I am not sure what causes this, but I know a way around it!
First, I would just like to say, very nice render with good lighting!
To fix the problem I would "cheat" the depth of field using the compositor,
Option 1, Use the Defocus Node
Here is a tutorial on how to use the defocus node
Option 2, Use the Mist Pass combined with the Guasian Blur
- Open the Scene Tab (The scene tab is one right of the camera icon):
- Enable mist pass
- Select the camera, and under the camera tab enable mist pass limits
- In the world tap mess with the start and end to tweak the "Depth" of the mist.
- Hit render
- Open the compositor
- Add in the following nodes: color ramp, blur, mix.
- Connect the mist output to the color ramp and the color ramp output to the fac input on the mix
- Connect the image output to the blur input and the blur output to the one of the mix input.
- Connect another image output to the other mix input.
- Mess with the blur values to increase defocus
- Mess with color-ramp to change defocus range
Please let me know if you a picture for the node setup