Question about Canon DSLR bodies w/150-500mm or 150-600mm

Started Jan 23, 2014 | Discussions thread
WilbaW
WilbaW Forum Pro • Posts: 11,562
Re: Question about Canon DSLR bodies w/150-500mm or 150-600mm

Jeff Peterman wrote:

It is both - a limitation of the camera and of the amount of light. When the lens can only open to something smaller (in size, not number) than f5.6, the amount of light starts becoming too little for the camera to use for focusing. This means that autofocus becomes very slow, and can become unreliable.

It's not because the illumination is 1/3 of a step dimmer, it's because the view the AF sensor has of the scene starts to get blocked. An AF sensor is like two telescopes looking through a window. No worries if the window is wide enough but a problem if it is narrower than the "width" between the telescopes.

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