Well, it's that time again! Did you get it right or not? I'm talking, of course, about last week's weekly quiz question.

The question was: "Which of the following lenses will not autofocus on the EOS 40D at it' maximum focal length?" 

The answer was: Sigma 18-200mm f/3.5-6.3 DC

The results were interesting. It seems 56% of you got it right, but 39% thought it was the Tokina and 5% said it was the Canon lens! 

The question is, why does the Sigma not autofocus? The answer lies in the AF sensitivity of the camera. Quite simply, the EOS 40D needs a lens with a maximum aperture of f/5.6 to have enough light for the autofocus system to work. With the Sigma, at the maximum focal length, the aperture drops to f/6.3 and therefore the AF system will not function correctly. 

The only cameras on which that lens will autofocus reliably are the EOS-1 Series models which can still perform autofocusing from the central point when the maximum aperture of the lens drop to f/8. All other cameras in the current Canon range are the same as the EOS 40D in terms of sensitivity. 

It's important to know this if you plan to use Extenders as well - if you put a 1.4x Extender on an EF300mm f/4L IS US lens for example, the maximum aperture drops to f/5.6 and AF will still function on all EOS cameras. However, if you go to a 2x Extender, the maximum aperture will drop two stops to f/8 and you'll be back to needing an EOS-1 Series camera to achieve focus. More commonly, this question applies to the EF100-400mm lens. At the maximum focal length it's f/5.6, so any form of Extender requires the use on an EOS-1 Series camera for focusing.  

 There'll be a new quiz question up later, so keep your eyes out....!