Field Tested: the Canon 300mm f/2.8L IS II

[Roger Clark]

:). Hey Roger, Here's a related question for you: I feel comfortable recommending the 1.4X TC with the 70-200 f/2.8L IS II lens for folks with pro-sumer bodies like the 7D and the 50D but not the 2X TC but have no compunction recommending the 2X for folks with pro bodies like the MIV. Is there any scientifically justifiable reason for that or have I been wrong all along? (It seems likely as the AF motor is strictly lens related and has nothing to do with the camera body...)

Hi Artie,

Yes there is some scientific justification, though probably a somewhat gray area. For example here are more numbers of AF speed with 2x TC (I posted these a year or so ago on BPN):

Experiment to determine focus acquisition speed: Time to move from minimum focus to infinity and lock on a target at infinity.
Canon 300 mm f/2.8 L IS lens with canon 2x TC.

1D Mark II: 4 to 5 seconds
5D Mark II: 4 to 5 seconds
7D: 4 to 5 seconds
1D Mark IV: 2 to 2.5 seconds.

So AF speed with 2x TC's is not great pre 1DIV (I didn't have a 1DIII to test).

A second factor is that the crop sensors have smaller pixels, so will be affected more by image quality degradation than the larger pixels of a 1DIV or 5DII.

But having said that, a 2x TC, especially on a 300 f/2.8, or other f/2.8 lens, can give more pixels on a subject, so if the AF can lock on, it might still be work the effort.

Roger