plane/3d object depth of field problem I have a 3d frog and a plane with an image of a foot.

but despite both being equidistant to the camera, foot is out of focus except for where it is in front of froggy.
If you look at the image, you can see what I mean, especially near his right eye.
Changing the focus distance doesn't help, I dont want to change the depth of field.
It seems to me that there is something untoward happenign, because the foot is obviously 'in focus' but only when there is frog behind it.
further clarification: its not specifically where it intersects with frog, as his right shoulder is further back than the eye. the plane is in front of the shoulder, not touching
If you want deeper focus you need a higher number for the f-stop.
Or you can render the foreground image and deal with the background and it's sharpness through compositing.
How to blur background image.?
How to Blur a Video Background in blender?
An alternative would be to work at a larger scale, so that the model won't be as close to the camera, and the focus won't be as critical.
Depth of Field F-stop not doing anything?
A different solution also is to work with a much smaller sensor, so that you work with even smaller lens size to achieve the same Field of View, and gain a bit of depth of field.
How do I set focus range?