Newbie DSLR question

I was just experimenting with my new Canon Rebel xsi and had a question about two photos I took.

HTTP response: 404 | MIME changed to 'image/gif' | Byte size: ZERO | PHOTOBUCKET ERROR IMAGE

More information is required.

What lens/focal length were you using and, at what distance were you from the tyre.

If you plug those values into this DOF calculator, you will probably have your answer.

Welcome to the Forum.

It's hard to tell from the small pics but the f/8 one almost looks as if there is a bit of motion blur.

If they where both shot at 18mm or near then both of them will probably have near infinite focus.

If you managed to take one of them at 3.5 with the kit lens, i guess you had to have it at 18mm.

The f/8 one is not so clear for different reason, it looks a bit overexposed compared to the other one, and maybe a bit of flare from the top right is killing some of the contrast (and therefore detail) so it looks a bit washed out. Or could be motion blur as mentioned above.

Lesson #1, from personal experience; when comparing anything in photography,
use a tripod, with a remote shutter and mirror lockup.

This should probably be in the "Talk About Photography" section

Yes, it should be shallower, but I think you're too far away from the focal point to tell the difference. Try the same thing again but this time get as close to the tire as you can (i.e. minimum focusing distance away) and the difference should be more pronounced.