Stupid question:

How can a lens be HD vs. SD? Does it have something to do with the purity of the glass in the lens? Thanks for explaining.

not so much with the purity of the glass as with the actual quality of the lens elements used(shape, number, coatings). Because of the higher resolution required for HD, lensmakers have to be more precise in the grind and prescription than for SD lenses.

How can a lens be HD vs. SD?It's determined primarily by the resolving power of the glass. But it's not simple -- in fact, the question you've asked is actually highly complex. The short answer is three words long and we can even abbreviate it to three letters:

Modulation Transfer Function or MTF

It's a characteristic of a lens and basically it's what separates SD from HD. Canon Broadcast has assembled an entire series of white papers which discuss HD lens design in detail and the importance of MTF in particular, all thanks to optics guru Larry Thorpe. These documents are available on the web at:

http://www.usa.canon.com/html/industrial_bctv/whitepapers.shtml

In particular you'll want to learn about MTF in the second paper of the series:

http://www.usa.canon.com/html/industrial_bctv/Whitepapers/WhitePaperLens_2FINALMarch4th05.pdf

Hope this helps,

Chris:

I believe you really meant optical transfer function, which is the measurement of angular separation points as a function of frequency, or essentially resolving power.

The MTF is a measurement of contrast attenuation as a function of frequency for both the sensor and a lens system. This defines a lens to sensor contrast relationship critical for the perception of a sharp image.

Both are critical for a lens and sensor system.

All I can say is I'll bet a lot of our consumer camcorders would have produced better video if we had better lenses. My Sony Hi8 Handicam is a terrific machine that does a good job, close-in, and is still going strong. Too bad the lense was made from something that came out of a Cracker Jacks box! I'll bet just about any of the new low-cost hi-def tape camcorders produce better DV images than their DV only counterparts. The lens is very important!

I really like the audio metaphor for this explanation.
Consider putting some very expensive Bang and Olefssen audio speakers on a dime store am radio. There's not much point in doing that because the radio will never provide these precision speakers with a quality audio signal.

It's the same with videocams. What's the point in giving them glass that can resolve more than the CMOS or CCD can resolve?