Focus problems

[ar14]

Camera: Canon EOS 20D
Lens: Canon EF-S 17-85mm
Exposure: 30 sec (30)
Aperture: f/6.3
Focal Length: 17 mm
ISO Speed: 100
Exposure Bias: 0/2 EV
Flash: Flash did not fire

I would like to get some advice about focusing. I wanted to focus on the tower in front, but the camera couldn't get a lock in AI FOCUS mode. The only way to take the shot was to select AI SERVO, but now the front tower is not completely in focus.
Any suggestions on how to improve the focusing capabilities?
Is this maybe a limitation of the lens?

[zetson]

I think the problem here is that it was not enough light for the AF to work properly. I don't know why it worked on servo, but maybe it found another area to focus on instead?

[wulf]

I've moved this to the technique section since you seem to be focusing () on an aspect of technique rather than asking for critique on the merits of this particular photo. Presumably the scene was very dim before you captured it with a 30 second exposure, so the camera was having to pretty much work in the dark?

Manual focus may be the way to go. One option is to use a strong light on the subject to give the autofocus something to work on and then flip the lens into manual focus mode before taking your shot. Another would be to caculate the rough distance and use markings on the lens to make a good guess. Review the result and, if necessary take some more shots.

At that distance, DoF should be quite wide. Indeed, plugging your figures into an online calculator suggest that you can rely on the hyperfocal distance - focus at about 8' away and everything from 4' to infinity should come out sharp.

Wulf

[ar14]

So I should carry a torchlight with me

Thanks for your help.