Do the two letters mean High Definition? Are they called HD lenses? How do they differ from normal (SD?) lenses? Please include a plenty of details in your answer..
Yes, the two letters together mean High Definition. It is just a marketing term like many others. Pentax at some point changed the coatings on 5 of their lenses and added HD to their names and changed a color around the edge from green to red. You will find the press release for the coating here. There is a copy of the press release for the lenses here.
While we can probably deduce that the HD version of a lens is slightly better than the non-HD version. That is all we can tell from these two letters. Camera and lens manufacturers often quality their new products with acronyms to be able to talk about them but it is simply marketing. It does not reliably say that something is better and the lack of an acronym does not mean that the item does not have that quality or feature. In other words, you may find HD coating on a lens which is not called HD as it ends up becoming more common.
Take for example ED which stands of Extra-Low Dispersion. Initially, lenses with such glass elements had ED appended to their names but as it became more common, they used it less in the name, even though that type of glass is effectively used in the majority of lenses by now.
Just remembered that Panasonic added the HD label to several lenses with no physical change, only by providing a firmware update for the purpose of making them more suitable for video by reducing the noise made by AF and improving image stabilization. This is a great example on how the HD term and others can be used at a whim from the manufacturer.
This depends on the make and model of lens. But often 'HD' means it is specifically designed for recording HD video.
So features will often include:
- power zoom
- fast focusing, and focus tracking
- image stabilisation
- smooth aperture changes
- quiet operation
Not all of these features will apply for every HD lens. Though optimised for video, some of this could also be useful for still photography. eg fast focusing is handy for action photography, and quiet operation if close to wildlife etc.
What does it mean when 'HD' is printed on a lens?
Excellent question. My answer:
We all know what "HD" stands for, but what does it mean with respect to lenses? Well, the meaning of the "HD" label is not well-defined. Manufacturers are free to put a "HD" label on any lens they want (and they very often do; many cheap add-on lenses have it). A "HD" lens may sound cool, but that's all it is - a meaningless term that sounds cool. A manufacturer can't be accused of false advertising if "HD" doesn't actually signify anything specific. You may be misled into thinking it is something to do with the lens' resolving power (HD vs SD resolution), but this is not the case.
Since the term doesn't actually signify anything about the lens, then, I say, it means absolutely nothing.