-Grays of Westminster Glossary

Ai lens and an Ai-S lens - what is the difference?

The Ai-type Nikkor lens was introduced during 1977, and was the first major change to the famous Nikon F mount since it was launched in 1959. The designation is derived from the name of the method used to couple the lens with the TTL metering system of compatible Nikon cameras, which is known as Aperture Indexing (Ai).

Previous designs of Nikkor lens, now usually referred to as pre-Ai, have to be indexed by manually engaging a coupling fork on the lens aperture ring with a prong protruding from the cameras viewfinder prism head, followed by turning the aperture ring to the smallest aperture, then the largest aperture value.

The Ai system automatically indexes the lens with the camera via a ridge on the rear edge of the aperture ring, which enables much easier and quicker lens mounting and changing.

Apart from this narrow indexing ridge Ai lenses can be distinguished from earlier pre-Ai Nikkor lenses by the following:

  • A second smaller row of aperture values engraved below the main scale on the aperture ring, known as the Aperture Direct Readout (ADR) scale, which is visible via a small window in the viewfinder of Ai compatible cameras.
  • The coupling fork often referred to as the rabbits ears, which was retained to allow Ai lenses to be mounted on non-Ai cameras, has two small cutouts to improve the illumination of the ADR scale.
  • The maximum aperture indexing post, which is a solid metal lug that is located at the rear of the lens next to, and extending beyond, the rear element. This is used by; the EM and FG-20 cameras as part of the system employed to control automatic flash exposures; the FG, F-301 and F-501 cameras to assess the range or aperture values available in the Program exposure mode; the FA camera to assess the range or aperture values available in its Program and Shutter-priority exposure modes; and FA and F4 cameras for matrix metering.

Introduced from 1980/1981 onwards, the Ai-S type lenses are very similar to the Ai-type versions of the same lens, except they incorporate a number of changes to the mechanical linkage that connects the lens to a camera body. These changes were required to enable improvements in the level of accuracy and range of automated exposure options on forthcoming cameras, principally the Nikon FA that was launched in 1984; this was the first Nikon camera with a multi-pattern TTL metering system, known as AMP. These days we know the system that has evolved from AMP as Matrix metering.

The single most important change between the earlier Ai and Ai-S type Nikkor lens is the linear relationship between movement of aperture coupling lever and the blades of the iris diaphragm used in the latter. In Shutter-priority and Program exposure modes this permits the camera to accurately stop down the lens to the required aperture by moving the aperture-coupling lever a known, precise, and equal distance for each increment of aperture value. This system permits the Nikon FA and the Nikon F-501, which was introduced a year later, to operate Shutter-priority and Program exposure modes with a far greater degree of accuracy compared with non-Ai-S lenses.

Other features found on Ai-S type Nikkor lenses but which are absent from the Ai type include the following:

  • A small semi-circular notch milled out of the lens mount bayonet flange, which indicates to the FA, F-301, F-501 and F4 camera models that an Ai-S type lens is attached.
  • The minimum aperture on both the main and ADR aperture value scales is marked in a bright orange.

On lenses with a focal length of 135mm or longer, the focal length indexing ridge, a flange that is located around the outside edge of the rear element, extends past the indentation in which the lens locking pin seats. Again this is used by the FA and F501 cameras to detect when a telephoto lens is attached in order that the camera body will select the appropriate option in the Dual-Program exposure mode; the camera automatically switches to P-Hi (high-speed Program) that sets a bias toward higher shutter speeds to reduce the effects of camera shake. When lenses with a focal length shorter than 135mm are attached to these cameras normal Program exposure mode is selected.

On most standard and wide-angle Ai-S lenses the throw of the focusing ring in shifting focus from infinity to the minimum focus distance is reduced by about half compared with the Ai version, so that the former only require a quarter turn as opposed to a half turn for the latter. Focusing is quicker with Ai-S lenses in this range of focal lengths but as a consequence the depth-of-field markings on the lens barrel are closer to together, which makes assessing depth-of-field and the hyperfocal distance less precise.

With the exception of a few lenses (notably the Micro-Nikkor 55mm f/f.8, Micro-Nikkor 105mm f/2.8, and the 85mm f/1.4 lenses) Ai-S types have the depth-of-field scale markings engraved on the chrome ring set around the lens barrel.

The only two camera models actually make full use of the Ai-S lens mount: the Nikon FA and F-501, both of which have been out of production for many years. The F-301 and F4 will detect that an Ai-S type lens has been mounted but both these cameras operate with Ai lenses in an identical way. All other Nikon cameras make no differentiation between the Ai and Ai-S mount.

The Ai-S lens mount represents the pinnacle of Nikons design and engineering in terms of the mechanical linkage and control of a lens diaphragm. Introduction of the AF-Nikkor lenses, which use electronics to control the lens and provide other information such as the focus distance for exposure computations have obviated the need for further development of the mechanical system.

The optical construction of most Ai lenses is either identical or very similar to the equivalent Ai-S variants, and they operate with the same functionality on compatible Nikon camera bodies. It is worth bearing this in mind when buying used equipment since, generally, the Ai-type Nikkor lenses are less expensive.

Simon Stafford