Depth of field problem

[Richard DeYoung]

I am photographing an exhibit for a master gardner presentation. The object is out doors and is 15x 3 feet. I want photograph the 15 foot side and throw every thing beyond it out of focus. I set my 40D on Av mode f 2.8, the shutter speed is calculated automatically, focus the lens at 17mm and shoot. The object is in focus as well as everything beyond it. I understand depth of field but for some reason its not working for me at this job. The lens is a Tamron 17x50 1:28 Aspherical (IF).
Any help will be appreciated.

[CaptainTom]

http://www.tawbaware.com/maxlyons/calc.htm

http://www.dofmaster.com/dofjs.html


You would need to use a 22mm lens to get the DOF you want
You need to be 15 feet from the subject to get an angle of view of 15 ft.

You are still going to get a pretty hefty DOF.

[kenw]

this might help:
http://www.dofmaster.com/dofjs.html

kenw added 0 Minutes and 36 Seconds later...Double Post Merged Below

Cap'n, we think a like. And simultaneously. One of us should be very worried....

[JohnT]

What is the distance between you and the subject? That 17mm is going to be a tough one. Wide angles have an inherent deep DOF (even at 2.8).

[CaptainTom]

In order to get minimum DOF you need to be as close as possible to the subject. The farther you get away, the more DOF you get.

With your 17mm lens you have to just back away until the subject fills the frame and shoot at minimum aperture. This is the best you will get.

[AndrewCCM]

Consider backing away and using a zoom (along with the calculations mentioned above). You will get more pleasing bokeh/blur much easier this way..

[Richard DeYoung]

tHANKS FOR ALL THE HELP. i AM GOING TO TRY MY 70-200 AND SET IT AT 4.5 AND SEE WHAT i can do.