Nikon released the D40 in 2006 as a continuation of its line of entry-level DSLR cameras. But the D40 marked a notable shift in the Nikon entry-level plan. To save costs and attract more relatively budget conscious DSLR enthusiasts, Nikon removed the auto-focus motor from the camera body and removed the focus pin from the lens mount. Costs came down -- by about $300 from the D50 kit -- but the selection of auto-focusing lenses was slightly more limited.
Nikon Lens Classifications
Since 1986, Nikon has released four different classifications for lenses: AF/AF-D, AF-S/AF-I, VR and G lenses. There are subcategories on top of these basic classifications, such as "DX" for smaller-format sensors. As a general guide, lenses in the last three categories, AF-S and -I, VR and G, will work with full functions on the D40. The letters can be combined too, and AF lenses can have VR and G sub-features.
AF-S/AF-I Lenses
Nikon began releasing AF-I lenses in 1992 and AF-S lenses in 1998. "AF" stands for auto-focus and "I" for internal motor. These lenses work with cameras without an internal drive motor, since the focusing is driven from inside the lens. The coupling contacts will communicate with the D40 and focus on auto mode. The "S" in "AF-S" stands for Silent Wave Motor, another internal focus drive that is quieter than the AF-I drive motor and more flexible than AF-I lenses. You can manually focus these lenses while the camera's focus setting is on "A." All AF-S lenses work on the D40.
VR and G
"VR" stands for vibration reduction. Nikon's VR lenses are designed to stabilize shots taken at longer exposures, such as in low-light settings. VR is usually at the end of the name of the lens and is most commonly found in lenses that zoom and not in fixed focal length lenses. The "G" designation stands for gelded. The G lenses don't have aperture rings, meaning that the camera, not the photographer sets the maximum aperture. Most casual photographers won't notice much of a difference here. The "G" always appears in Nikon's Nikkor lens names after the aperture value, such as in "AF-S NIKKOR 50 mm f/1.4G." The D40 handles all VR and G lenses.
Understanding Lens Names
Sometimes, lens names can get fairly complicated. For example, consider the AF-S DX Nikkor 18 to 55 mm f/3.5 to 5.6G VR. "Nikkor" is the name of Nikon's lenses. This lens has a focal range of 18 mm -- wide angle -- to 55 mm -- about what you can see with the naked eye -- so it has limited zoom abilities. It's a "G" lens, so there's no aperture ring, and the widest possible aperture ranges between f/3.5 and f/5.6. It's also a VR lens, so the camera will help you stabilize shots at any focal range. DX is a digital format for smaller sensor cameras, like the D40. The most important consideration for the D40 is the AF-S, which guarantees full auto-focus support on the D40.
Third-Party Lenses
Nikon isn't the only company that makes lenses for the D40. Tamron, Sigma and a handful of other third-party manufacturers produce lenses for most DSLR cameras. These companies all offer specific lens compatibility charts that list every camera their lenses are compatible with. Check before buying and note, where specified, which lenses require an auto-focus motor.
Tags: AF-I lenses, AF-S AF-I, AF-S lenses, auto-focus motor, drive motor, DSLR cameras