If companies make more money on lenses why don't they make for other companies?

[thefonz78]

If companies such as Nikon and Canon make more money selling lenses, you would think they would go down the same route as sigma and tamron and make there lenses with fittings for all the brand slr's. It doesn't seem to have done leica any harm making for lumix.

[tonyvicenzo64]

any experiences with Tamron Lenses?? especially the 18-200 mm F/3.5-6.3 Di II VC PZD.....

HOW WOULD IT PERFORM COUPLED WITH A CANON 600D

[thefonz78]

Tamron lenses are usually pretty good. I'm not sure i would buy a super zoom lense myself but people love them.