Nikon have produced a variety of autofocus (AF) Nikkor lenses since the first mainstream AF Nikkors were introduced during 1985 to compliment the F-501 camera, which was Nikon's first mass produced AF SLR camera. Over the years, as AF camera and lens technology has evolved, Nikon has introduced new types of AF Nikkor lenses, although most of the earlier versions still remain compatible with current AF Nikon cameras, including the digital SLR models.
There are six general designations that Nikon use to identify the features and functions available with each category of AF Nikkor lens; AF, AF-D, AF-I, AF-S, AF-G, and AF DX.
The designation 'AF' was used on the first generation of AF Nikkor lenses, and whilst this type of lens has long since been discontinued there are many still in use and they remain compatible with most functions available all current Nikon autofocus SLR cameras. The introduction of autofocus technology and the continuing development of more sophisticated through-the-lens (TTL) metering systems required a far greater level of communication between a lens and the camera body. This could only be achieved by use of electronics, so each AF Nikkor lens contains what Nikon call a central processing unit (CPU), although to be strictly accurate it is in fact an encoding microchip that passes this information to and fro.
It is easy to distinguish between an AF Nikkor lens and a manual focus Nikkor as the former has the 'AF' designation marked on the lens barrel and a series of electrical contact pins arranged around the rear flange of the lens mount.
The essential difference between the original series of AF Nikkor lenses and all AF lens types introduced subsequently, from the AF-D versions onwards (see below), concerns the compatibility of a particular lens with Nikon's enhanced Matrix metering system known as 3D Matrix metering that was introduced during 1996. All Nikon cameras, film or digital, which support 3D Matrix metering require the focus distance information to be transmitted from the lens in order that the metering system can operate to its full potential. If an early type AF Nikkor lens (i.e. one that does not communicate the focus distance to the camera) is used with a camera that supports 3D Matrix metering the camera's metering system defaults to the less sophisticated standard Matrix metering.
Note: During early 1983 Nikon launched the F3AF variant of the F3 camera that had two dedicated AF Nikkor lenses: the AF 80mm f/2.8 and AF 200mm f/3.5 IF-ED. However, these two lenses are not compatible with any of the current Nikon AF cameras.
To provide full functionality with the 3D Matrix metering system that has been incorporated in Nikon camera models introduced since 1996, which uses the focus distance information as a component in calculations for both ambient light and flash exposures Nikon introduced the AF-D series Nikkor lenses. Although externally these lenses look very similar to the earlier type of AF Nikkor the internal microchip performs the additional function of communicating the focused distance of the lens to the camera body, hence the 'D' (D - distance) designation. Cameras compatible with Nikon's 3D Matrix metering system use this information to improve the accuracy of their built-in through-the lens (TTL) ambient and flash exposure metering systems.
All current Nikon autofocus lenses communicate the focused distance information to compatible cameras capable of performing 3D Matrix metering and therefore conform to the AF-D specification, these include the AF-I, AF-S, and AF-G types. AF-D lenses can be identified by the 'D' designation in the description of the lens; this is marked on the lens barrel, and generally it is also shown as a suffix to the lens aperture value, which is also displayed on the lens except in the case of AF-G lenses.
To improve the focusing speed of their autofocus lenses, particularly long focal length, wide aperture telephoto types, Nikon introduced the AF-I (I - internal) series of Nikkor lenses. Rather than using the focusing motor built-in to the body of an AF Nikon camera, which relies on a mechanical drive shaft link between the camera and lens to operate the focusing action, the AF-I type Nikkors have a coreless electro-magnetic focusing motor built in to the lens, which improves both the speed and accuracy of autofocus, significantly.
Only four AF-I lenses were released: the AF-I 300mm f/2.8D, AF-I 400mm f/2.8D, AF-I 500mm f/4D, and the AF-I 600mm f/4D. All are clearly identified by the designation shown on the descriptive plate attached to the barrel of each lens that also displays the serial number of the lens.
The successor to the AF-I type Nikkor lens uses an enhanced version of the internal focusing motor, which converts ultrasonic sound waves to generate a rotational energy to drive the focusing action. This type of motor offers even greater speed and accuracy of autofocus action combined with near silent operation. Nikon refer to this type of focusing motor as a Silent Wave Motor (SWM) and give Nikkor lenses that contain them the designation - AF-S (S - silent).
Note: Nikon's choice of the AF-S designation is somewhat confusing since the same designation is used consistently throughout Nikon literature to denote the Single-servo AF focusing mode available in all AF Nikon cameras.
Note: As mentioned above all AF-I and AF-S type Nikkor lenses conform to the D-type specification as they transmit focus distance information to a 3D Matrix Metering compatible camera body.
The AF-G type Nikkor lenses differ from all other AF Nikkor lenses, significantly in one respect - they have no aperture ring.
Apparently, based on the results market research, fewer and fewer photographers will use a lens aperture ring, if their camera/lens combination has the ability to adjust the lens aperture value by 1/3-stop electronically via a command dial on the camera, as this method is far more accurate than a manual aperture ring adjustment, regardless of the photographer's experience.
As a consequence, during 2000, Nikon introduced the first examples of the AF- G (G - genesis) type AF Nikkor. Initially these lenses appeared as entry level models with a specification and price point intended for the consumer market, because they offered an number of benefits including being simpler to use by less experienced photographers, lighter, easier to construct, and therefore cheaper to mass produce.
However, Nikon has extended the inclusion of the AF-G specification to several new professional AF-S lenses, such as the award winning AF-S VR 200mm f/2G IF-ED, and it is expected that this trend will continue across the Nikkor range as further lenses are released.
Note: Earlier Nikon camera models that lack the ability to set the aperture value from the camera body require a Nikkor lens with an aperture ring to obtain full compatibility with all the exposure modes available on the particular camera in use.
Optimized for the DX format (15.6 x 23.7mm) sensor used in Nikon digital SLR camera models, which was first introduced with the D1 model during 1999, all six of the DX type Nikkor lenses released so far (mid-2005) are AF-G type lenses, and communicate the focused distance information to compatible cameras capable of performing 3D Matrix metering.
The optics used in a DX type Nikkor lens is designed to ensure that after light leaves the lens it strikes the camera's digital sensor at the optimum angle. This requires the light rays to be perpendicular, or as close to perpendicular as possible relative to the surface of the sensor. Furthermore, since a DX type lens need only project an image circle that covers the smaller area of the DX format sensor rather than the larger area of a 135-format film frame they can be made smaller and lighter.
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