reduce the sharpness of monochrome images. Chromatic aberrations are most evident in supertele lenses, but can also occur in shorter designs. Low-dispersion and extra-low-dispersion glass elements can minimize chromatic aberrations. These elements have designations such as LD, ED, ELD, SD, SLD, HLD, FLD and UD. Fluorite elements are especially good at reducing chromatic aberrations, but are very costly to produce, and thus found mostly in high-end pro lenses. Canon’s Diffractive Optics (DO) also effectively reduce chromatic aberrations, while also permitting more compact lens designs.