DOF

I recently took some bird photos from my truck with the birds only a few feet beyond the close focal point of the lens. I took some of the photos with a 1.4 TC. The DOF seemed shallow on all these photos. My question is how does the DOF vary at a fixed aperture with the distance from the lens?

It's a quadratic function.

For a 500mm lens at f/5.6 on a 5D mkII, that function looks like the above plot.

See this for more info: Depth of Field calculations

I you're sure the problem is just limited DOF, and not other types of fuzziness, ignore the rest of this reply. However, if the effect could be motion blur caused by your equipment moving (not a moving bird), be sure to stop the engine and anything else jiggling your truck. With the tele lens, effects of equipment movement will be exaggerated. Even though my vehicle vibrates very little when the engine is running, I found out there is enough movement to contribute to blur when taking photographs with a full frame sensor camera and 600mm lens at about 17 ft from the subject. This disappears when the engine isn't running.
---John

The chart explains it very well. Thanks for the info.