tamron focusing problem

[biggin]

has anyone had problems with focus assist on the 20d using a tamron 28-75?

[tim]

I haven't had a problem with that equipment, though I use a 550EX not the built in flash for focus assist. What problem are you having?

[Marshy]

I haven't had a problem with that equipment, though I use a 550EX not the built in flash for focus assist. What problem are you having?
Got a 300D not 20D and only had the tamron for a couple of days but tried to take a picture of the 300D box with the tamron to put on e-bay . The box is black with grey writing on it , couldn,t get the tamron to auto focus at all until i realised it doesn,t like the black , on the grey bits it is very quick . I know autofocus doesn,t like flat colours but the grey is dark quite close to the black but the lense can tell the difference .

[tim]

Autofocus ability has very little to do with the lens, it's all in the camera AFAIK. The camera needs contrast in what you're shooting to be able to lock focus, for why have a read of this (http://electronics.howstuffworks.com/autofocus.htm).

Fast lenses (for me, F2.8 or faster like F1.8) focus more quickly in low light, because there's more light available to hit the sensor.