The EF 85mm f/1.8 USM is a consumer level lens. It is the shorter sister to the EF 100mm f/2.0 USM, and is designed very similarly. It is constructed with a plastic body and a metal mount. This lens features a distance window with infrared index. An 8 blade, maximum aperture of f/1.8, gives this lens the ability to create depth of field effects. The optical construction of this lens contains 9 lens elements, without any special lens elements. It uses an internal focusing system (meaning that the front of the lens neither rotates nor extends when focusing), powered by a ring type USM motor. Auto focus speed of this lens is very fast.
In addition to portraiture, the lens is also suited for indoor sports photography, particularly on a cropped sensor, due to its focal length, fast autofocus, and fast aperture, and for photos in social situations, due to its fast aperture (thus usable in dark interiors) and quiet operation (hence minimizes disruption).
In terms of aberrations, the lens suffers from noticeable vignetting when used wide-open on full frame cameras, though this is significantly reduced when used on a cropped sensor or stopped down to f/2.8 or f/4. The lens also suffers from purple fringing in high-contrast lighting, such as chrome, water, and black-and-white.
Read more about this topic: Canon EF 85mm Lens