How do you calculate where you should focus with your DOF on a zoom lens? Ive heard (dont own one myself, yet) that fixed lenses have distance settings you can calibrate on the lens in conjunction with focal length. However zoom lenses dont have these distance settings. I have heard that focusing 1/3 of the way into the photo is a good rule of thumb... but find that most times i follow this rule, there is just not quite as much DOF as id like. Im refering to photos i take with an aperture of f/22 on my D80 with the Nikkor 18-135mm f/3.5-5.6 at about 20-25mm. Thanks.