jdramirez said:
Also, look at sigma and their issues with AF consistency. Canon doesn't give them their secret recipe, so Sigma reverse engineers it... and only somewhat effectively.
So if canon made lenses for Sony or Nikon or pentax, or even sigma... what happens if they don't share the Colonel's secret recipe... then the Canon brand is hurt as well as their reputation.
I have no issues with recommending either a Canon or a Nikon to a beginner... but it doesn't take much for someone to sour on a brand... because they heard a few bad things. I still haven't bought a Sigma lens even though I have had a boner for both the 35mm art and the 50 art... so there's that to consider.
When Canon adds a lens to their stable, they add characteristics of the lens to their firmware.
For example, let's say you are shooting with a 1DX and Canon comes out with a 200-600F5.6 lens. The lens would report itself as such to the body, and the body would say "I don't know you, but I will treat you like a 100-400" and the camera would focus the lens. Then Canon comes out with a firmware update for the 1DX and the focusing characteristics of the lens are in that firmware. Now the 1DX knows that if the AF sensor is X distance off that it has to drive the AF motor for time Y and it now focuses the lens faster with less hunting.
The problem with a Sigma (or Tamron) lens is that Canon does not put other manufacturer's characteristics into the Canon Firmware. Sigma (or Tamron) have to cheat and tell the camera body that their lens is whatever the closest Canon lens is to their characteristics. The AF can never be as good as a Canon Lens.