Sunday, July 25, 2010

Lenses: What's in a name?



Camera lenses are usually designated by names that look somewhat like this: Nikkor AF 55-200mm F/4-5.6 VR.

So what does it mean?

AF means that the lens will work in autofocus mode. AF-S is more recent and usually more desirable; it means that the autofocus mechanism is driven by the camera instead of relying on its own internal motor. AF-S lenses will usually focus faster, but they will not autofocus on cameras which do not have the capacity of driving autofocus. Examples of such cameras include the Nikon D40, D60 and D80.

The designation AI means that the lens can handle light metering as we know it (provided the camera can be set up to accept this particular lens), but focusing will be manual. The designation MF means manual focus, but the lens may or may not be capable of light metering. Both the AI and MF designations are usually seen in listings for secondhand lenses; if you’re thinking of buying one of those, you should do some online research and make sure the lens is compatible with your camera before you put in your bid on eBay.

The figure expressed in millimeters, such as 35mm or 55-200mm, refers to the focal lenght of the lens. Single figures mean that you’re looking at a prime lens with a fixed focal lenght, while a pair of figures indicates the range of a zoom lens.

The F number refers to the maximum aperture of the lens, or in other words how much light it can allow into the camera. On eBay listings and camera store web sites, it will usually be expressed as, for instance, F/1.8. On the lenses themselves, it will usually appear as 1:1.8, as in the example above. The lower the number, the bigger the maximum aperture, and the more light can be allowed into the camera.

For zoom lenses, a maximum aperture expressed as a single figure, like F/2.8 or 1:2.8, means that the maximum aperture will remain the same throughout the lens’ focal range. A pair of figures, like F/4.5-5.6 or 1:4.5-5.6, means that the maximum aperture will decrease as the focal lenght increases, allowing less light into the camera at 200mm than it will at 55mm.

As a general rule, lenses with very wide maximum apertures (such as F/1.4 and F/1.8) will be significantly more expensive. Zoom lenses with constant aperture throughout their focal range will also be more expensive than variable-aperture zoom lenses.

The rest of the alphabet soup in a lens’ name depends on the vocabulary used by the various manufacturers. In Nikon world, for instance, VR is Nikonese for Vibration Reduction. Some of the codes in lens names may refer to the type of glass, special coating, absence or presence of a manual focusing ring, and too many more characteristics to be able to list them here. Before you go ahead and drop a chunk of money on any lens, new or used, you should make sure you understand what you’re buying and whether it will work with your camera.

ADDITIONAL RESOURCES

Ken Rockwell has a more extensive description of Nikon lens types on his web site, including notes on camera compatibility.

For a more technical discussion of Nikon lenses, see Making Sense of Lens Acronyms by Thom Hogan.

If you know of any relevant Canon, Pentax or Sony resources, please send them on and I'll be happy to add them to the list.

NEXT WEEK: FOCUSING

Tune in next week as we discuss the various factors involved in taking sharper pictures.

Happy shooting!

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