I looked for some tutorials on how to manage depth of field in Poser 11. I found here on Rendo one link that unfortunately take me to pages containing other articles (maybe they are deleted after a while?)
Thanks

I  am not at my Poser computer at the moment, but to achieve depth of field, you need to set your camera f-stop, focusing crosshair for distance, and setting your render engine for depth of field.  Someone else may provide more details on exact location of all the features.  You can get more information from the online manual unless there is something specific you need clarified.

Select the camera with which you wish to achieve DOF.

Go to your top menu. Select Display-->Guides--> Focus Distance Guide.

This will make the guide crosshairs appear. Now go to the camera parameters in your parameters window and move the Focus Distance to where you want the focus to be. Everything that is closer or farther will be blurred. The degree of blur can be adjusted with the fStop value--I find around 5 gives a nice soft blur, but ymmv.

Make sure you have a decent amount of pixel samples set (above 10, mine is usually between 50 and 75).

Hope this helps.

If you are familiar with photography, the fStop value, aperture value, determines how much light reaches the sensor plate and how much distance is in focus before and behind the your point of focus.  Since lighting is not influenced by fStop in 3D programs, the aperture value range will determine the focal depth distance.  On a camera lens, a large fStop value will make the aperture opening small thus increasing your depth of field and making the image foreground and background appear sharper.  Decreasing the fStop value will open the aperture wider, making the depth of field shallower and blurry.  Depth of field is also dependent on the lens focal length but I believe that is not apply to Poser.

Thanks everyone! Tomorrow I will do some experiments......

Thanks again. Finished a little render HERE

You're correct that lens focal length does affect the depth of field. You'll also find there's no infinity focus distance. There's no distance where everything behind the focal point is in focus. You have to keep focus all the way to the end of space.