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Generally photographers put UV filters on DSLRs for the larger purpose of protecting their lenses. Lenses can cost hundreds or thousands of dollars, whereas UV filters are generally less than $40, and sometimes less than $10. 

UV filters are practically useless (image-wise) on DSLRs since digital sensors are not nearly as sensitive to UV light as film. So don't spend $150 on a "premium UV filter" if you're shooting digital. Just grab a $20 one off amazon and use it as a lens protector. 

Some digital cameras can adjust for lens focal problems. Sometimes the focus was sharp enough but other times, especially at f/2.8 (limited depth of field) the shots were soft. This was a film camera and no local calibration was possible. 

The lens manufacturer starts with a mixing of chemicals and fusing them in high temperature, making the glass. The glass is fused, coooled, cutted and polished, and you have a lens. 

Compared to the body, certain types of lenses can be extremely hard to make. Quality aside, if you want a 1.4 aperture 300m telephoto lens, that will be extremely expensive to make (the resulting lens will be huge with lots of glass and components, and most likely impossible to build practically).