they can be used together to get a better quality pic and you can use them seperately also. Its for high definition Where did you get the idea that there are lenses made from crystal?
The only lenses ever made with a crystal substance were made by Canon (A number of lenses have a fluorite element right behind this filter, such as the 600/4L, 500/4.5L, 400/2.8L II and 300/2.8L) but were discontinued because they were too fragile. The elements could shatter when the lens was bumped (as in being hit by a 300 pound football player or in very cold weather when shooting for Ski Magazine). Now the fluorite lens elements are used on internal portions of the lens to protect them from the hazardous environment we as photographers sometimes find ourselves.
There may be some macro lenses that contain fluorite elements, but in the past they were used in long lenses to correct for chromatic aberration.
Many microscope lenses do use fluorite crystal lens elements.
To find Canon lenses that still have fluorite lens elements you can do a search for them using google. | 