F100: autofocus very sluggish

I have been shooting with a F100 for a few months now and cannot help but notice that the autofocus seems to be very unreliable and is unable to spot a significant difference in distance.

Today i shot a building that was an estimated 20m away from me. From the same position i decided to focus on the roof, which was easily 35m distance. The camera simply could not spot the difference in distance so the lens didnt move.

While we at it, could this issue also be coming from the lens and not the camera? I am using a Sigma 28mm 1.4 II (aspherical).

Thanks in advance!

With a 28mm lens there is no difference between focusing at 20m and 35m. With 28mm, anything beyond some 5 or 10 meters is infinity.

Ron789 said:

With a 28mm lens there is no difference between focusing at 20m and 35m. With 28mm, anything beyond some 5 or 10 meters is infinity.

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Ron is correct. Put on a longer lens and see. My 50mm lens shows 20m on the distance scale, then infinity so for a 28mm lens there will be next to no difference.
Does it focus ok from close (1m) to mid distance?

Thanks you guys, it makes pefrect sense. Too bad i didnt think of this beforehand, haha.