10D and autofocus at larger than f5.6

[joeprimus]

Help me with my theory -- my mind is fuzzy: With a Tamron 28-200 zoom (f3.5-5.6) -- with the Canon 10D, the 200 mm setting becomes a 380mm effective focal length. Does the f5.6 aperture then become smaller as well? And does that mess up the autofocus? Thanks for thoughts.

[Belmondo]

First, the factor is 1.6X, so your 200mm lens will become the equivalent of 320mm---not 380.

The aperture is not affected. The smaller sensor in the 10D only has the effect of cropping the image. The optical characteristics of the lens remain the same. The narrower field-of-view is then the same as it would be in a lens 1.6 times longer. In your case, the camera will be recording the same FOV as a 320mm lens would capture in a full-frame camera.

The aperture controls the amount of light passing through the lens. Because of the cropping effect of the smaller sensor, some of it is wasted, but the focal ratio is still the same.

[megaweb]

all canon non-pro SLR cameras does have AF for all AF lens at all aperture

all canon non-pro SLR cameras does not have AF for all AF lens at f8 or smaller e.g. f11 when mounted with Teleconverter (1.4x or 2x)

all canon non-pro SLR cameras include 10D , D60 , D30 , EOS 30 , etc ...