When it comes to first vs third party lenses for digital SLR cameras, there's usually a tradeoff: price vs. quality.
Many of the third party lenses are not as expensive as those made by camera manufacturers and when it comes to lenses you do get what you pay for.
However, many of the third party lens companies have recently produced exceptional lenses that aren't matched by any similar first party companies.
This makes them a tempting alternative for those who don't want to spend a week's salary on a camera lens.
Now my question is why do they call it 3rd party lenses and not 2nd party lenses? Or should I ask why there is no 2nd party lenses?